US3113033A

US3113033A - Light filter

Info

Publication number: US3113033A
Application number: US29331A
Authority: US
Inventors: John P Hoxie; Alan J Werner
Original assignee: Corning Glass Works
Current assignee: Corning Glass Works
Priority date: 1960-05-16
Filing date: 1960-05-16
Publication date: 1963-12-03
Anticipated expiration: 1980-12-03
Also published as: GB966567A

Description

Dec. 3, 1963 J. P. HOXIE ETAL 3,113,033

LIGHT FILTER Filed May 16, 1.960 2 Sheets-She's: 1

,3? .aa .39 A xm 42 .43 a4 .45

INVENTORS JOHN P HOX/E AND ALA/v J WERNER Dec. 3, 1963 Filed May 16. 1960 J. P. HOXIE ETAL 3, ,033

LIGHT FILTER 2 Sheets-Sheet 2 73am A/JM/TTA/VC'E //v Psecawr WAVELENGTH-MILL/M/CRONS 0 INVENTORS .1 4. 5 JbH/v l? b ox/E A/vo ALA/v Z' WE NER Arra/Z/v United States Patent Ofiice 3,1 13,533 Patented Dec. 31, 1963 3,113,033 LIGHT FTLTER John P. Hoxie and Man J. Werner, Corning, N.Y., as

signers to Corning Glass Works, Corning, N.Y., a corporation of New York Filed May 16, 1966, Ser. No. 29,331 2 Claims. (Cl. 1(i654) This invention relates to the art of illumination and more specifically to the provision of improved illumination for medical and surgical purposes. It is particularly concerned with a glass filter adapted to transmit light from an incandescent source in a color corrected form with the non-visible heat radiations removed by absorption. The filter may take any conventional form, such as a globe or closed cylinder, adapted to be mounted in a lighting Ware fixture to enclose an incandescent light source.

Incandescent lamps are particularly desirable inasmuch as they provide a more balanced or continuous spectral energy distribution and a closer approximation to black body illumination than readily available are or fluorescent lamps. However, incandescent illumination presents two serious problems, chromaticity characteristics outside specified limits due to low color temperature of the illuminant and infrared radiations as a source of heat.

Conventional incandescent lamps employ as an illuminant a tungsten filament operating at a color temperature of 2854" K., or slightly higher depending on lamp design. However, the chromaticity characteristics of a black body illuminant having a color temperature of 3500 K. or higher are recognized as more satisfactory for many purposes, the human eye apparently being better adjusted to the making of color judgments with such illumination.

This is of particular significance in medical and surgical diagnosis where quick and/ or accurate color comparisons or discriminations must frequently be made in determining minor, but vital, changes in, or differences between, skin and/ or tissue colors. In recognition of this situation, a Federal Specification, W-F-4l6, Surgical Lighting Fixtures, has been established to define acceptable illumination in terms of chromaticity values. It requires that the conventional x value of chromaticity be no greater than 0.40, the corresponding y value no greater than 0.41.

A substantial portion of the radiation from a conventional incandescent light source is in the infrared region of the spectrum. This may result in severe overheating, particularly when concentrated by reflectors or other means employed to provide intense or spot lighting. It is desirable therefore to provide a filter that will selectively remove the infrared radiation and also color correct the transmitted visible portion of the radiation in accordance with prescribed specifications. Such a filter should be composed of a heat resistant material having a low coefiicient of thermal expansion to avoid fracture from thermal stresses during rapid and/ or non-uniform changes in temperature. The material should also be adapted to fabrication, as by press molding, into the shape of a hollow enclosure such as a cylinder or globe.

Heretofore, filters have been molded from low expansion borosilicate glasses containing ferrous oxide as an infrared absorbing agent. Such glasses have undesirably high y chromaticity values, i.e. a greenish color character, when suflicient iron oxide is added for complete infrared absorption. This greenish character cannot be adequately overcome with color additives to provide a satisfactory color correcting filter. Even in the partial absorbing filters heretofore employed, the y chromaticity value has approximately the upper limit of 0.41 whereas somewhat lower values are considered more desirable as explained subsequently.

Attempts have also been made to use combinations of filter glasses fabricated in the form of flat sheets or plates.

The resulting multiple, flat plate filter assemblies are cumbersome, expensive to construct and difficult to maintain and use. It has not heretofore been considered possible to produce a heat resistant glass having substantially complete heat absorption, adequate color correction, and adapted to molding as a hollow filter.

It is the primary purpose of this invention to provide an improved filter in which these various requirements are adequately met. A more specific purpose is to provide a single, heat absorbing, color correcting filter in the form of a hollow enclosure for an incandescent light source. A further purpose is to provide a glass filter having a low thermal coefiicient of expansion that adapts it to use in a lighting unit operating at high temperatures. Other advantages and benefits of the invention will become apparent from the description that follows.

The invention resides in a light transmitting filter for incandescent illumination that absorbs infrared radiation from such illumination and color corrects the remaining transmitted portion to simulate illumination from a light source having a color temperature of at least 3500" K. and chromaticity characteristics such that the x value is not greater than 0.40, and the y value is not greater than 0.41, the filter being formed from a phosphate glass containing ferrous oxide as a heat absorbing additive and an oxide of cobalt as a color correcting additive.

It will be appreciated that the effectiveness of these oxide additives, both for heat absorption and color correction, is dependent on filter thickness as well :as on concentration of the oxides in the glass. It is desirable to minimize the amount of glass additives both for economy and for ease of glass melting and quality control. Accordingly, it is customary to employ filters having a thickness of about 6 millimeters. Subsequent reference to glass additive concentrations will be with reference to filters of such conventional thickness unless otherwise indicated. It will be understood that corresponding effects can be achieved in filters of lesser or greater thickness by employing correspondingly greater or lesser concentrations of glass additives.

In general, there should be incorporated in a phosphate base glass filter of conventional thickness about 15% iron oxide computed as FeO and about 0.005-0.015% cobalt oxide computed as C00 to provide an adequate degree of heat absorption and color correction. The pre cise amount of each additive will depend on desired chromaticity characteristics as well as on filter thickness. However, the weight ratio of FeO/CoO should exceed about to 1 in any event. The amount of oxide additive to the base glass will depend on the oxidation level, i.e. equilibrium with higher oxidation states such as ferric oxide, in the glass. Accordingly, some adjustment may be necessary between appreciably difierent base glasses and for variations in melting conditions as explained later. However, such adjustments are readily determinable by one experienced in glass melting.

The invention is further described with reference to the accompanying drawing in which FIG. 1 is a view, partly in section, of a lighting fixture in accordance with the invention,

FIG. 2 is a graphical illustration of chromaticity characteristics, and

FIG. 3 is a graphical illustration of visible transmission through a filter of the invention.

FIG. 1 illustrates a conventional surgical lighting fixture embodying an incandescent tungsten filament lamp 1!), a metal capped, glass cylinder 12 enclosing the lamp, and a generally elliptical reflector 14 adapted to concentrate light in a focal plane. In accordance with the invention, cylinder 12 constitutes a heat absor bing, color correcting filter for radiations from incandescent source it). Depending on the particular type of lighting fixture or application, the filter may take various other physical forms such as a globe.

In accordance with the invention, light filter 12. is molded from a phosphate glass characterized by the presence of ferrous and cobalt oxides in amounts indicated above. In melting the phosphate glass from which the filter is molded, mild reducing conditions are employed to maintain the iron content of the glass in a reduced ferrous state. Such reducing conditions may be maintained by conventional procedures, such as the use of ferrous oxalate as a source of iron in the glass and/ or addition of minor amounts of a reducing agent, such as starch, sugar, or an ammonium compound in the glass batch. In the absence of other glass colorants, ferrous iron-containing phosphate glasses have a greenish cast or tint.

The term phosphate glass is here used in its conventional sense to denote a glass containing P as the primary glass-forming oxide and further containing glass modifying and flux materials, particularly divalent metal oxides. Other conventional glass forming materials such as SiO B 0 and A1 0 are present in compatible amounts for purposes of improved melting and working as well as stabilization of the glass against weathering. Easily reducible oxides, such as lead oxide, and colorants other than those specified should generally be avoided because of their tendency to interfere with the color correcting effects of cobalt and iron oxides. In general, glasses suitable for molding of filters in accordance with the invention, exclusive of the cobalt and iron oxide additives, consist essentially of 45-80% P 0 up to 20% of one or more divalent metal oxides, preferably oxides of Mg, Zn, Ca and Ba; 525% A1 0 up to 30% SiOg; up to 20% B 0 and, optionally, up to or more of alkali metal oxides. The latter are generally avoided in the interest of maximum infrared absorption and lower thermal coeificient of expansion. A low expansion is desired to render the filter heat resistant and is preferably on the order of 50x 10* or below.

By way of further illustration, 2. number of specific glass compositions suitable for present purposes are illustratively shown in the following table in terms of percent by weight together with their average thermal expansion coeificients between 0 and 300 C. The latter is shoum as a number which when multiplied by 10' gives the actual expansion coelficient per degree C.

. but:

In melting phosphate glasses for present purposes, conventional batch materials and melting practices may be employed except as otherwise indicated. The glass batches may consist of a mixture of oxides, phosphates, fluorides and a reducing agent properly proportioned to produce the desired glass composition. The batch mixture may be melted in a pot type melting unit at a maximum temperature of about 1400 C. for a time suificient to produce a completely fused and adequately fined glass for molding purposes. The molten glass is then cooled to a suitable working temperature and delivered to a glass press or other type of molding equipment.

The color correcting characteristics of the present filter are illustratively described with particular reference to the graphical illustration of FIG. 2. In this illustration, progressively increasing and y chromaticity values x are shown respectively along the horizontal and vertical axes of the graph. Horizontal line A and vertical line B on the graph define the boundaries or limits established by Specification FW4l6 referred to earlier. Light from an illuminating fixture in accordance with this specifica tion will have chromaticity values falling below line A and to the left of line B. For adequate transmission, it is generally desirable to keep both the x and y values above about 37, i.e. the extreme left corner of the graph.

Curve C illustrates the relationship between the chromaticity values of uncorrected light from Planckian or black body radiators at various indicated temperatures. It will be observed that the values of uncorrected light from a conventional tungsten filament having a color temperature of 2854" K. is well to the right of the values acceptable for present purposes.

We have discovered that the chromaticity of such incandescent light can be corrected to bring it within accepted limits, as defined by lines A and B, by including ferrous and cobalt oxides in a phosphate glass filter in proper proportions. The color correction effects attainable are illustrated in FIG. 2. Chromaticity characteristics of incandescent light transmitted through 6 mm. thick filter molded from glasses having the compositions set forth in the above table are shown by circled dots identified by numbers corresponding to the composition numbers in the table. As indicated earlier, the chromaticity values are dependent on filter thickness as well as on the ratio of colorant oxides in the glass. Thus, for glass filter thicknesses greater than 6 mm., there would be a decrease in the x chromaticity value of a given glass. Correspondingly, thinner filters would be characterized by an increased x value. The y chromaticity value is comparatively little affected by change in thickness and the plotted chromaticity points may be considered as moving from right to left on the graph along an almost horizontal line with increase in filter thickness. In general, an increase in the cobalt oxide content of a filter will greatly decrease the x chromaticity value and produce a relatively much smaller decrease in the y value. Increasing the ferrous oxide content tends to increase the y value and decrease the x value. By proper control of ferrous and cobalt oxide contents, as well as filter thickness, chromaticity values may be varied as desired in a given glass.

It has further been found that the optimum chromaticity characteristics are not those of a true black body illuminant. Rather, characteristics having a slightly higher y value are more acceptable. These are defined by a polygonal area generally designated as D on the graph of FIG. 2. This area is roughly parallel to the black body illuminant curve but ofiset along the y axis.

In addition to satisfying chromaticity characteristics or limits as explained earlier, a color correcting filter should also provide a controlled transmittance across the visible portion of the spectrum. In the lighting art, this is commonly defined with reference to visible transmittance curves. Such curves are graphical plots of transmittance at wavelengths across the visible portion of the spectrum, usually taken as 400750 millimicrons. 'For proper color correction to permit accurate color comparisons or discriminations, the transmittance curve should be a substantially smooth or unbroken curve although varying in transmittance at different wavelengths. Ideally, the transmittance curve will progressively decrease from a high value at a low wavelength to a lower value at a high wavelength with a substantially constant rate of variation. In other words, the curve should not be characterized by sharp or prominent dips, i.e. a plurality of maxima and minima values. A transmittance curve for the glass shown as Example 3 in the composition table is plotted in FIG. 3 to better illustrate this characteristic of preferred filters in accordance with the invention. The curve shows spectrophotometric measurements of percent transmittance through a six mm. thick filter across the range of visible radiations. Except for small dips of minor consequence, as at about 535 millimicrons, the curve is essentially smooth and unbroken, an indication of desired color correction.

What is claimed is:

l. A phosphate glass, light transmitting filter for an incandescent lighting unit characterized by a combination of color correction additives consisting of about 1-5% ferrous oxide and about 0.005-0.0l5% cobalt oxide on the basis of a 6 mm. thickness of glass, the ratio of ferrous oxide to cobalt oxide being at least 150:1 in the glass, the amount of ferrous oxide being sufiicient to absorb infrared heat rays from incandescent illumination and the cobalt oxide, in combination with the ferrous oxide, being present in an amount effective to color correct the transmitted portion of the illumination to simulate illumination from a light source having a color temperature of at least 3500 K. and chromaticity characteristics such that the x value is not greater than 0.40 and the 3/ value is not greater than 0.41, the phosphate glass, in addition to the cobalt oxide and iron oxide additives, consisting essentially of 45-80% P 0 up to of at least one divalent metal oxide, 525% A1 0 up to SiO and up to 20% B 0 2. A filter in accordance with claim 1 having chromaticity characteristics within the area designated as D in FIG. 2 of the drawing.

References Cited in the file of this patent UNITED STATES PATENTS 2,194,784 Berger Mar. =26, 1940 2,278,501 Tillyer et al. Apr. 7, 1942 2,359,789 Pincus Oct. 10, 1944 2,441,853 Stanworth May 18, 1943 2,7 8,006 Kreidl et al May 29, 1956 2,900,264 Brown Aug. 18, 1959

Claims

1. A PHOSPHATE GLASS, LIGHT TRANSMITTING FILTER FOR AN INCANDESCENT LIGHTING UNIT CHARACTERIZED BY A COMBINATION OF COLOR CORRECTION ADDITIVES CONSISTING OF ABOUT 1-5% FERROUS OXIDE AND ABOUT 0.005-0.015% COBALT OXIDE ON THE BASIS OF A 6 MM. THICKNESS OF GLASS, THE RATIO OF FERROUS OXIDE TO COBALT OXIDE BEING AT LEAST 150:1 IN THE GLASS, THE AMOUNT OF FERROUS OXIDE BEING SUFFICIENT TO ABSORB INFRARED HEAT RAYS FROM INCANDESCANT ILUMINATION AND THE COBALT OXIDE, IN COMBINATION WITH THE FERROUS OXIDE, BEING PRESENT IN AN AMOUNT EFFECTIVE TO COLOR CORRECT THE TRANSMITTED PORTION OF THE ILLUMINATION TO SIMULATE ILLUMINATION FROM A LIGHT SOURCE HAVING A COLOR TEMPERATURE OF AT LEAST 3500*K. AND CHROMATICALLY CHARACTERISTICS SUCH THAT THE "X" VALUE IS NOT GREATER THAN 0.40 AND THE "Y" VALUE IS HOT GREATER THAN 0.41, THE PHOSPHATE GLASS, IN ADDITION TO THE COBALT OXIDE AND IRON OXIDE ADDITIVES, CONSISTING ESSENTIALLY OF 45-80% P2O5, UP TO 20% OF AT LEAST ONE DIVALENT METAL OXIDE, 5-25% AL2O3, UP TO 30% SIO2, AND UP TO 20% B2O3.