CN108896175B - High-resolution and high-numerical-aperture imaging spectrometer for vegetation weak fluorescence passive detection - Google Patents

Abstract

The invention discloses a high-resolution and high-numerical-aperture imaging spectrometer for vegetation weak fluorescence detection. Wherein the first to sixth lenses constitute a collimating lens group; the eighth to thirteenth lenses constitute a focusing lens group. The aperture diaphragm is positioned on the plane transmission grating; the collimating lens group projects the slit emergent light on the plane transmission grating, the plane transmission grating disperses the collimated light, and the collimated light is formed into continuous dispersion spectrum image by the focusing lens group and projected on an image surface. The optical system has the advantages of high numerical aperture, high spectral resolution, simple and easily-processed components, easy assembly and excellent imaging quality, the full-field full-waveband root-mean-square dot diagram radius value is less than 6.5 mu m, and the imaging distortion is less than 0.5 percent.

Description

High-resolution and high-numerical-aperture imaging spectrometer for vegetation weak fluorescence passive detection

Technical Field

The invention belongs to the technical field of ultrahigh spectrum imaging, and particularly relates to a high-resolution and high-numerical-aperture imaging spectrometer for vegetation weak fluorescence passive detection.

Background

The high-resolution and high-numerical-aperture imaging spectrometer plays an important role in the research field of vegetation sunlight-induced weak fluorescence passive detection. Although weak fluorescence emitted by vegetation subjected to sunlight irradiation is weak, the photosynthetic capacity of plants can be accurately reflected, the tolerance capacity of the plants to environmental stress and the damage degree of the stress to plant organs can be further reflected, the characteristic enables the fluorescence to be completely used as an early probe for the damage of plant health conditions and photosynthetic functions, the early probe can be used for carrying out early prediction when crops are damaged by stress factors through passive detection of the weak fluorescence, the physiology and growth conditions of the plants can be quantitatively, quickly and nondestructively monitored, and the method has important significance for modern vegetation ecological research and precise agricultural application. The passive detection of the weak fluorescence of the vegetation is carried out by using the hyperspectral imager, so that the extraction of the fluorescence information can be realized, and the relevant research can be better completed. However, the existing hyperspectral imager with common performance still has the following problems in the passive fluorescence detection of vegetation:

1. the detection mechanism of vegetation weak fluorescence is different from that of general hyperspectral detection, detection is carried out by utilizing a Fraunhofer line of the sun, false identification and confusion are easily caused by general spectral resolution, and the spectral resolution of more than 0.3nm is realized by an instrument optical system;

2. the radiation intensity of the fluorescence spectrum is extremely weak, and only accounts for 1% -3% of the total energy absorbed by the vegetation leaves, so that the requirements on energy transmission and signal-to-noise ratio of the instrument are high, and the numerical aperture of the system is required to reach more than 0.25;

3. on the premise of ultrahigh spectral resolution and high numerical aperture, good optical imaging capability is difficult to realize.

Disclosure of Invention

In order to solve the problems in the prior art, the invention provides an optical system of an ultrahigh spectral resolution imaging spectrometer, which has high numerical aperture and good optical imaging capability, is a telecentric system, has excellent imaging quality, and has a numerical aperture of 0.265 and a spectral resolution of 0.2nm in a fluorescence characteristic observation waveband of 670nm to 780 nm.

The technical scheme adopted by the invention for solving the technical problem is as follows:

the imager is sequentially provided with a slit 1, a first lens 2, a second lens 3, a third lens 4, a fourth lens 5, a fifth lens 6, a sixth lens 7, a planar transmission grating 8, an eighth lens 9, a ninth lens 10, a tenth lens 11, an eleventh lens 12, a twelfth lens 13, a thirteenth lens 14 and an image plane 15. The first lens 2, the second lens 3, the third lens 4, the fourth lens 5, the fifth lens 6 and the sixth lens 7 form a collimating lens group; the eighth lens 9, the ninth lens 10, the tenth lens 11, the eleventh lens 12, the twelfth lens 13 and the thirteenth lens 14 form a focusing lens group. The aperture diaphragm is positioned on the plane transmission grating 8; the collimating lens group projects emergent light of the slit 1 on the plane transmission grating 8, the plane transmission grating 8 disperses the collimated light, and the collimated light is formed by the focusing lens group to form continuous dispersion spectrum imaging and projected on an image surface.

The invention has the beneficial effects that: the optical system has the advantages of high numerical aperture, high spectral resolution, simple and easily-processed components, easy assembly and excellent imaging quality, the full-field full-waveband root-mean-square dot diagram radius value is less than 6.5 mu m, and the imaging distortion is less than 0.5 percent.

Drawings

FIG. 1 is a block diagram of a high resolution, high numerical aperture imaging spectrometer for the passive detection of weak fluorescence of vegetation in accordance with the present invention;

FIG. 2 is a full-field full-waveband root-mean-square radius point-and-column diagram of a full-field full-view imaging spectrometer for vegetation weak fluorescence passive detection according to the present invention;

FIG. 3 is an image surface speckle imprint diagram of a high-resolution, high-numerical aperture imaging spectrometer for vegetation weak fluorescence passive detection according to the present invention;

FIG. 4 is a field curvature and distortion plot of a high resolution, high numerical aperture imaging spectrometer for the passive detection of weak fluorescence of vegetation in accordance with the present invention.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and examples.

As shown in fig. 1, the invention relates to a high-resolution and high-numerical aperture imaging spectrometer for vegetation weak fluorescence passive detection. The method comprises the following steps: the lens comprises a slit 1, a first lens 2, a second lens 3, a third lens 4, a fourth lens 5, a fifth lens 6, a sixth lens 7, a planar transmission grating 8, an eighth lens 9, a ninth lens 10, a tenth lens 11, an eleventh lens 12, a twelfth lens 13, a thirteenth lens 14 and an image plane 15. The first lens 2, the second lens 3, the third lens 4, the fourth lens 5, the fifth lens 6 and the sixth lens 7 form a collimating lens group; the eighth lens 9, the ninth lens 10, the tenth lens 11, the eleventh lens 12, the twelfth lens 13 and the thirteenth lens 14 form a focusing lens group. The aperture diaphragm is positioned on the plane transmission grating 8; the collimating lens group projects emergent light of the slit 1 on the plane transmission grating 8, the plane transmission grating 8 disperses the collimated light, and the collimated light is formed by the focusing lens group to form continuous dispersion spectrum imaging and projected on an image surface.

In order to meet the requirement of passive detection of vegetation fluorescence, the working waveband of the imaging spectrometer is 670nm-780nm, the waveband is a fluorescence characteristic spectrum concentrated region of vegetation chlorophyll photosynthesis, and is perfectly superposed with a solar Fraunhofer line, so that the mechanism of passive detection of fluorescence is completely met. The invention completes the design of each component element in the system through the research of imaging aberration theory and the analysis of the system power distribution. In the aspect of system model selection, a reflective system is difficult to meet the requirement of high numerical aperture, an aspheric surface reflector is needed when high optical performance is achieved, processing is not easy, and cost is high, so that a transmissive system is selected as a system configuration. The number of available CCD pixels is 1024 multiplied by 2048, the pixel size is 13 microns, the scribing density of the transmission grating is 1200l/mm, in order to meet the spectral resolution requirement of more than 0.3nm and approximately meet the magnification ratio of 1:1, the focal length of a collimating lens group and a focusing lens group of an imaging spectrum system is required to be about 260 mm; the numerical aperture of the system is finally set to be 0.265, the system can be matched with a telescope with the F number of 1.8, and the energy collection efficiency and the signal-to-noise ratio of the system are ensured.

Considering the cost of an optical system of an imaging spectrometer, the physical and chemical properties and the processing performance of materials, and simultaneously, in order to realize the distribution of the focal power of the system, a material with high Abbe number is needed for a positive lens, and a material with low Abbe number is needed for a negative lens, and finally three materials of H-ZK9, H-ZF2 and H-K9L are selected from the aspects of engineering usability and material applicability.

The basic system adopted by the invention is a double-gauss lens system, the original double-gauss lens system consists of a front lens group and a rear lens group, the two lens groups respectively consist of three spherical lenses, the front lens group has negative focal power, and the rear lens group has positive focal power. The invention mainly optimizes the double-Gaussian lens group through material change, curvature radius change and the like, so as to redistribute the focal power of the system, correct the high-order aberration amount and chromatic aberration of the system and realize the optimization of image quality. Fig. 1 shows the optimized structure of the optical system of the final imaging spectrometer. Table 1 shows the optical parameters of each lens after final optimization.

TABLE 1 high resolution, high numerical aperture imaging spectrometer optical element parameters

FIG. 2 shows the distribution of the full-field full-band root-mean-square radius dot-column diagram of the design system. The dot sequence diagram comprehensively reflects the design evaluation result of the system. It can be seen that the root mean square radius value of the full-field dot array is less than 6.5 microns under the full-wave band, namely the size of an image spot can be fully surrounded by CCD pixels, so that the system is designed to realize very good imaging quality under the full-field full-wave band.

Fig. 3 shows the distribution of the image surface image spot imprint, according to the image, we can analyze that the image surface width occupied by the working spectrum band with the bandwidth of 110nm of the spectrometer in the dispersion direction is 22.35mm, and about 1719 pixels are obtained, so the spectrum sampling of each pixel is 0.064 nm. The width of the slit of the optical system is 39 micrometers, the magnification is 1.14, the width of the slit image on the image surface is 44.4 micrometers, and the occupied spectral sampling is 0.218 nm. And calculating the spectral resolution of the designed system to be 0.142nm according to the spectral resolution of the imaging spectrometer. The spectral resolution of the system is extremely high.

Fig. 4 shows the distribution of the field curvature and distortion maps of the designed system, in which the center wavelength and two edge wavelengths are selected for analysis. The dotted line represents the meridional direction of the field curvature, and the solid line represents the sagittal direction of the field curvature. In field curvature display, the wavelength generating the maximum field curvature is the edge wavelength of 780nm, and the defocusing amount is about 0.13 mm; while in the distortion display, the maximum distortion of the system is below 0.3%, these values demonstrate that the system achieves good distortion and curvature of field design control results.

Claims (1)

1. An imaging spectrometer for vegetation weak fluorescence detection is characterized in that: the collimating lens group comprises a slit (1), a first lens (2), a second lens (3), a third lens (4), a fourth lens (5), a fifth lens (6), a sixth lens (7), a plane transmission grating (8), an eighth lens (9), a ninth lens (10), a tenth lens (11), an eleventh lens (12), a twelfth lens (13), a thirteenth lens (14) and an image plane (15), wherein the first lens (2), the second lens (3), the third lens (4), the fourth lens (5), the fifth lens (6) and the sixth lens (7) are sequentially arranged to form a collimating lens group; a focusing lens group consists of an eighth lens (9), a ninth lens (10), a tenth lens (11), an eleventh lens (12), a twelfth lens (13) and a thirteenth lens (14); the aperture diaphragm is positioned on the plane transmission grating (8); emergent light of the slit (1) is projected on a plane transmission grating (8) by a collimating lens group, the collimated light is dispersed by the plane transmission grating (8), and forms continuous dispersion spectrum imaging through a focusing lens group to be projected on an image surface;

the working waveband of the imaging spectrometer is 670nm-780nm, the numerical aperture is 0.265, the focal lengths of the collimating lens group and the focusing lens group are 260mm, the spectral resolution is 0.142nm, the pixel spectrum sampling is 0.064 nm/pixel, the root mean square radius value of the full-field dot-column diagram is less than 6.5 microns under the full waveband,

TABLE 1 high resolution, high numerical aperture imaging spectrometer optical element parameters

The parameters of the components of the system are shown in table 1.

Images