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Abstract
This study proposes an ultraviolet-visible composite optical target simulation technique based on a liquid crystal display (LCD) spatial light modulation device to solve the problem of not being able to satisfy the demand for optical target simulation for both ultraviolet and visible light operating spectral ranges in a single system when composite simulation of multi-source spatial targets is performed. We establish a composite light source model of an ultraviolet light emitting diode (LED) and a xenon lamp to enhance the energy simulation of the ultraviolet portion, and the light is mixed and homogenized by an integrating sphere. We analyze the light transmission principle of LCD display devices and derive the equation for the relationship between its working band and transmittance. We design a transmission-type projection system with a wide spectral range and simulate the transmittance of the whole system, and demonstrate the optical target simulator can realize the simulation requirements of a wide working spectral range, high interstellar angular distance accuracy, and high magnitude accuracy.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Space remote sensing cameras (SRSC) intended for deep space exploration are often integrated and multifunctional, aiming to minimize mass, reduce costs, and prolong the lifespan of satellites [1]. Ultraviolet radiation plays a crucial role as part of cosmic rays. Serving as a complement to visible light, it enables SRSC to capture target and star profile information even in low-light conditions, thereby providing additional redundancy in terms of information. Consequently, this enhances the autonomous operation of spacecraft in orbit [2,3]. Therefore, the development trend in space remote sensing imaging is leaning towards multi-band integrated optical systems [4].
The optical target simulator is a ground calibration equipment for simulating space targets, and the simulation level of the dynamic optical target simulator determines whether the SRSC can effectively tested and calibrated various properties and indexes on the ground [5–7]. Because of the complexity of the multi-source space target composite simulation system structure, it is necessary to ensure the integration and miniaturization of each wavelength band. So the design of a composite optical target simulator that can cover the ultraviolet and visible light is particularly important [8–10]. However, reflective dynamic spatial light modulation devices are large in size under the same technical specifications [11], which cannot meet the needs of multi-source spatial target composite simulation. Transmissive dynamic spatial light modulation devices are affected by the transmittance band and have the problem of a narrow simulation spectral range, which cannot realize a wide-band working spectral range covering the ultraviolet and visible widths [12].
Aiming at the simulated spectral range of optical target simulators, some scholars have investigated related techniques. For example, the Italian Institute of Physical Astronomy [13] designed an LED static star simulator, which uses a light source of specified brightness to create a circular projection environment to simulate stars with the required accuracy and magnitude range, with an operating spectral range of 0.365 −0.94 µm. However, the static star simulator is unable to realize the dynamic simulation of star charts as well as the functional test of the algorithm of the SRSC for the extraction of star points and the recognition of the chart; Meng Yao et al. [14] designed a dynamic star simulator based on Liquid Crystal on Silicon (LCOS)splicing technology, using LED as the light source, with a spectral range of 0.5-0.8 µm, interstellar angular distance error better than 18$^{\prime\prime}$, and a simulated magnitude range of -1-+7 Mv. This study is mainly aimed at eliminating stray light to improve the contrast, but does not carry out an in-depth study of the simulated spectral range, which is not able to simulate a wide spectral range. Chen Na [15] designed a high-precision star simulator optical system based on LCD with an operating spectral range of 0.5-0.8µm, the interstellar angular separation of better than ±25$^{\prime\prime}$, and the simulated magnitude range of +2-+6 Mv and the magnitude simulation accuracy error of less than ±0.1 Mv. The study mainly focused on the magnitude simulation accuracy in-depth. Still, it did not analyze the relationship between the working wave band and LCD transmittance and could not achieve the working requirements of the LCD star simulator in the ultraviolet band. Some other scholars, such as Zhang Jianzhong and Yuge Huang et al. [16,17] have also studied the projection system of LCD, but the working spectral range is in the visible range, which can not realize the simulation of dynamic spatial targets in a wide spectral range. Meanwhile, some scholars have carried out research on LCD. For example, Anna Linnenberger et al. [18] introduced a liquid crystal-based hyperspectral projector, which can be applied to a wide range of spectral ranges by slight modification of the LCD. However, only a theoretical model of the design was presented, and there was no practical application. 2023, Wu Peisen et al. [19] theoretically analyzed the factors affecting the transmittance performance of LCD and prepared an LCD with a bright-state transmittance of more than 40% and a contrast ratio of better than 1200:1 under the condition of a central wavelength of 0.55µm. The above studies show that LCD spatial light modulation devices can realize the simulation of a wide spectral range of ultraviolet-visible light. However, no detailed discussion on the design method of an optical target simulation technology covering the wide spectral range of ultraviolet-visible light in the previous studies on the design of an optical target simulator.
Therefore, this study proposes an ultraviolet-visible composite optical target simulator technology based on LCD to improve the coverage spectral range of ultraviolet-visible composite optical target simulator in the ultraviolet-visible, mid-wave infrared, and long-wave infrared multi-source optical target simulators. We put forward the proposal of ultraviolet LED and xenon lamp composite light source as the light source of composite optical target simulator, deduce the relationship between the working wavelength and transmittance rate of LCD display device, analyze the requirements of LCD display device during the design process of the composite optical target simulator, and design the optical system of the optical target simulator that can transmit the spectral range of 0.32-0.76µm. To solve the problem of the narrower working spectral range of the optical target simulator. It basically realizes the ground simulation test of the optical remote sensing camera for the spectral range and image stabilization accuracy, which provides a reference for the future optical target simulator and its multi-scenario application.
2. Components and working principles of composite optical target simulator
With the purpose of increasing the working spectral range of the composite optical target simulator, the optical system of the ultraviolet-visible composite optical target simulator is designed to be composed of an ultraviolet LED light source, xenon light source, integrating sphere, LCD spatial light modulation device, and projection system set of mirrors, as shown in Fig. 1.
An ultraviolet LED light source is compounded with a xenon lamp light source to provide the target wavelength spectrum. The light source is illuminated and enters the integrating sphere; after being mixed and homogenized by the integrating sphere, the broad-spectrum light beam is uniformly illuminated onto the LCD surface. The simulated target emitted from the LCD is collimated by the mirrors of the projection system to form a parallel light and is provided to the SRSC for functional detection. Among them, the LCD is an important part of the dynamic optical target simulator, analyzing the working principle of the display device, deducing the relationship between its working wavelength and transmittance, and then improving its working spectral range. Meanwhile, a projection system with high imaging quality is designed to improve the simulation of interstellar angular separation accuracy and magnitude accuracy by the optical target simulator.
3. Relay and lighting system design
3.1 Analysis of the transmittance principle of LCD
The relay system includes the LCD spatial light modulation device, which consists of two glass plates. The glass plates are each about 1 mm thick and are evenly spaced apart by a liquid crystal material containing 5µm. On the outside of the substrate are two polarizers, which are coated with a light-filtering film that acts similarly to an optical switch, and the LCD must rely on the polarized light for imaging.
When no voltage is applied, as shown in Fig. 2(a). As the direction of travel of light and liquid crystal optical axis and molecular long axis, the incoming polarized light through the liquid crystal is not affected by birefringence, which means no light through the analyzer. When the applied voltage exceeds the threshold voltage, as shown in Fig. 2(b). The long axis of the molecule will deviate from the direction of the electric field by a certain angle, which increases with the increase of the voltage, making the incoming ray polarized light become elliptically polarized due to birefringence so that the light passes through the analyzer. LCD is based on the presence or absence of this high or low voltage so that the panel can achieve the effect of the display.
Fig. 2. Component structure of LCD: (a) arrangement of molecules without voltage; (b) rotation of liquid crystal molecules induced by voltage addition.
The transmittance of a system represents the system's obstruction of light, defining the transmittance of an LCD as ${T_{LCD}}$, the transmittance of a polarizer as ${T_P}$, and the transmittance of a twisted liquid crystal as ${T_L}$, which is related to each parameter of the designed liquid crystal device. When calculating the optical properties of twisted birefringent layers of liquid crystals, they are usually divided into m layers, each of which can be approximated as a wave plate arranged parallel to the pointing vectors. The neighboring wave plates are rotated at an angle to each other, and the Jones matrix of each layer is multiplied to obtain the Jones matrix of the liquid crystal twisted birefringent layer as a whole [19]. The Jones matrix of the twisted birefringent layer as m tends to infinity is expressed as:
where $c ={-} {b^\mathrm{\ast }}$, $d = {a^\mathrm{\ast }}$.According to the definition of transmittance, there is:
For Twisted Nematic (TN)-type LCD, $\phi = \pi /2$, the transmittance axes of the two polarizers ${P_1}$ and ${P_2}$ are perpendicular to each other, and ${n_1}/{/}{P_1} \bot {P_2}$, and there are $\theta = \pi /2$, $\gamma = 0$. Then the transmittance of the LCD is shown in Eq. (6), and the relationship between the transmittance of each part and the wavelength is shown in Fig. 3.
The transmittance of the LCD is maximized when T = 1. By analyzing Eq. (5), there are multiple points where the modulation amount $2\Delta nd/\lambda $ meets the requirement. When $2\Delta nd/\lambda \textrm{ = }\sqrt {4{n^2} - 1} (n = 0, \pm 1, \pm 2\ldots )$, T = 1. According to Eq. (5), when designing the liquid crystal with the center wavelength $\lambda \textrm{ = }0.52$µm, the relationship between the refractive index of the liquid crystal and the thickness should satisfy $\Delta nd\textrm{ = 0}\textrm{.26}\sqrt {4{n^2} - 1} (n = 0, \pm 1, \pm 2\ldots )$, and at this time, the relationship between the working band and ${T_L}$ is shown in the curve (a) of Fig. 3.
Combined with the experimental transmittance test results of the selected polarizer, the relationship between its transmittance and wavelength is shown in curve (b) of Fig. 3. According to Eq. (6), the relationship between the transmittance and wavelength of the LCD is shown in Fig. 3(c), and the transmittance of the LCD in the target spectral band is always greater than 20%, which is a good light transmittance.
3.2 Illumination system design and determination of the overall structure of the optical target simulator
The illumination system consists of an ultraviolet LED light source, xenon light source, and integrating sphere, whose role is to provide the LCD with an incident beam that meets the spectral conditions and has uniform illumination. Currently, commonly used light sources are LED lamps, tungsten halogen lamps, xenon lamps, etc. [20]. LED lamps have a narrower spectral distribution, and due to process reasons some of the bands are more difficult to achieve, and it is difficult to achieve the accuracy of spectral matching. Tungsten halogen lamps and xenon lamps have relatively low energy in the short-wave band and cannot meet the requirements when used alone. Due to the wide spectral range of the simulation, according to Fig. 3 can be seen, the transmittance of the LCD in the ultraviolet band is low, and a single light source in the ultraviolet band of energy is relatively low. According to the law of independent propagation of light, different light sources meet at a point in space, do not affect each other, and the beams propagate independently, as in Eq. (7). Therefore, the light source selection of ultraviolet LED lamps to simulate the ultraviolet wavelength spectrum, xenon lamps to simulate the visible spectrum of the hybrid light source approach as a broad spectrum of light sources to facilitate the regulation of ultraviolet light energy alone.
where ${\tilde{E}_1}$ and ${\tilde{E}_2}$ are the complex amplitudes of the different beams.According to the working principle of the system, it is known that the illumination source outlet illuminance uniformity needs to be ensured because the light source outlet irradiates the display device. The integrating sphere is a cavity sphere coated with white diffuse reflecting material on the inner wall, and the inner wall of the integrating sphere should be a good spherical surface [21], the basic principle of which is shown in Fig. 4.
A beam of light with radiant flux $\Phi (\lambda )$ enters the integrating sphere through the incident port ${S_2}$ and is projected on the inner wall ${S_3}$. The radius of the inner sphere of the integrating sphere is R. The light is diffusely reflected in the integrating sphere by the coating for several times, a small part of which is emitted through the incident port ${S_2}$. The rest is uniformly irradiated on the inner surface of the integrating sphere. $E(\lambda )$ is the total illuminance, which consists of the diffusely reflected illuminance of all the points several times and the direct illuminance of the light coming out from the point ${S_3}$. The total illuminance at point M can be expressed as Eq. (8).
This formula shows that under ideal conditions when a beam of light enters an integrating sphere, the illuminance at any point on the inner surface of the sphere (except for the projection surface ${S_3}$) is independent of position. Instead, it is determined by the geometry of the sphere, the diffuse reflectance ratio of the coating, and the radiant flux entering the sphere. Since two light source beams are needed to shoot into the integrating sphere, mix the light, and then shoot out of the integrating sphere from another opening and illuminate the LCD, a total of three openings are needed. To prevent the light beam directly from the LCD port, according to the above principle of the integrating sphere, two baffles are designed inside the sphere so that the ultraviolet LED light source and the xenon light source undergo multiple diffuse reflections, fully mixing and homogenization of light, and then uniformly irradiate the LCD.
The relationship between the size of the integrating sphere and the opening is shown in Eq. (9), and usually, the opening ratio should be less than 5%.
According to the above analysis, the lighting system based on a composite light source shall consist of an ultraviolet LED light source, a xenon lamp light source, and an integrating sphere with three openings and a radius of 21 mm.
After analyzing the relay system and the lighting system, the basic structure of the optical target simulator can be determined by combining it with the projection system, as shown in Fig. 5. The composite light source is selected to be a mixture of xenon light source and ultraviolet LED light source, and an integrating sphere is used as the homogenizing system. Three ports are opened in different positions of the integrating sphere, and two baffles are placed in the integrating sphere in the same direction as the two light sources. Upon lighting up the light source, the integrating sphere mixes and homogenizes the light, which exits through one of its ports located at a certain distance and is connected to an LCD, behind which the projection system set of mirrors is placed. This setup allows for the simulation of broad-spectrum ultraviolet and visible light.
4. Projection optical system design and image quality evaluation
The optical target simulator should simulate the infinite distance of the space target, so the projection system design selects the collimated optical system. The collimated optical system should ensure that the image point has a wide working spectral range while having a high imaging quality. Therefore, a wide-spectrum, high-precision, and high-image-quality projection system should be designed for the purpose of improving the working spectral range of the optical target simulator.
The projection system must have a very high imaging position accuracy, and any aberrations produced by the system will impact the interstellar angular separation accuracy and the magnitude simulation accuracy. The imaging position accuracy mainly depends on the amount of aberration of the system. At the same time, considering the identification of SRSC is to take the energy center of each image point, the optical system design should also focus on controlling the energy center and the main light deviation. The main reason for the center of the image point offset is that the collimated optical system has asymmetric vertical axis aberrations such as magnification chromatic aberration. The transmissive structure mainly utilizes the refraction of optical materials for imaging, in which the double glued telescope objective is simple in structure, easy to manufacture, with small loss of optical energy, and can correct spherical aberration and chromatic aberration, so the transmissive optical path structure is chosen for the design of the broad-spectrum optical target simulator.
The test result of the interstellar angular separation accuracy is the interstellar angular separation error, which measures the deviation between the theoretical value and the actual value of the positional relationship between the two stars. When the optical target simulator simulates the points of stars with different interstellar angular separations, it is the same as imaging the objects with different precise fields of view, and the formula of the interstellar angular separation error is as follows:
where $\alpha $ is the interstellar angular distance error, $d = 19.8$µm is the LCD single pixel size, f is the focal length of the projection system, $c = 21.7$ mm is the diagonal length of the LCD, and $\theta = {6^ \circ }$ is the field of view.The magnitude simulation accuracy is the deviation between the actual magnitude and the theoretical magnitude. The position and size of the star point in the projection system will produce aberrations, resulting in a certain change in the luminous brightness of the star point so that the dynamic light simulator simulation of star point magnitude simulation accuracy is reduced in the star map, the surrounding star point grayscale value rises. Star point (x,y) into the grayscale diffusion formula is as follows:
Through the calculation of Eq. (10), the focal length of the system is 207 mm, the interstellar angular distance error is 19$^{\prime\prime}$, and the aperture of the system is known to be 60 mm. Meanwhile, through the analysis of Eq. (12), it can be seen that the smaller the dispersion spot of the system is, the higher the simulation precision of the magnitude of the star. Therefore, the designed optical target simulator projection system is shown in Fig. 6, and the optimized system consists of three single lenses and two sets of double-glued lenses. Considering that the symmetric structure not only possesses better image field flatness but corrects optical aberrations such as coma, magnification chromatic aberration, and aberration [22], the double Gaussian structure is retained. The use of conventional glass material broadens the used band to 0.32-0.76µm, which improves the imaging quality.
After the design of the projection system, the relationship between the transmittance of the projection system and the working wavelength can be obtained, as shown in Fig. 7(a). Combined with the LCD transmittance curve in Fig. 5, the relationship between the transmittance of the entire optical system and the wavelength can be obtained, as shown in Fig. 7(b), it can be seen that the transmittance of the entire optical system in the 0.32-0.76µm is greater than 5%, but the transmittance in the near-ultraviolet is lower. Therefore, the energy of the strong ultraviolet LED can be increased separately to improve the energy of the corresponding band.
Fig. 7. Transmission rate versus wavelength curve for projection systems and optical target simulators.
According to the simulation parameters of the ultraviolet-visible composite optical target simulator, the optical system should have the characteristics of low chromatic aberration, small aberration, energy concentration, etc., and in the system aberration correction, it should be ensured that the field curvature of the system is small, the MTF is close to the diffraction limit, and the dispersion spot is uniform.
As shown in Fig. 8(a), the system MTF ≥ 0.7 at the cutoff frequency $\upsilon = 40lp/mm$ of the optical system, which is close to the diffraction limit. As shown in Fig. 8(b), the maximum distortion of the system does not exceed 0.1%, i.e., for the LCD of 2048 × 1556, the maximum displacement of its single pixel pixel does not exceed 1 pixel. As shown in Fig. 8(c), the RMS radius values of the full-field-of-view spot of the dot-array map are all within 4µm, the energy is relatively concentrated, and the single-pixel pixel size of the LCD is 14µm. The designed system meets the equipment requirements. As shown in Fig. 8(d), the color difference of the vertical axis of the system at 6° of the maximum field of view is smaller than that of the Airy spot, which shows that the imaging quality of the system is favorable for the acquisition of star points by the ACSC.
Fig. 8. Aberration map of the optical system of the optical target simulator: (a) MTF curves; (b) field curves and distortion curves; (c) spot diagram; (d) vertical axis color difference curve.
After the above analysis and design of the projection system, its working spectral range is 0.32-0.76µm, and the interstellar angular separation error is better than 19$^{\prime\prime}$. The imaging quality of the system is better; the maximum aberration is not more than 0.1%, which realizes the design of the projection optical system with a wide spectrum, high precision, and high image quality.
5. Experimental validation of an optical target simulator
The interstellar angular separation error and magnitude accuracy are the main indicators of the performance of an optical target simulator. To verify the advantages and disadvantages of the ultraviolet-visible composite optical target simulator in realizing the wide working spectral range and the rest of the main performance indexes, the interstellar angular separation error, simulated magnitude range, and magnitude accuracy of the optical target simulator are also verified in the experiments at the same time as the working spectral range of the optical target simulator is tested. According to the overall design results to establish the ultraviolet-visible composite optical target simulator system, the system is installed and adjusted after the completion of the wide spectral range of the optical target simulator of the physical map shown in Fig. 9.
A space target picture is input to the LCD, and the simulated picture is received by the ultraviolet detector and the visible light detector of the optical remote sensing camera, as shown in Fig. 10(a) and Fig. 10(b), respectively. The test results show that the optical system of the broadband optical target simulator designed in this paper can fully meet the requirements of the ultraviolet and visible target detection system.
Fig. 10. Received images of ultraviolet and visible detectors of the optical remote sensing camera; (a) ultraviolet light, (b) visible light.
5.1 Spectral range tests
The operating spectral range is the main index for the design of the ultraviolet-visible composite optical target simulator. To verify the spectral transmittance of the whole optical target simulator system in the range of 0.32-0.76µm, and whether the designed LCD-based wide-band optical target simulator meets the requirements of the spectral range, a spectrometer was used to test and calculate the spectral transmittance and simulated spectral range of the optical target simulator. The spectral transmittance of the optical target simulator and the simulated spectral range were tested, collected, and calculated using a spectrometer during the experiment. The spectral range of the spectrometer is 0.2-1.1µm, and to avoid errors caused by external environmental factors, the transmittance and simulated spectral range of the broadband optical target simulator are shown in Fig. 11 after the light source is stabilized and measured at the same position with and without the optical system. Which are measured with and without the optical system at the same position after the light source is stabilized.
Figure 11(a) shows the output spectral curve of the optical target simulator, and Fig. 11(b) shows the spectral transmittance curve of the whole system of the optical target simulator. From the curve of Fig. 11(b), it can be seen that the spectral transmittance of the broadband optical target simulator at 0.32µm in the desired band is the lowest, but it is greater than 5%, which can satisfy the working requirements of the optical target simulator in the broadband of ultraviolet and visible light. Meanwhile, the simulated spectral range covering 0.32-0.76µm can be seen from the curve in Fig. 11(a), which meets the simulation requirements of the spectral range of the optical target simulator in the wide spectral band.
5.2 Interstellar angular separation error tests
When the optical target simulator simulates star points with different interstellar angular distances, it is equivalent to imaging objects under different precise fields of view, which requires precise measurement of the system focal length. The image plane of the dynamic optical target simulator is divided into equidistant 11 × 11 grid points. The grid intersections are tested using a latitude and longitude meter with an accuracy of 0.5$^{\prime\prime}$, first horizontally, then vertically, for a total of 121 points. Ten tests are performed at each stage [14], respectively, and the error values of interstellar angular distances computed after the measurements are shown in Fig. 12.
In the testing process, the coarse error is eliminated, according to the test of the interstellar angular distance error, as shown in Fig. 12(a), Fig. 12(b) for the interstellar angular distance of the positive and negative error values, respectively, the results show that the maximum error of the test is ±18$^{\prime\prime}$ and its absolute value is less than 19$^{\prime\prime}$, it can be obtained that the starlight out of the precision in line with the design requirements.
5.3 Magnitude and magnitude accuracy tests
The high-contrast optical target simulator can mainly realize the magnitude simulation at all levels of magnitude. The simulated magnitudes are measured using the magnitude calibration equipment of the National Astronomical Observatory of China (NAOC), which can directly read the magnitudes of the simulated star points, The operation is simple, and the detection of the magnitudes in the 0.3-1.1µm band can be realized by replacing the filters. The detection accuracy is ±0.01 Mv, and the observed magnitude range meets the test demand of +2-+8 Mv. The comparison of the test results with the theoretical magnitude is shown in Fig. 13.
As can be seen from the above figure, the simulation range of the LCD-based wide-spectrum optical target simulator reaches +2-+8 Mv, and the simulation accuracy is better than ±0.2 Mv, which meets the requirements of the technical specifications.
6. Conclusion
A design method for an ultraviolet-visible composite LCD optical target simulator is proposed with the aim of improving the operating spectral range of the optical target simulator. A hybrid light source of ultraviolet LED light source and xenon light source was designed, and an integrating sphere was utilized for light mixing and homogenization. The relationship between the transmittance of LCD and the operating band is deduced, and the optical target simulator with LCD as the spatial light modulation device can transmit a wide spectral range of 0.32-0.76µm is realized. At the same time, a projection system with high imaging quality applicable to a wide working spectral range is designed, and the subsystems cooperate with each other to solve the problem that the previous LCD-based optical target simulator cannot simulate the ultraviolet wave. The experimental results show that: the ultraviolet-visible composite optical target simulator realizes the simulation of 0.32-0.76µm wide spectral range, and the transmission rate of ultraviolet wavelength is improved from 0% to better than 5%; the interstellar angular distance error of the simulation of the dynamic star map is better than ±18$^{\prime\prime}$; and it realizes the simulation of the magnitude range of +2-+8 Mv, and the accuracy of the magnitude is less than the technical specification of ±0.2 Mv. The developed ultraviolet-visible composite optical target simulator breaks through the limitations of previous optical target simulators on the simulation of the spectral range, and can be widely used in space target detection systems, wide/multi-spectral detection systems and other detection systems on the ground detection.
Funding
Jilin Province Innovation and Entrepreneurship Talent Funding Project (2023QN13); Science and Technology Development Program of Jilin Province (20210201034GX); National Natural Science Foundation of China (61703057); 111 Project of China (D21009).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but can be obtained from the authors upon reasonable request.
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