CA2121464A1 - Portable optical reader system - Google Patents

Abstract

A portable optical reader system for reading optical information over a substantial range of distances includes a casing having a light receiving opening for alignment with optical information.
Further included is a reading sensor for converting a reflected light image of optical information into an information signal and reflected light optical elements for forming a reflected light image. The optical system is adjustable to tend to focus a reflected light image of optical information, located within a substantial range of distances, onto the reading sensor. A light beam generator is associated with the casing and directed relative to the optical elements and the light receiving opening such that a light beam generated thereby will impinge on an information carrier, having optical information to be read, and will be reflected therefrom through the light receiving opening and via the optical elements to the reading sensor. Since the position of impingement of the reflected light beam on the reading sensor is a function of the range of the information carrier from the image sensor, the optical system may be adjusted to focus a reflected light image of optical information associated with the information carrier, at least in part, according to the position of impingement of the reflected light beam on the reading sensor.

Description

W0 93/144~0 ' PCI~/~S9~/~6157 '',, 21 214 6~
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POATABLE OPTICAL READER SYSTEM

Technical Field ' ' ' `' The present invention is directed to optical information readers and `" ~ ~ -more particularly to portable optical reader systems for instantaneously ' ' ', reading optical information over a substantial range of distances.
Description of the Pr~or Art Many industnes designate~ their~ ~products w,ith optically readable information such as bar code symkols consisthg of a series of lines and spàces~ ~of varying widths. Various bar code readers~ and laær scan~ing '`, systems have~ been employed- to decode ;the symbol pattern to a ;muItiple digit representation for inventory and checkout purposes.
Old in the art are conventional conhct and non-contact wand and pen ..
, bar code readers. However! tbese devices are incapable of instantaneously 1 3 ; :~: reading optical information.
Portable~ instantaneous ~optical information readers sre also known to~
the a~,~e.g",,as represented by;~Da~elson and~Durbin U.S. Patent N~ 4,877,949 ' assigned~to the assignee~herein.
Therefore,~ it ~ls ~;a~ principal object ~of the present invention to provide:
20~ an improved ;por~able optical~ reader system for reading optical info~mation over~ a substantial range of distances.
A~other obje~t of the present ~ven~on is to~pro~de~ a new ranging s)rstem for portable optical readers which~enables op:~ al infolmation to be read instantaneously over a ;substantial range of distances.

- 2 5 ~ ~ Another object of the pre,sent invention is to provide~a por~able optical reader system which is~capable of rapid and ef~icient alignment with optical information~ located a substantial distance from the reader.

wo 93/14470 PCr/US92/06157 - 2~2~6~
A further object of the presen~ invention is to provide a portable optical reader system which is easy to use, simple in construction, and trouble free. Other objects will be apparent to those skilled in the art from ~ -the detailed disclosure. -Summarv of _e l~vention ~-The present invention provides a novel system capable of instantaneously reading optically readable information over a substantial - ~ ~ -range of distances. A prefeITed embodiment cornprises a hand-held data collection terminal with a combined reader and RF module which includes a casing having a light receiv~ng opening for alignment with optical information. Housed within the casing are optical means which include light generat~ng means and reflected light collecting means for forming a reflected light image. Also included are reading sensor means for converting a reflected light image of optical information into an informa~ion signal. The optical means is adjustable such that a reflected light image of optical information, located within a substantial range of distances, is focused onto the rea~ding sensor means.
]Light beam genera~ing means is provided for range finding. The light beam generating means is associated with the casiIlg and directed relative to the optical means and the light receiving opening so that a light beam generated by the generating means may be caused to impinge on the surface of an information earrier displaying op~ical inforcllation to be read and will then be reflected from the ca~ier into the light receiving ope~ g through -~
the collect~ng means to the reading sensor means. The system is so 25 configured that the position of impingement of the reflected light beam on the reading sensor means is a function of the range of ~he info~nation ~ -~
carrier. Accordingly, focal adjustments may be made based on position i~formation from the reading sensor for focusing the image o~ optical information onto the reading sensor means. I~ a preferred arrangeme~
30 control means may be pro-~ided fvr accomplishing such adjustme~ts according to the position of impingement of a pair of reflected light beams on the reading sensor means.
Detaiieti DescriDhon of the Drawin~s . ,.`,.~ -. ;'-' ' " '~:

W0 93/14470 2 ~ 2 1 4 ~ ~ PCI/US92/06157 Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which~
Figure 1 is a top plan dia~rammatic view of an embodiment of a S portable optical reader system for reading optical information over a substantial range of distances;
Figure 2A, 2B, and 2C are graphical represe~tations of waveforms illustrating the output of a CCD image sensor under various conditions of focus adjustment and object distance, and with the distance measurement beams active;
Figures 3A, 3BI and 3C are graphical representations of waveforms as a function of time illus~rating the output of a CCD image sensor at zones A~
B, and C, respectively, of Figure l; ~
] igures 4A and 4B are ray diagrams illustrating the viewed range of an object located at a given distance from the window of an embodiment of the present inverltionl Figure 4B is a side elevational traci~g, and Figure 4A is a top plan tracing;
F igure ~ is a side elevational view of an optical system configuration for adjustably imaging optically readable informatton on a reading sensor;
Figures 6 and 7 are side elevational views of a preferred optical system illustrating the optical system of the present invention focused at zero inch and (twenty) inches respectively;
Figure 8 is a graphical representation of a focus quality gradient;
Figures 9A and 9B are graphical represen~ations of a family of 2 5 overlapping focus quality functions for a ~ber of equally spaced positions of a focusing mechanism;
Figures IOA and lOB are graphical representations of waveforms illustrating the output of a CCD image sensor;
Figure 11 is a graphical repreæen~ation of a waveform wherein W is the pulse width at a threshold level, T i~ ~he pulse threshold level, and A is the pulse amplitude;
Figure 12 is a graphical representation illustrating the separation of incident LED spot images on white paper in pixels versus object position in motor steps wherein: line (1) defines this function where the lerls position is WO 93/14470 PC~r/US92~06157 ~ 1 2 ~
zero; line (2) defines this flmction where the lens posi~ion is located at 20 motor steps; and line (3) defines this function where the lens position is located at 40 motor steps;
Figure 13 is a graphical representation o~ the wavefolm of the LED
S spots superimposed on a focused barcode, wherein LED spot centers were located at 285 and 652;
Figure 14 is a graphical half-scale represe~tation of the waveform of Figure 13 wherein the waveform has been processed;
Figure 15 is a graphical representation of a focus quality gradlent 10 wherein ~he focus quality of an object located at a position 12 i~ches from the invention is shown at va~g lens positions as determined by (1) a pulse modulation algorithm, (2) a ~naximum slew rate algorithm. and (3) a max~mum slew rate algorithm wherein th~ signal of every 10 pixels is averaged;
]Figure 1~ is a graphical representa~io~ of a focus quality gradient utilizing a pulse mvdulation algorithm~wherein the lens was focused a~ 9 inches, 13 inches, and 20 inches respectively;
]Figure 17 is a is a graphical representa~ioIl of a focus quality gradient whereiu the focus quality of an ob,~ect located at a pos~tion 20 inches from the invention is shown at va~g lens posieions as determinedby (1) a pulse modulation algorithm, (2) a maximum slew rate algorithm, and (3) a maximum slew rate algorithm wherein the signal of every 10 pixels is averaged;
F}gure 18 is a graphical represe~tatioIl of a focus ~uality gradiellt 2S wherein the focus quality of all object loca~ed at a positio~ 2 inches from the inven~ion is shown at varyi~g lens positions as dete~mined by (1) a pulse modulation algorithm, (2~ a ma~mum slew rate algorithm, (3) a maximum slew rate algorithm wher~ the signal of every 10 pixels is averaged, and (4 a rela~ive length of cuIve algoritbm;
Figure 19 is a graphical representation of a focus quality gradient wherein the focus quality of an object located at a posi~ion 4 i~ches from the invention is sh~wn at va~g lens positions as determined by (1) a pulse modulation algorithm, (2) a maximum slew rate algorithm, and (3) a ': ;~.-,., wo 93/14470 21214 6 ~ pcr/us92/o61s7 -maximum slew rate algorithm wherein the signal of every lO pixels is averaged;
Figure 20 is a graphical representation illustrating the separation of incident LED spot images on a bar code label in pixels versus object position 5 in motor steps wherein: line (1) defines this function where the lens positionis zero; line (2) defines ~his function where the lens position is located at 20motor steps; and line (3) defines this function where the lens position is located at 40 motor steps; :-Figure 21 is a graphical representation of the waveform of two LED
10spot images superimposed, slightly out of focus, and incident on a dense bar code label wherein the lens was located at 40 steps and the label was located at 38 steps (illuminations was liigh with peaks detected at 326, 576);
]Figure 22 is~a graphical representation of the waveform of two LED
spot images superimposed, out of foeus, and incident on a dense bar code 15label wherein the lens was located at 40 steps and ~he label was located at 30 stepls (illuminations was high with peaks aetected at 271, 644);
Figure 23 is a graphical representation of ~he waveform of two LED
spot imlages superimposed, In focus, and incident on a dense bar code label wherein the lens was located at 30 steps and the label was located at 30 ~:
20steps (illuminations was high with peaks detected at 288, 646);
Figure 24 is the processed scan illustrated by Figure 23 showing the :
uneven illumination of ~he bar code label;
Figure 25 is a graphical representatio~ of the waveform of two LED
spot images superimposed, ou~ of focus, and incident on a slense bar code 25label wherein the lens was located at 30 steps and ~he label was located at 20 steps (peaks detected at 198, 723~
Figure 2C is a graphical representation of the waveform of two LED
spot images superimposed, in focus~ and i~cident on a llense bar code label wherein the lens was located at 20 steps a~d the label was located at 20 30steps (peaks detected at 195, 738);
Figure 27 is a graphical represelltation of the waveform of two LED
spo~ images superimposed, out of focus, and incident on a dense bar code label wherein the lens was located at 20 steps and the label was loca~ed at 10 steps (pealcs detected at 114, 813);

2~2~
WO 93~14470 PCr~US92/061~7 Figure 28 is a graphical representation of the waveform of two I~D
spot images supeIimposed, in focus, and ineident on a dense bar code label wherein the lens was located at 10 steps and the label was located at 10 steps (peaks deteeted at 107, 831);
Figure 29 is a graphical represeIltation of the waveform of two IED
spot images superimposed, out of focus, and iIlcident on a dense bar code label wherein the lens was located at 0 steps and the label was located at 10 steps (peaks detected at 84, 843);
Figures 30 and 31 are side elevational views of a preferred optical system utilizing a moveable prism;
Figures 32, 33, and 34 are graphical representations of focus quality gradients prepared using a pulse modulation algofithm wherein the lens was focused at 18 inches, 9 inches, and 13 inches respe~ively;
E igures 3~, 36, 37, and 38 are graphical representations of the waveform of a reflected light image of a bar code label wherein illumina~ion was constant and focus quality was changed;
F igure 39, 40, and 41 are graphical representations of the wavefo~m of .. .
a reflected light image of a bar code label wherein focus quality was constant ~ -and illumination was changed;
Figure 42 is a graphical representation of a focus quality gradient wherein the focus quality of an obJect located at a position 20 inches from ... .
the invention is shown at va~g lens positions as determined by (1) a pulse modulation algorithm, (2) a maximum slew rate algorithm, (3) a maximum -:
slew rate algorithm wherein the signal of every 10 lpixels is averaged, and (4) : ~ :
a relative length of curve algorithm;
~ . .
Figure 43 is a grapbical represeIltation of a focus quality gradie~t wherein the focus quality of an object located at a pssi~ion 5 inches from ~the i~vention is shown at varying lens positions as determined by (1) a pulse modulation algorithm, (2) a maximum slew rate algo~ithm, (3) a maximum slew rate algorithm wherein the signal of eve~ 10 pixels is averaged, and (4) a relative length of curve algorithm; a~d Figure 44 is a graphical representatiorl of a focus quality gradient wherein the focus quality of an object located at a position 2 inches from the invention is shown at varying lens positions as determined by (1) a pulse :;

wo 93/14470 2 ~ 2 1 ~ ~ ~ PCr/US92/06157 modulation a~gorithm, (2) a maximum slew rate algorithm. (3) a maximum slew rate algorithm wherein the signal of every I0 pixels is averaged, and (4) a relative length of curve algorithm.
Figures 45 - 48 depict a flow diagram of an exemplary ins$ructio~ set 5 to be used by the microprocessor to carry out the processes of the present invention.

While the invention will be described with a preferred embodiment, it will be understood that it is not ~ntended to limit the Lnvention to that 10 embodiment On the contrary, it is intented to cover all alternatives, modifications and equivalents as may be included uithin the spirit and scope of the invention as defined by the appended claims.
Best Mode for Ca~ving Out the Invention ~ ;
Fig. 1 is a diagrammatic illustration of a presently preferred 15 embodiment of the instant inventiolL While the reading system of Fig. 1 could b,e contained within a housing such as indicated in the third figure of U.S.PatentNQ4,877,9~9,andasdescribedindetailLnl:J.S.PatentNQ4,570,057, it is hilghly advantageous to be able to house ~he reader system of Fig. 1 within a compact modular casing such as generally indicated in the thirty-20 second figure of the international patent application published under thePatent Cooperation Treaty as International Publication N8 WO90/16033 with a publication date of December 27, I990. Where an RF module as shown in the thirty~sixth figure of WO90/16033 might have a depth of about three centimeters, a ~width of about seven centimeters, arid a length of about eight 25 centimeters (exclusive of RF antenna), a combined RF and bar code reader module utilizing the reader sys~em of Fig. 1 might have the same length and width, and a depth of about six ce~timeters, or less. The reader system of Fig. 1 including associated electronics would thus1 preferably, occupy a volume of less than 168 cubic centimeters. Rectangle I0 i~ Fig. 1 may 30 represent a module casing havi~g a length of about eight centimeters, a width of about seven centimeters, and a depth of about six centimeters, constructed to i~terchangeably engage with a base housing such as utilized with the RT 1000 radio data terminal of Norand Corporation, and containing an RF transceiver in addition to the reader system of Fig. 1.

WO 93/14470 P~us92/lK1~i7 2 1 ~
As diagrammatically indicated in F~g. 1, ~he module easing 10 may have a frontal end lOa with a light transmissive window lOb accommodating marginal light rays, such as 11, 12. In the event ambient ligh~ alone is insufficient to obtain a valid reading, an elongated light source î4 may be electrically energized from a lamp dr~ver 16 to illuminate the region bounded by marginal rays 11,12. For the case of a bar code, the maximum length of bar codes which may be read depends on the distance of the bar code carrier from the reader frontal end. At a minimum distance, a bar code of a length corresponding to the width of the light transmissive window (60mm) may be successfully read, while at a distance of 1.5 inches, the bar code may have A
length of ninety millimeters, and at a distance of three inches, the bar code may have a length of one hundred ~hirty millimeters.
Bar codes occupying locations within the field defined by marginal rays 11~12, and lying between zones A and C in Fig. l, may be focused onto a reading sensor 20 by adjustment of an optical system diagrammatically indicated a~ 22 and as represented by double headed arrow 24.
]ln ~he illustrated embodiment, it is contemplated that the reading sensor 20 ~e.g., a CCD type image sensor with a length of abou~ thir~
millimeters and between 2,000 and S,000 elements), may be driven via clock buffers 26 from a microprocessor 3a, and that each pixel value in the output from the sensor 20 may be converted to digital form (e.g., a binary ~umber) with a gray scale range of substantial extent for processing, either by a high speed ex~errlal analog to digital converter 32, or by comparable on-chip means associated with microprocessor 30. The :microprocessor is indicated as controlling Focus via a stepper driver 34 a~d stepper motor 36.
Visible light beam generators 41, 42 are indicated as being electrically energized under the control of microprocessor 30 via a d~iver 44. The beams 51, 52 from beam generators 4i, 42 preferably produce sharply defiIled spots on a bar code carrier within the operating range of the lens system.
When the reader system is activated e.g., by manually operating a trigger button such as indicated in WO90/16033 (e.g., at 651 ill the thirty-second figure), the microprocessor may flash the ~eam generators 41, 42 to generate beams 51, S2. These beams assist the user in aiming the reader toward a bar code. Further, when the beams 51, 32 are directed so as ~o wo 93/14470 PCr/US~ 7 2~21~9 impinge on a bar code carrier, the reflected ~ight from the beams will in part follow a path through ~he light transmissive window lob at frontal end lûa, and via optics system 22 to impinge on reading sensor 20. Even though the reflected beams may not be sharply focused Oll the reading sensor 20, it is conceived that ~he reading sensor may be utilized to supply the output generated by the reflected beams (with light source 14 inacitive~ and that such output may be processed by the microprocessor 30 to obtain an indication of the direction and amount to move the lens system to improve focus.
Furthermore, as focus improves, a further flashing of beams 51, 52 may result in a more accurate measure of the positions at which the reflected beams impinge on the reading sensor. In this way, the microprocessor may quickly determine the diseance ~to move lens system 22 so as to bring the bar code within the depth of field range of the lens system.
Experimental Work Re~ardin~ Fi~
I.n ~esting the conception of the use of marker beams 51, 52 for obtainiIlg a distance indication. an optical system was used having an ~-m~mber for irlfini~ of four and requiring a change in the optical path distance from the lens to the sensor of about fourteen millimeters (14mm).
Such a system was determined to be capable of focusing over distances from lens to an object between nil and 578 millimeters t578mm). Beam sources corresponding to sources 41, 42 were light emitting diodes that created light spots at a distance of twenty inches which were about one inch square. At close up distances between zero a~d one inch, such light spots were about one-quarter (~) inch square. Everl with such light emitting diode~, it was found that distance measurem~ts were feasible ~ased on outputs from the CCD sensor such as represe~ted in Fig. 2A. Figs. ~, 2B and 2C show waveforms representing the output of a CCD image sensor using ~he beaIn sources just described.
The sensor S~o~ output was di~itized by a flash six bit A~D conver~er (32) and acquired by a personal computer (PC). The PC was programmed to store the image iIl memory, display the image on a monitor, and then print a graphical representation of the image Oll paper. These graphs show the images of the spots of light produced by the two pointing beams (51, 52) for various distances to an object. Fig. 2A was taken when the dist~nce was Wo 93/lq470 PCr/US92/06157 lo 212~
twenty inches and the lens 22 was focused for zero inch object distanre.
Gradual slopes of the pulses 53, 54 may be seen in Fig. 2A, but pulses due to the beams are clearly identifiable. The distallce between the pulse centers is small (about 189 pixels). On the graph of Fig. 2B, there is show~ an image 5 of the same spots at the same object distance of twenty inches, but the lens is now focused also at twenty inches. The slopes of the pulses 55, 56 are sharper. The distance betweeri cen~ers is a bit smaller ~about 170 pixels) due to an increased reduction coefficient, (i.e., the distance between the lens and sensor was reduced during focusLng). For accurate range measurements a 10 correction measure, in order to compensate for this effect, must be performed by a microcomputer.
Fig. 2C shows the image of the spots 57, 58 when the object is positioned at a five inch distance and the lens 22 is focused far zero inch object position. In this configuration ~he distance between ~he spots is about 15 416 pixels indicating a smaller distance to the targ~t.
It may be noted that a light emit~ing diode with a special lens could be use~d for the beam source to provide a much more sharply collimated - beam. Further of course, solid state laser diode sources could provide beams which would produce spots of one to two millimeters diameter at twenty 20 inches. Based on the experimental results, it is concluded that such more expensive beam sources should not be necessary for the purposes of a low cos~ embodiment according eo Fig. 1.
Description of TABLES I and Il. FIGS. 3A, ~ C. 4A & 4~
In explaining an exemplary mode of operation, it is useful to refer to 25 the TABLES I and II on the following pages~
~''~ ''.~ . . ' WO 93/14470 PCrtUSg2/~6157 11 212~6~ ~
TABLE I :
D distance from window to ob,ject (Lnches) ~ --B distance from the lens to the object (mm) b barwidth (mils) ~:
5 K magnification R viewed range in mm (inch) :
A distance from the lens to the sensor (mm) P14 size of a pixel image for sensors with 14 microns per pixel (lL~ce TCDl 201 ) 10 P11 size of a pixel image for sensors with 11 microns per pixel (l~ce TCD1250) P8 size of a pixel image for sensors wi~h 8 microns per pixel (llke TCD1 300) lp required lens resolution in line pairs per mm at 0 deg pitch and 50%
modulatio~ to read corresp~nding bar width b lp(~D55 required resolution at 55 degree pitch and 50% modulation to read corresponding bar of wid~h b @) 55 ~-__ r_ _ __ __ ~ __ _ D B b I; R R AP14 Pl I PJ
(Inehe~ ~ (~' ~ ~IDche~)~n) ~ll~r ~ ~mlb) , _ . _~_ _ _ _ O 70 1.7 51 (2.0) 41.Z .94 .7~ .54 . _ __ . .
106 4 3~6 95 (375~ 34.2 1.74 1.37 10 3.0 146 5 4.62 139 (5.S) 31.7 æss 2.0 1.4 _ _ . __ __ . . __ .. ; ~
6.0 222 75 7.55 227 (9.0~ 29.5 ~16 3.27 ~38 _ , _ . _ _~ . o_ . ~ ~ . . . , ~, ~
451 2~ 16.35 460 (lQ0) 27.6 Q0 7.04 5.15 , __ _ _ _. __ . _ __ ., _ , ~ _ 576 40 7 23 637 ~25) 2723 117 al6 669 o~e mil = 0.001 Inch ::

SUE~STlTlJTE SH~
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W093/1q470 1~ 2121~ 6~

TABLE II
__ ___ _ ____ D b baP201p- b@55 Ip Ip~?SS
. .. . _ . _ ~
1.5 4 3.11 2.3 lS.6 27.1 . . .
3.0 5 4.55 2.9 18.2 31 _ . ._ __ _ , __ ~. ......
S 6.0 7.5 7.43 4.3 19.8 34.5 :
. ... . . . __ . ~ .. . _ IS 20 16.1 ll.S 16.1 28 _ _ _ _ . __ . _ __ :
90 20.1 23.0 ~0.5 18.2 :::
, _ , . ~
max variation of distance A is 14mm ~b@201p is the width of a bar which can be resolved with 50%
modulation at distance D using a lens quality of twenty l~ne pairs per ; ~ ;~
millimeter .: :,, .

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SUaSTITUTE SHE~T
- :-wo s3tl447~ u~92to6ls7 13 2 1 ~ 4 .
pixel size: 14 microns ¦ 11 microns ¦ 8 microns ~ ~
_ ~
D b pixels per narrow bar; 0 deg pitch , . .. _ . . _ . . ~
1.5 4 2.3 2.9 4 . .. _ ___ -. . . __ . . ...
3.0 5 1.96 2~5 3.42 .,. _ . . . _ __ 6.0 7.5 1.8 2.29 3.15 _ ~ _ 15.0 20 2.2 2.82 3.88 ._ _ .
20.0 40 3.42 4.35 5.98 _ _ . __ pixels per narrow bar; 55 deg pitch . , ... _ .... , , . .

4 1.32 1.66 2.29 ,. .__ _ ".
1.12 1.43 1.96 . . _ ., , . ...
7.5 1.03 1.32 1.81 , ....... ~ _ . .. . . . _ .
1 26 1062 2.23 1.96 2 50 3.43 pixels per narrow bar; 45 deg pitch . . ,, . ~
4 1.63 2.05 2.83 ~-_ , . . . .
1.39 1.77 2.42 . _ __ ~
7.5 1.27 1.62 2.23 ~_ ; .~ . .
1.56 1.99 2.74 .. . ......... _ . ". ~.
24~ 308 423 ~-~

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wo 93/14470 Pcr/us92t~6157 - -14 2 ~ 2 1 ~

Assum~ng that the optical system is initially set for focusing a bar code at Zone C ~e.g., D - 20 inches in Table I), when the reader trigger is actuated, microprocessor 30 is awakened from a sleep mode by means of a host system via interXace buffers 60. At this time, the microprocessor 30 has stored in its memory the focus condition of the lerls system (e.g., A - 27~23, Table I), e.g., by virtue of a read only memory having this parameter stored therein as a sta~ up value.
The microprocessor 30 may emit a pulse to the LED driver 44 of predetermined duration sufficient to create visible marker spots at a distance of twenty inches, but of brief duration to avoid detrimental blurring. The reflected light due to beams Sl, 52 is evaluated based on the output from the image sensor 20. If the output from the sensor 20 resembles Fig. 3C, where pulses 61~ 62 have a relatively close spacing (in time) Pc relative to total sensor readout time Ts computer 30 could conclude ~hat the focus adjust~ment was proper without activa~ing steppi~g motor driver 34. In this case, with the beam sources 4l, 4~ de-energized, the clock buffers 26 could ., ~.
be activated to clear the image sensor 20 and effect a readout therefrom. The output frorn A/D converter 32 would then be analyzed by the microcomputer, and thle corresponding rectangular waveform bar code signal (analogous to a conventional emulated wand bar code signal) could be transmitted to buffers component 60. If microprocessor 30 concluded that a valid bar code had been transmitted, or upon receiving a command from ~he host, microprocessor 30 could retum to a stalldby or sleep mode, leaving the lens system at its existing position. In this case, since the stepper driver had not been actuated, the microcomputer would recognize that the lens ad~justment remained at its irlitial value (A Y 27.23 mm, Table I), and this current lens adjustme~t value would be stored i~ a lens conditiorl register (e.g., maintained by baetery power while the reader remained in a stan~by mode).
If the initial distance reading based on beams ~l, 52 showed an output like Fig. 3A or Fig. 3B, the microprocessor would then activate stepper driver 34 to effect focus adjustment. At a later instant of time9 the microprocessor could take a reading from sensor 20 with the beam sources 41, 42 off.
Normally at this time, the user might not yet have aligned the reader with ~he wo 93/14470 pcr/us92/o61s7 15 2 ~ 2 ~
bar code, but the ou~put from converter 3~ would provide information which could be used to determine whether the au~liary light source 14 should be pulsed via lamp driver l6.
A next operation of the microcomputer might be either to again pulse S beam sources 41, 42 to maintain the marker beam and distance monitor function, or to pulse the light source 14 and make a further attempt at a bar code reading. The driving pulse supplied to the light source 14 could have a time duration such as tO provide for a va]id bar code reading even with stepper motor 36 operating at top speed.
l 0 With each successive distance measurement with focus improving, the microprocessor 30 could refine the target lens ad~justment. As such target focus c ondition was approached, the microcomputer would follow an optimum deceleration program to bring the lens system to a stop at an optimum focus position. As previously mentioned, the microprocessor 15 would maintain a current register for lens focus posi~ion as pulses were suppliesl to the c:river 34, and, for use in a subsequent reading operation, this register could then show the final position of the lens system where a valid bar code reading had been obtained.
From the foregoing, it wDI be understood that the microprocessor 30 20 is preferably programmed so as to provide periodic flashes of the beam sources 41, 42 while a reading operation is in progress so that the operator is adequately guided in aiming of the reader toward a bar code. Particularly when the motor 36 is operating at relatively high speed, the operation of auxiliary light source 14, where indicated, will be brief enough to avoid 2 5 detrimental blur. To conserve ba~tery power, the light source 14 is preferably only flashed, as may be needed, to verify the microcomputer's evaluation OI
operating conditions until such time that the microcompllter determines that proximity to optimum focus has been reached, e.g., where the object is located within the depth of focus of the lens system for the current le~s 30 system adjusted position.
In any case, the microprocessor may freque~ly examine the output of the image sensor 20 using ambient hght during focus adjustments since this does not require the use of lamp driving power, and since the readings can provide useful information concerning the need for the auxiliary light source WO 93/14470 Pcr~us92/o61~7 ~
16 2121~
and/or supplemental information conceFning the focus condition of the optical system. It is preferred, howe~er, that bar code information only be sen~ to a host system where such information appears to represent valid bar code information which could be decoded by the host. An alternative would 5 be for the microprocessor to decode the incoming signal from converter 3~
prior to sending bar code information to the host. In this case, microprocessor 30 would genërate a binary ~rectangular waveform) bar code signal to represent the decoded data, or of course, merely send the bar code number as decoded.
l 0When the reader system is deactivated, microprocessor 30 may activate the stepper motor 36 to place the lens system 22 in such initial condition, e.g., focused at a distance where the next label is most likely to be found based on the experience of the last few reading operations.
The following is a summary of the preselltly preferred sequence of 15 steps to be taken by the system of Fig. 1 to achieve and maintain optimum focus ~a mirror is moved to effect focusing e.g., as shown ~n Figs. 6 and 7 to be described hereafter~
STEP (1) Power up, system in~tialization, the mirror moves to an i~itial positiun providing focus for remote zone (20 inches~
20STEP (2) System receives ~irst request to read label and the computer turns the pointing beams ON;
STEP (3) Image taken with pointing beams and the distance between the pointing beam cen~ers of area is dete~ed (the current range is calculated from the known function of ~he distance between the pointing 25beam cerlters of area);
STEP (4) A comparison of the focusing mirror's existi~g position with the desired focusing mirror position is made and the computer calculates the amount and direction of corrective motion and se~ds the necessary control signals to the motor driver;
30STEP (5j An image of the object without the poL~ing beams is taken, processed and made available for decodi~g;
STEP (6) The cycle may be repeatecl in order to correct the focu~ing mirror position by taking ~n image with the pointing beams "ON" for wo 93/14470 PCr/US92/061~7 17 2t2~
remeasuring the interval between the spots (the computer recognizes the spot images since they are substantially different from barcode pulses); and STEP (7) After successfully decoding label data, ~he reading system, including the microcontroller, goes into a power down mode for co~s~ving 5 ~he battery energy, leaving the focusing mirror in the latest position.
The nex~ time the system is activated, it begins the cycle from the previous focusing position. T}ie pointing beams may be generated by either bright visible LEDs or visible solid state lasers. Of course, both LEDs and lasers may be of an infrared type if their purpose would be limited only to 10 range measurement.
A single beam (instead of the two beams used ill the preferred embodiment) might also be use~ for the same purpose. The position of the center (of area of the image) of a single beam relative to a fixed point on the image sensor would then correspond with the distance to the target.
15 Howeverl a target surfacc pitch angle might produce an error since the point of ~mpi~gement of a sir ~eam might not represent the average distance of the label ~rom the re~
For optimum results two pointing beams are preferred. Two beams provide the computer with the information necessary to determlne the 20 distance to the target independently in two points, i.e., the centers of the two pointing spots. The d~stance between spots provides knowledge pertaining to the distance to the middle point between the spots, but if necessary, the individual distance to each spot may be determined and used for reading long labels at significant pitch angles by sequentially focusing at both halves 25 ~ and reconstructing the full label data.
Currently~available visible LEDs may be utilized without additional lenses to distances of about 1 to 2 feet. If a longer range is desired, visible lasers or collimating lenses (external to LED) may be used. For cost reductlon, two beams may be created with a single laser with a beam splitter 30 and mirror.
:
EsemDIan Det~ils of A~ ian Liebt Sollrce ~Fi~. 5 While the light source 14 may be of tne type disclosed in the aforementioned Laser et al. U.S. Patent 4,570,057, it is conceived that the `~
light source might comprise a tungsten wire 70 with an associated reflector WO 93/1447~ Pcr/us~2/o~57 : : ~ 18 2 1 2 ~
71 (as indicated in Fig. 5). Tension springs may act on the opposite ends of the w~re in order to absorb the elongation of the wire when dIiven to emit a light pulse. Microcomputer 30 may monitor the pulse excitation of the wire 70 so as to take into account wire temperature du~ing a further electric S current pulse, and may fur~her limit the dura~ion of the electFic current pulse to only that time interval required to produce the desired light output.
The type of auxiliary on-board light source may be, for ~xample, either: (1) an array of bright LEDs, (2) a long f~lament incandescent bulb powered m a supercharge mode, or (3) a strobe light.
The use of a long filament incandescent bulb, powered in a supercharge mode, has been tested. A significantly bright pulse of light may be regularly produced by applying a 2 ms pulse with a voltage eight times the nominal lamp voltage. T4e energy spent in such a pulse was determined to be 0 15 J. This approximates the same energy used by the strobe light found in the product described in Laser, et al., U.S. Patent N8 4,570,057.
Withou~: undue testing, data may be determined regarding: (1) the necessary voltage and power requirement per pulse, (2) energy storage devices, (3) filament modification, and (4) bulb life.
I~candescent lamps are capable of producing pulses of light with 20 durations in milliseconds. Incandescent lamps provide the following advantages: (1) small size, (2) power efficiency, (3) utilization with a concentrated flux and small reflector (a thin filament presents an accurate ligh~ source), (4) long life, ~5) durability, (6) high energy flash output, anà (7) the amount of light per pulse may be co~trolled.
2 5 Exist~ng line filament lamps exhibit a typical life of between llQOO to 25,000 hour:s. Even in pulse mode where lifetime would bc reduced ten times, the number of 3 ms pulses available would be over 50 millio~
Strobe lights are also commonly utilized as light sources. Strobe lights may be the best option where very fast label motion is required. In order to .. ~, , : achieve a 10 inch per second non-blu~Ted image speed twith a label having ~ ::
5.0 mil bars) the CCD integration time should not exceed 250 microseconds.
However, such an integration time is not a problem if an electronic shutter ~-is utilized. ln such a case, the required level of label illumination will be very high. The strobe light hardware described in the Laser, et al., U.S. Patent N~
. -:., ,:
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wo 93/14470 PCr/US92/061~7 19 ~121~6~
4.570,057 might be utilized for the instant application. Where fast label motion is required a customized xenon tube with a significantly reduced diameter would produce a much more accurate light beam that would be easy to form and direct efficiently. Such a customized xenon tube would 5 reduce power consumption, component size, and proYide an increased firing rate.
EsemDIar~ O~tical S~st~J~,~
In an exemplary embodime~ the lens 81 should be aspheric with an F-number of approximately five.
In such an embodiment ~he variable parameter for focusing is the distance between the sensor 82 and the lens ~1. A light weight mirror 84 is inserted in the optical path between the sensor 82 and the lens 81 to serve tw~ goals: (1) to shorten the required length of the housing, and (2) to alter the distance by moving a low mass component.
The second goal is the most critical of the two. For fast acting focusing mechal~isms the amount of movi~g mass is a crucial consideration In an exemplary embodiment the mi~or 84 and suppor~ing bracl~et 85 may be made as a single metallized plastic part with a mass of less tha~ 0.5 grams. In the proposed mechanism, when the~mirror moves toward the 20 sensor, it slightly rotates counter-clockwise due to the difference in the distances between the top and bottom pairs of axes. This rotation helps to keep direction of view from changing in the plane perpendicular to the direction of CCI3 scanning. In a~ exemplary embodiment this rotational movement ls accomplished by~pivotally mounting tlhe~mirror support braclcet 25 85 ~to a ~drive quadra~t 86. The drive quadrant is driYably connected to a stepper motor 87.
A single photosensor array 82, mounted within a housing, is utilized for both ranging and imagi~ig purpo~es. The housL~g indudes a light recehiing opening for alignm~ with optical informatio~ or ehe like. Optical 3() ~ mea~s are associated with ~he array 82 such that the image of optical information may be focused on the array 82.
The optical means includes a lens 81 associated with a primary mirror 83 (Fig. ~, 6, & 7). Light entering the housing through the opening is refractedby the lens 81 onto the primary mirror 83. Light refracted by the lens 81 onto wo ~3/14470 P~r/u~s2/o~ls7 2 1 2 ~ ~ 6~
the primary mirror 83 is then reflected back through the lens 81 onto the focusing mirror 84.
The focusing mirror 84 may be adjusted such that both the angle of separation and distance between the mirrors may be changed (Fig. C & 7).
5 The focusing mirror 84 is held by the bracket 85 which is pivotably connected to a drive quadrant ~6. Ro~ation of a drive motor shaft causes the posi~ion of the focusing mirror 84 to change in relation to the array 82 (Fig. 6 & 7).
In operation, the image of a bar code, OI the IL~ce, is focused from any distance, within a substantial range of distances, onto the array 82 by lO operation of the stepper motor 87.
Esem~lnrv Detai_s for Distance Mea~urement and Autofocu~ Control An ultra high brightness ~ Fn ls preferably used for distance measurement. Such an TEn is commercially available as part ANDl90CRP
from A~ND Corporation of Bur~ingame, California. The AND19OCRP may be l 5 utilized in pulse mode. In pulse mode cu~ren~ may be increased up to ten times m order to achieve a corresponding increase in brightness. Since available power is limited to five volts, a current increase nf ~hree to four times may be expected in pulse mode.
Mirror positioning might be based on a number of approaches. The 20 auto-focusing algorithm rnay have several branches with various priorities depending on the instant situation.
Focusing data might be derived from the distance between the two red Pointer spots. With a pair of ~Kilobright" LEDs the narrow beams could be turned on and off during altema~e scans so the image could be taken with -25 or without the LE13~beams.
Another source of focusing information might be derived from an analysis of sensor data. In~ a first case the steepness of slope~ may be studied for deciding which direction to drive the focusing mirror. II1 a second case, especially when a label lilce pattern is scanned, the modulation 30 coefficient` for different pulse widths could be measured and then compared with a similar measurement of a following scan.
In order to stay away from the need to recognize particular puIses or pulse sequences in different scans (which were taken in different points of time) the processed data could be sorted statistically in histograms, :

WO 93/14470 P(~/US92/061~7 21 212~
representing distribution of modulations Ln the pulse width domain. Then particular groups of pulse widths could be idelltified in different scans and their average modulations compared. This comparison would provide focusing quality (FQ) crite~a. The cnmputer could trace the history of FQ
S and make decisions regarding the amount and direction of focusing movement.
Metbods of Determini~Q Focu~ing~O~alitv Automatic focusing of a barcode reading system may be accomplished by analyzing, in real time~ certain qualities of an image signal produced by 10 a CCD sensor. A numerical value may be calculated to provide the quantitative estimate of the focusing accuracy Q This focus criterion Qmay then be used to estimate the amount of ad~justmen~ necessary to achieve an optimum focus. The following three methods may be used for determining Q
l. Co~trast Gradient 'Slopes of pulses represent transitions from dark to bright and are steeper for sharp images. The microprocessor 30 may determine the sharpest slope in a scan and record its value in volts per p~xel. The slope rate is a function of focus quality. The slope rate function may be reshaped 20 into a Qvalue by using a table.
2. Distributio~ of Modulation~
Since the MTF (modulatio~ transfer function) of all optical systems changes shape significantly depending on the position of best focus r~atively to the position of an object, tbe degree of modulation of pulses present in 25 a bar cod~image signal depends on the width of those pulses.
Depending on the speed performance of the u~ilized processor, either a portion or entire length~ of a data string may be used for analysis. The modulations for each bar in an area of i~terest may be measured as:
m~A/E
30 where:
A is an amplitude of a pulse, measured from its root to peak value, and E is an average of peak values for bright and dark pulses in the area of interest.

-~ .

WO 93/l447~ P~r/U~i92106157 22 2 1 2 ~ ~ 6 ~
In an exemplary embodiment modulations may be stored temporarily along with values of corresponding widths. All pulses may ~hen be sorted into two groups, e.g., a narrow pulse group and a wide pulse group. The limit between the groups may be calculated as~
S Wl = k Ws where:
Wl is the largest width permitted for the narrow group, W2 is the shortest pulse width in a scan, and k is an empirically ad~usted coefficient equal to 2.5 + 0.5.
The average modulations for each group (M" for a narrow and M, for a wide group) may then be calculated.
Finally, focusing quali~y Q, rnay be derived through modulation measurements as~
Q~n = Mn / (Mw x Wl ) Since this value is statistical in nature, normalized for average brightness, the value does not depen~d on the signal or con~ent of the image.
3. Dispersio~ sf E~tremes An alternate method of focus quality measurement is based on~the measurement of the dispersion of extreme values. This method is also statistical in nature and based on the fact that in an image, consisting of pulses of various widths the amplitudes of short pulses degrade faster then the amplitudes of wider pulses as image focus quality decreases. Thus, amplitudes become wider as focus quality decreases. Focus quality under this method may be calculated as~
~ ~; Q - ~ (B~ - Bo)/nB~ + ~ (DI - DO)tnD~
Where Q, is a measure of focus quality w~ch represents a normalized dispersion of the extreme values of a signal. Without loss of functionality the squaring of deviations may be omitted in order to increase processing speed.
A, is a current extreme value of a bright pulse, A, is an average value of ~right extremes ((l/r B, is a ellrrent extreme value of a dark pulse, B~, is an average value of dark extremes ((l/~)~B,~, and a is the number of pulse pairs.
-' ..

W0 93/14470 PCr/US92J06157 ~3 2 1 2 1 ~ 6 ~
This s~mple and fast method can give reliable results when the number of pu~ses exceeds some minimum (obviously, for a single pulse the dispersion is nil).
For any particular position of a focus adjusting mechanism the focus 5 quality depends on the position of an object. Fig. 8 shows a typical curve.
When an object is in the best focus position (p), focus quality reaches its maximum, Q=100%. At any other position of the object, the focus quality decreases. Since a small decreases in focus quality do not diminish image quality below that necessary for successful decoding, a depth of focus zone lO (~ f) around the best focus (p) position may be used.
Even though this function exhibits duality, it is still very useful for determin~ng the position of the best focus by measuring its value at a current ~osition, e.g., if it is possible to measure a current value q of the focus quality function in the current position of the focusing mechanism (pl or p2) 15 then the distance to the best position p may be found, and the duplicity problem could be resolved by tracing ~he direction and result of a previous move.
T:he function of focus quality very often is descrl~ed in terms of a modulation transfer function, which gives a complete characteiization of the 20 quality of image produced by an optical system in a given focusing situation.Fig. 9A and Fig. 9B illustrate a family of overlapping focus quality functions for a number of equally spaced positions of a focusing mechanism.
It should be apparent that bar code density must decrease as distance is increased. The further the target with a label, the wider the minimum bar 2 5 (or space) of a bar code. ThiS corresponds with the function of a fixed focal length lens where the distance from the lens to the sensor is ad~justable for focusing.
- Fig. 9A best illustrates focus quality as the positiorl of best focus moves towards longer ranges. However, the focusing mechanism would 30 ~ove an equal amount Uor example 3 steps of a stepper motor) between ~.
~each of the shown positions. Correspondingly, in the image space (space between the lens and the sensor) projections of these focus quality functions (representing acceptable depths of field at various ranges) all the curves will Wo 93/14470 pcr/us92/o61s7 : ~ -24 2 ~ 2 1 ~ 6 ~
be about the same and equally spaced, as shown on the Fig. gs. This adds to the simplicity of the focusing mechanism design.
While the invention has been described with a certain degree of particularity, it is manifest that many changes may be made in the details of 5 construction and the arrangement of components without departirlg from the spirit and scope of the disclosure. It is understood that the invention is not limited to the embodiments set forth herein for pu~poses of ~xemplification, but is to be limited only by the scope of the appended claims including the full range of equivalency to which each element thereof is entitled.
Thus, there has been shown a~d described an improved portable optical reader system for reading optical information over a substantial range of distances which accomplishes at least all of the stated objects. ~ :

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Claims (19)

I claim:

1. A portable optical reader system for reading optical information over a substantial range of distances, comprising:
(a) a casing having a light receiving opening for alignment with optical information to be read;
(b) optical means comprising reflected light collecting means for forming a reflected image and reading sensor means for converting a reflected image of optical information into an information signal, said optical means being adjustable such that said collection means tends to focus a reflected light image of optical information located within said substantial range of distances onto said reading sensor means;
(c) light beam generating means associated with said casing and directed relative to said optical means and said light receiving opening such that a light beam generated thereby will impinge on an information carrier having optical information to be read and will be reflected therefrom through said light receiving opening and via said collecting means to said reading sensor means, the position of impingement of the reflected light beam on said reading sensor means being a function of the range of the information carrier from said image sensor; and (d) control means for adjusting said optical means to tend to focus a reflected light image of optical information associated with the information carrier at least in part in accordance with the position of impingement of the reflected light beam on said reading sensor means.

2. The portable optical reader system of claim 1, wherein said reading sensor means comprises a solid state photoelectric sensor array capable of resolving elements of optically readable information simultaneously incident thereon.

3. The portable optical reader system of claim 1, wherein said optical means include at least one lens.

4. The portable optical reader system of claim 3, wherein said optical means further include a first mirror for collecting light from said lens and reflecting said light.

5. The portable optical reader system of claim 4, wherein said optical means further include a movable mirror for effecting focusing of light reflected from said first mirror onto said sensor means.

6. The portable optical reader system of claim 5, wherein said optical means further include means for adjustably moving said mirror in relation to said sensor means without change of the aiming axis of the optical means which is to be aligned with the optical information.

7. The portable optical reader system of claim 6, wherein said mirror is adjustably movable about at least one axis.

8. The portable optical reader system of claim 6, wherein said mirror is adjustably movable about at least two axes.

9. The portable optical reader system of claim 1, wherein said control means controls movement of said sensor means to effect focusing of the reflected light image.

10. The portable optical reader system of claim 1, wherein said lens means comprises a lens, and a lens system component, and said control means controls movement of at least one of said lens and said lens system component to effect focusing of the reflected light image.

11. The portable optical reader system of claim 1, wherein said optical means comprises:
a lens associated with said casing; and a mirror mounted within said casing such that light entering said li ght receiving opening passes twice through said lens.

12. The portable optical reader system of claim 1, further comprising:
a carriage having a focus adjusting mirror movable therewith to effect focusing;
pivot means for pivotably connecting said carriage to said casing;
drive linkage means associated with said carriage; and electric motor means for drivingly moving said carriage about at least two axes via said drive linkage means.

13. The portable optical reader system of claim 1, further comprising:

a carriage having a focus adjusting mirror movable therewith to effect focusing;
drive means for movably adjusting said carriage relative to casing; and cam means associated with said carriage such that said carriage moves said focus adjusting mirror about at least two axes during movable adjustment of said carriage.

14. A portable optical reader system for reading optical information over a substantial range of distances, comprising:
(a) a casing having an opening for facing optical information;
(b) adjustable optical means, disposed in said casing, for imaging at a predetermined reading position in said casing an image of optically readable information facing said opening by light reflected therefrom, said information located on the surface of an optically readable information carrier;
(c) reading sensor means, disposed at said predetermined reading position and having a light receiving plane, for converting an image into an electric signal;
(d) means for emitting a light beam, positioned in said casing, such that said light beam is reflected from a surface of an optically readable information carrier without being refracted by said optical means, said carrier at an unknown distance from the plane of said reading sensor means, such that said reflected light beam is refracted by said optical means onto the plane of said reading sensor means such that the optically refracted and reflected light beam is converted into an electric signal;
(e) means for obtaining a measure of any needed focus adjustment based in part on said electric signal produced by said reading sensor means from the refracted light beam reflected from said carrier surface, (f) means for adjusting said optical means in accordance with said measure of needed focus adjustment such that said optical means focuses the image of optically readable information, located on the surface adjacent said information carrier, at said predetermined reading position; and (g) means for producing a digital information set corresponding to the image of said optically readable information focused at said predetermined reading position.

15. A portable optical reader system for reading optical information over a substantial range of distances comprising:
(a) means for emitting a light beam, such that said light beam is reflected from a surface of an object at an unknown distance from said emitting means, said light beam having a center and a peripheral edge;
(b) receiving means, including a receiving surface disposed in a plane, for receiving at least a portion of said reflected light beam and converting at least said portion of said reflected light beam into an electrical signal;
(c) means for obtaining a measure of any needed focus adjustment based in part on said electrical signal produced from said light beam;
(d) an adjustable optical system disposed to receive the reflected image of said object and adjustable to affect focus of said image at said receiving surface;

(e) means for adjusting said optical system in accordance with said measure of needed focus adjustment such that said optical system focuses the image of said object on said receiving surface; and (f) means for producing a digital information set corresponding to the image of said object focused on said receiving means.

16. A method of determining focusing quality according to a contrast gradient, comprising:
(a) reflecting a light beam from an object having an image to be focused;
(b) optically directing the image of said reflected light beam such that said light beam impinges on the surface of a photosensitive array;
(c) processing the output of said photosensitive array such that an image contrast gradient is determined; and (d) focusing said image with said contrast gradient wherein focus quality is a function of the slope rate of the contrast gradie

17. A method of determining focusing quality according to a distribution of modulations, comprising:
(a) optically directing the image of an object to be focused such that the reflected light image of said object impinges on the surface of a photosensitive array;
(b) determining the modulation transfer function of at least certain areas of the image of said object from the output of said array;
(c) sorting the pulse modulations of said at least certain area of the image of said object by pulse width;
(d) separating the sorted pulse modulations into a group of narrow pulse widths and a group of wide pulse widths;
(e) calculating the average pulse modulation of each group; and (f) focusing said image with said average pulse modulation for each group wherein focus quality is a function of the ratio of the narrow pulse width group and wide pulse width group.

18. A method of determining the the focusing quality of an image of pulses of various widths, comprising:
(a) optically directing the image of an object to be focused such that the reflected light image of said object impinges on the surface of a photosensitive array;
(b) determining the extreme values of bright pulses (B1) and dark pulses (D1) of the image of said object from the output of said array;
(c) determining the average values of bright extremes (B?) and dark extremes (D?);
(d) determining the number of pulse pairs (n);
(e) statistically evaluating said pulse data according to the following expression:

Qa = .SIGMA.(A1 - A0)/nA0 + .SIGMA.(B1 - B0)/nB0 wherein:
Qa is a measure of focus quality, A1 is a current extreme value of a bright pulse, A0 is an average value of bright extremes ((l/n).SIGMA.A1), B1 is a current extreme value of a dark pulse, B0 is an average value of dark extremes ((l/n).SIGMA.B1), and n is the number of pulse pairs.

19. The method of determining focusing quality of claim 16, 17, and 18, further comprising the step of controlling the signal level of said array output such that said signal is maintained within a dynamic range.