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Abstract
This study proposes an optical illumination system design based on vector diffraction characteristics and the Scheimpflug principle to determine an optimal relationship between illumination uniformity, energy utilization, and system size in an infrared scene projector. We investigate the influence of digital micromirror device (DMD) diffraction efficiency at different incidence angles on energy utilization rate and establish a two-dimensional diffraction grating model to determine the optimal incidence angle of the DMD beam. We demonstrate that the optical illumination system of a long-wave infrared (LWIR) scene projector based on diffraction characteristics can simulate an infrared scene with a compact structure, high energy efficiency, and high uniformity.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Infrared scene projection is essential for evaluating the performance of infrared imaging guidance and detection technologies. As a supplement to visible-light scene projection technology, it can provide more information redundancy and avoid the influence of the weather. Moreover, it can reduce the number of field tests by 30–50%, save 10–40% of the development costs, and shorten the development cycle by 30–40% [1]. The optical illumination system is the key to the optical engine design of an infrared scene projector. The advantages and disadvantages of infrared illuminating optical systems are generally measured using two indicators: energy utilization rate and illumination uniformity [2]. Unlike visible or infrared light, long-wave infrared exhibits a severe diffraction phenomenon when incident on the DMD, resulting in a significantly reduced energy utilization rate with no light entering the projection system [3]. Therefore, high optical efficiency and high uniformity of the illuminating optical system are key desirables in designing a compact LWIR scene projector.
Some scene projection systems have been designed in previous studies. For example, Xueqiong et al. [4] proposed a method for designing the illumination system with an efficient uniformity projector suitable for visible light using a fly-eye lens array and the Scheimpflug principle to improve the uniformity of the image surface. The energy efficiency of different DMD chips was also verified. The results showed that the image uniformity and optical efficiency of the illumination system were 97% and 62%, respectively. However, because of the considerable difference between visible light wavelength and the DMD single-pixel image element size, the effect of diffraction can be neglected. Yue et al. [5] designed an infrared dual-band scene simulator that uses a three-dimensional spatial layout to avoid interference between different optical paths and improve the integration and energy utilization of the optical engine. A simulation uniformity of 94% was realized in the 3.7–4.8 µm and 8–12 µm bands with a medium wave optical efficiency of 0.464. However, the back intercept of the illumination system reaches 165 mm, which is large and not verified experimentally. Yang et al. [6] studied the total internal reflection(TIR) prism set of a compact LWIR scene projector, increased the contrast, reduced the system size, and designed the structure of the illumination system. However, the diffraction pattern of LWIR was not considered, and no detailed analysis or design of the illumination system was reported.
Owing to the diffraction characteristics of DMD, Dudley et al. [7] equated the dip angle of the DMD microlens to the blazed angle and established a blazed grating model of DMD. Rice et al. [8] proposed a two-dimensional planar grating model for the DMD and experimentally verified the incompletely blazed state of the DMD. Qing et al. [9] converted the DMD to an equivalent two-dimensional blazed grating. The results showed that the infrared target simulator had the best contrast ratio and imaging quality in the LWIR band (8–12 µm) when the incident azimuth and altitude angles were 0° and 44–48°, respectively. However, the system must have a large back intercept. However, no previous study has designed an optical system based on diffraction characteristics or established a relationship between the diffraction characteristics of DMD and the energy utilization rate. This study on the diffraction characteristics of the infrared scene projection system and DMD shows that the illumination system is essential in an infrared scene projector. A high energy utilization rate and uniformity can significantly improve the simulation ability of infrared scene projectors. However, no detailed discussion on the design method of an infrared optical illumination system with high uniformity and high energy utilization was reported in the previous studies on the design of an infrared scene projector.
Therefore, this study proposes a compact optical illumination system design method based on vector diffraction characteristics and the Scheimpflug principle for an infrared scene projector with a wavelength range of 8–12 µm to improve the irradiation uniformity and energy utilization rate while reducing the system size. To reduce the system size, a total internal reflection (TIR) prism was designed to improve the contrast and lower the back intercept. To improve the energy utilization rate, the optimal incident azimuth angle and incident azimuth angle of the incident DMD beam, along with their corresponding diffraction efficiencies, were determined. To improve the uniformity of the irradiation, the light source, Kohler illumination structure, and Scheimpflug principle were studied. Thus, these three objectives were comprehensively considered to realize the abovementioned objectives. With the gradual miniaturization of the spatial light modulator's pixels, the target's analog band, affected by the diffraction characteristics, gradually moves closer to visible light. Therefore, it is essential to study the diffraction characteristics of the projection system, including the projector and target simulation. The method provides research ideas for the design of target simulators or target scene simulations when the target simulation wavelengths is close to the single-pixel element size of the spatial light modulator element used in the design of projection systems
2. Optical system composition of the LWIR scene projector
An LWIR scene projector optical system is designed to improve lighting uniformity and energy utilization while reducing the system size of the optical illumination system of the LWIR scene projector, which mainly consists of a black-body light source, long-wave infrared polarizer, concentrating prism group, TIR prism set, spatial light modulator (DMD), and projection system set of mirrors, as shown in Fig. 1. An infrared polarizer is used to transform a natural infrared beam into polarized infrared light. The concentrating prism group is used to converge the beam and shape it. The TIR prism set is used to reduce the system size and derive the DMD off-state beam from the projection system. A DMD is used to simulate the scene target, and the projection system mirror set is used for beam collimation.
When the black body is functioning, the light from the black body passes through the infrared polarizer and becomes infrared polarized light when passing through the concentrating prism after shaping; the light enters the TIR prism set and uniformly illuminates the DMD. The projection system collimates the simulated targets sent from the DMD to form parallel light, which is subsequently sent to the infrared deep space target detection system for functional detection. The optical illumination system is divided into an illumination light source and an illumination relay system. The illumination light source uniformly radiates infrared polarized light, and infrared polarized light uniformly irradiates the DMD surface after converging through the illumination concentrating prism. The incident DMD beam angle design can improve diffraction and energy efficiency, and the Scheimpflug principle can improve illumination uniformity.
3. Design method of the lighting optical system of the LWIR scene projector
3.1 Lighting optical system design
The design of the optical illumination system includes the selection of the light source, design of the TIR prism set, and design of the optical illumination system structure. A surface black body was selected as the light source. Owing to its advantages of high temperature control accuracy, high emissivity, and high lighting uniformity, it is widely used as the lighting source for infrared scene projectors [10]. The first choice of black body is based on the simulation temperature, which is calculated according to the energy utilization rate of the entire system and Planck’s formula [11,12]. Combined with the design requirements, a black body with a temperature range of +20−700 °C, temperature resolution of 0.1 °C, effective radiation aperture of $\phi 8mm$, and effective emissivity of 0.94 ± 0.04, was selected.
The illumination and projection systems did not overlap, and the structure was more compact. Using the principle of total reflection, a specially shaped TIR prism was designed to separate the incident and reflected beams [13]. To reduce the system size, three main design conditions are satisfied for the TIR prism set. First, the illumination system beam is fully reflected within the TIR prism set and uniformly illuminated by the DMD. Second, when the DMD-reflected beam passes through the TIR prism group, the DMD off-state and flat-state beams on the TIR prism are fully reflected simultaneously and are far away from the projection system, while the on-state beams refract and enter the projection system. Third, the light beam reflected by the DMD into the projection system are parallel to the optical axis of the DMD. After satisfying these conditions, the designed TIR prism set improves the contrast ratio of the system. In the design of the optical illumination system, the length of the prism ${G_1}$ used to fold the lighting beam and uniformly illuminate the DMD in the optical path of the illumination system should be calculated, as shown in Eq. (2).
The angle and thickness of the TIR prism set can be obtained using the total reflection calculation formula $C\textrm{ = }\arcsin \left( {\frac{1}{n}} \right)$, refraction law ${n_1}\sin {\theta _1} = {n_2}\sin {\theta _2}$, and geometric trigonometry. The optical path analysis and design results are shown in Fig. 2(a) and 2(b), respectively.
Fig. 2. TIR prism set: (a) Schematic of the TIR prism set optical path analysis; (b) Design results of TIR prism set.
The optimal incidence angle $\varphi $ of the illumination system is calculated using Eqs. (1) and (2). As it is assumed that the DMD is close to the surface of ${G_1}$ before calculating the thickness, and the DMD should be some distance away from it in practical applications, the value of ${L_1}$ should be appropriately increased during the optimization. Finally, the length L in the optical illumination system ${G_1}$ was determined.
The Kohler lighting structure is used to image the light source at the entrance pupil of the condenser, and the light emitted from each point of the light source is superimposed on the illuminated surface, thus further improving the lighting uniformity of the light source based on the uniform black body. In the design of the optical illumination system of the infrared scene projector, the initial structure of Kohler illumination is solved based on the optical imaging theory, and the image quality evaluation is considered a reference while optimizing the structure. The non-imaging optical theory is used to analyze and improve energy distribution, transmission, and numerical aperture connection of the optical system. The principle of the Kohler lighting structure is illustrated in Fig. 3.
According to the design index of the LWIR scene projector, the field of view is 6°, aperture is 58.5 mm, and back intercept is 20 mm. DLP7000 is used in the design. The deflection angle is ±12°, the size is 14 mm × 10.5 mm, the diagonal length is 17.5 mm, and the pixel size is 13.68 µm. The numerical aperture of the illumination system is expressed as Eq. (3).
where NA is the numerical aperture of the illumination system, $F/\# = \frac{f}{D}$ is the F-number of the system, and the focal length of the projection optical system is. $f = \frac{l}{{2\tan (\theta /2)}}$To ensure the uniformity of the image surface, the image height should be slightly greater than the diagonal length of the DMD. The design indices of the optical illumination system are summarized in Table 1.
Table 1. Design index of an optical illuminating system
3.2 Vector diffraction model of DMD
To analyze the blazed metal grating model of the DMD microlens, the blazed grating was first divided into several layers of rectangular gratings with the same thickness along the z-direction. In this case, the blazed grating was equivalent to several rectangular gratings with different duty ratios and the same period. The three-dimensional structure of the DMD is shown in Fig. 4(a). A schematic of the blazed grating layer is shown in Fig. 4(b).
Fig. 4. Schematic of DMD: (a) DMD three-dimensional structure; (b) Schematic of blazed grating layering.
The solution process for the rigorously coupled wave analysis of vector diffraction is divided into three steps [14]. The incident light wave can be decomposed into a superposition of a TE wave, whose electric field component is perpendicular to the incident plane, and a TM wave, whose electric field component is parallel to the incident plane. The calculation methods for the two polarization states are the same. When the light wave is incident with TE mode linearly polarized light, the wavelength of the incident plane wave is $\lambda $, and the electric field of the incident light wave has only the y-component, and the incident plane wave is in the plane $x - o - z$.
For region ${\rm I}$ (incident dielectric layer), the electric field of the incident dielectric layer also has only a y-component, which can be expressed as Eq. (4).
According to Maxwell's curl equation, the component of the magnetic field in the x-direction of the incident dielectric layer is given by Eq. (7), as follows:
By expanding the electromagnetic field components ${E_{{\rm I}{\rm I},y}}$ and ${H_{{\rm I}{\rm I},y}}$ in region ${\rm I}{\rm I}$ into Fourier series and deducing the differential equations, the normalized electric field amplitude ${S_{l,ym}}(z)$ and the normalized magnetic field amplitude ${U_{l,xm}}(z)$ of the spatial harmonic field in layer 1 of region ${\rm I}{\rm I}$ can be obtained by mimicking the method of solving the electromagnetic field in layer L of the rectangular grating.
In region II, at the boundary of each layer, Eq. (10) can be applied, as follows:
The electromagnetic field equations for each region can be obtained according to Maxwell equations, and the electromagnetic field relationship at the boundary can be obtained by combining the boundary conditions; the matrix form can be written by combining them, as follows:
As the vector diffraction theory can only be obtained through simulation, the optimal incidence angle and efficient energy utilization rate of the optical illumination system can be determined through a comprehensive analysis of the obtained angle value and diffraction efficiency.
3.3 Design of the optical axis angle between the illumination system and projection system based on the Scheimpflug principle
According to the working principle of DMD [15–18], an angle exists between the optical axis of the incident illumination light and that of the projection system. In the optical illumination system, if the optical axis of the illumination light is considered the optical axis of the entire optical illumination system, the optical axis of the projection system must not be parallel to the optical axis of the illumination light. That is, because of the insertion of the TIR prism set and the influence of the diffraction effect, the DMD chip is not located on the actual image plane. The Scheimpflug principle [19] is introduced to calculate the angle between the optical axes of the projection system and the illumination light. This principle indicates that when the object plane, image plane, and plane of the lens of the imaging system intersect at the same point in the two-dimensional plane, a clear image of the object plane can be formed with an infinite depth of vision, as shown in Fig. 5.
The included angle of the optical axis between the illumination and projection systems is shown in Fig. 6. Combined with the mathematical formula and relationship between the axial and vertical magnifications, Eq. (13) can be deduced as follows:
where ${\varphi ^{\prime}}$ is the included angle between the tangent plane of any point on the object plane and the vertical optical axis plane; $\varphi$ is the angle between the tangent plane and the vertical optical axis plane of the corresponding image point, namely, the angle of the optimal incidence altitude; and $\beta$ is the magnification of the optical system.In the infrared optical illumination system, and the object plane is the light source, the image plane is DMD, ${D_B} = 8mm$ is the effective radiation aperture of the black body. To ensure the design accuracy, we consider ${D_B} = 7mm$, and $\beta = {D_{DMD}}/{D_B}$. The angle ${\varphi ^{\prime}}$ between the optical axes of the incident illumination light and the projection system are obtained, as shown in Eq. (14):
In summary, the optical illumination system uses the Kohler lighting method to image the polarized light source emitted by combining a black body and a linear gate polarizer on the DMD. A TIR prism is used to connect the illumination and projection optical systems. When the DMD is in the on-state, the TIR prism refracts the light reflected by the light source through the DMD into the projection optical system. When the DMD is in the off-state, the TIR prism reflects the light reflected by the DMD from the projection optical system to simulate the target.
4. Simulation analysis
4.1 Simulation analysis of the illumination beam incident DMD angle
As the wavelength of the DMD pixel is similar to that of LWIR, it is equivalent to a special two-dimensional blazed grating; however, the approximate condition of scalar diffraction is no longer valid. TI corporation provides reference incidence angles for DMD applications in ultraviolet, visible, and near-infrared bands: the incidence beam azimuth ${\theta _i}$ is 45° (the microlens is deflected along this diagonal), and the incidence altitude angle ${\varphi _i}$ is 24° (the microlens is deflected at ±12°). In the LWIR band, the grating equation of the DMD two-dimensional grating model can be obtained from the two-dimensional grating diffraction theory [20].
Here, m and n are integers; (m, n) represent different major maxima of diffraction; $\lambda $ is the wavelength; ${\varphi _r}$ and ${\varphi _i}$ are the altitude angles of the outgoing and incident beams, respectively; ${\theta _r}$ and ${\theta _i}$ are the azimuth angles of the outgoing and incident beams, respectively; a and b are grating constants.
According to the above equation, the grating diffraction energy level distribution is determined using the incident wavelength $\lambda $ of the beam, the incident angle ${\varphi _i}$, and ${\theta _i}$ of the light. The two-dimensional grating model of DMD in LWIR is simulated using programming software, and the diffraction characteristics of DMD in different spatial incidence angles and wavelengths are analyzed. Then, $a = b = 13.68\mu m$ can be obtained such that the diffracted light reflected by two-dimensional grating mainly has four orders, and the rest diffraction orders do not exist, or the diffraction efficiency is close to 0. Figure 7 shows the reflection directions of each energy level of the light beam reflected by DMD at several typical incidence angles with different wavelengths.
Fig. 7. Relation curves of the reflected azimuth and altitude angles of each energy level with the incident azimuth and altitude angles at different wavelengths: (a) $\lambda \textrm{ = }8\mu m,\,\,{\theta _i}\textrm{ = }{0^ \circ },$ (b) $\lambda \textrm{ = }8\mu m,\,\,{\theta _i}\textrm{ = }{45^ \circ }$ (c) $\lambda \textrm{ = }8\mu m,\,\,{\theta _i}\textrm{ = }{90^ \circ }$ (d) $\lambda \textrm{ = }10\mu m,\,\,{\theta _i}\textrm{ = }{0^ \circ }$, (e) $\lambda \textrm{ = }10\mu m,\,\,{\theta _i}\textrm{ = }{45^ \circ }$ (f) $\lambda \textrm{ = }10\mu m,\,{\theta _i}\textrm{ = }{90^ \circ }$ (g) $\lambda \textrm{ = }12\mu m,\,\,{\theta _i}\textrm{ = }{0^ \circ }$, (h) $\lambda \textrm{ = }12\mu m,\,\,{\theta _i}\textrm{ = }{45^ \circ }$ (i) $\lambda \textrm{ = }12\mu m,\,{\theta _i}\textrm{ = }{90^ \circ }$
In a DMD infrared scene projection system, to ensure that the target projection system operates well, the incident beam angle should satisfy the following conditions:
- 1. When the DMD microlens is in the on-state, the diffraction beam after DMD modulation should satisfy the condition ${\theta _r} = {\theta _i}$;
- 2. When the DMD microlens is in the on-state (off-state), the altitude angle of the diffractive beam after DMD modulation should be lower than (greater than) the beam aperture angle of the infrared scene projector. In addition, as the F-number of the system is 2.44, the corresponding outgoing beam altitude angle ${\varphi _r}$ should be less than $\arctan (\frac{1}{{2F}})\textrm{ = }{11.58^ \circ }$ to ensure good operation of the DMD microlens.
Figure 7 shows that among the four diffraction angle orders, the light of (-1, -1) and (0, 0) orders cannot enter the projection system, and only part of the light of (0, -1) and (-1, 0) orders can enter the projection system. Conditions 1 and 2 are valid when the beam incident azimuth is 0° or 90°, and the incident altitude angle is 33–67°. The central wavelength is 10 µm, and all values are between 8 and 12 µm, thus representing an LWIR band. The design adopts the incident azimuth angle at 10 µm for simulation analysis. The relationship between the incident and reflected altitude angles and the diffraction efficiency of different polarization states (0, -1) is shown in Fig. 8. To make the beam emerge as far as possible along the center of the optical axis of the projection system, the relationship between the beam incidence angle and the diffraction efficiency of the order (0, -1) is analyzed. The beam incidence angle is 44–50°, and the polarization states of the incident light are TE and TM.
Fig. 8. Relationship curves of incident altitude angle and diffraction efficiency of different polarization states (0, -1).
As shown in Fig. 8, when the incident beam is TM linearly polarized light, the incident azimuth angle of the illumination beam is 0°, the incident altitude angle is 47°, and the diffraction efficiency is 0.62, it is suitable for the design of the projection system.
4.2 Simulation analysis of optical lighting uniformity of the illuminating system
According to the design index of the LWIR scene projection, when the incidence altitude angle is 47°, ${L_1} = 21.92mm$ is obtained by substituting Eq. (1) in Eq. (2). As the DMD is assumed to be close to the surface of the prism before the thickness calculation, and the DMD should be located away from the prism in practical applications, ${L_1} = 27mm$ is adopted during the optimization. Then, because the length of the TIR prism in the illuminating optical system is $50.9mm \le L \le 57.3mm$, $L = 54mm$ is considered in the calculations. By substituting these values into Eq. (14), the angle between the optical axes of the incident illumination light and the projection system is obtained as ${\varphi ^{\prime}}\textrm{ = 23}\textrm{.2}{\textrm{2}^ \circ }$. The condition of conservation of etendue is satisfied in the design. The most commonly used infrared materials, such as germanium and zinc sulfide, are selected as lens materials so that the black body light source can illuminate the DMD surface uniformly after passing through the illumination system. As lighting relay systems do not involve imaging, the requirements for aberration are not stringent. Usually, only spherical aberration and relative illuminance need to be corrected, and other aberrations are not considered. An optical design software is used for the iterative optimization, and the design structure of the optical illumination system is optimized using the ZEMAX software, as shown in Fig. 9.
The ZEMAX-designed illumination optical system is imported into TracePro for image plane uniformity analysis, which is based on the Monte Carlo principle of light tracing. The illumination system consists of a light source, condenser mirror set, DMD, and TIR prism set, and the corresponding simulation is shown in Fig. 10. Further, the same materials as those used in the ZEMAX design are used as the prism materials. To improve the energy utilization, a transmittance enhancement film with a transmittance of 98% is coated on the surface of each concentrator and TIR prism, and the DMD is modeled with the scaled structure in the software, which can also simulate the deflection angle of the DMD. To obtain simulation results that well-match the experimental ones, the parameters listed in Table 2 are used in TracePro to trace one million rays by considering the light-emitting position, direction, and DMD tilt angle.
Table 2. TracePro main parameter setting
Lighting uniformity on the DMD surfaces is calculated using the 13-point test method of the American National Standards Institute [21]. Typically, the illumination uniformity of the optical illumination system (N) is calculated using Eq. (17) as follows:
where 1 ≤ i ≤ 13, ${E_1}$–${E_{13}}$ represents the irradiance values of the points ${P_1}$–${P_{13}}$, respectively and ${E_a}$ is the mean value of irradiance at point ${P_1}$–${P_{13}}$.However, the calculated maximum distance from the mean is susceptible to noise and numerical errors. In particular, the edge points (10 to 13) can have particularly high (or low) values owing to noise. Therefore, to improve the accuracy of the illumination uniformity calculations, variance is used to calculate the values of the 13 points using Eq. (18):
The irradiance map of the DMD surface is simulated the using optical simulation software TracePro. The irradiance image of the surface is divided into nine equal rectangles, and the irradiance values at the midpoint of each rectangle are considered, as indicated by ${P_1}$–${P_9}$ in Fig. 11. Considering that the irradiance value of the edge is lower than that of the center, to more accurately measure the illuminance uniformity, the irradiance value of the four landing points on the diagonal is measured. The corner point is 90% of the half-diagonal length from the center of the diagonal, as shown by ${P_{10}}$–${P_{13}}$ in Fig. 11.
The simulated image plane has the dimensions of 14 mm × 14 mm, which is larger than that of the actual image plane, and the calculated area is 13 mm × 10 mm, without the simulation edge. The outlet irradiance charts obtained before and after applying the Scheimpflug principle are shown in Figs. 12(a) and 12(b), respectively.
Fig. 12. TracePro image uniformity simulation analysis: (a) Before applying the Scheimpflug principle; (b) After applying the Scheimpflug principle.
By substituting the simulated irradiance values into Eq. (18), the irradiation uniformity before and after applying the Scheimpflug principle is 96.32% and 98.97%, respectively. These values indicate that the system designed using the method proposed in this study can obtain satisfactory lighting uniformity.
4.3 Simulation analysis of the energy utilization rate of the illuminating optical system
The calculation principle for the energy utilization rate of the optical illumination system of the infrared scene projection is shown in Fig. 13. The energy utilization rate was calculated.
The input relative irradiance ${E_i}$ of the optical illumination system determines the value of the output relative irradiance ${E_0}$:
where r is the energy utilization rate of the illumination system.The possible energy loss of the optical illumination system of the infrared scene projector can be divided into six parts: the blackbody, LWIR gate polarizer, DMD diffraction loss, remaining DMD loss, TIR prism, and concentrating prism group. The energy utilization rate r is directly proportional to the effective emissivity of the blackbody ${\tau _b}$, the transmittance of polarizer ${\tau _p}$, the diffraction efficiency of DMD chip $\eta $, the energy loss caused by other factors of DMD ${\tau _{DMD}}$, the transmittance of TIR prism ${\tau _{TIR}}$, and the transmittance of condenser group ${\tau _l}$ [6], as follows:
According to Eq. (20), the energy utilization rate of the simulated system can be calculated after designing the optical illumination system. The effective emissivity of the blackbody is 0.96, transmittance of the polarizer is 0.5, and energy loss of DMD caused by factors other than diffraction efficiency is 0.21 [22]. In the optical system, each surface is coated with an anti-reflection film with a transmittance of approximately 0.98. The transmittances of the TIR prism and concentrating prism group are 0.75 and 0.69, respectively. When the incident azimuth and altitude angles of the illumination beam are 0° and 47°, respectively, the diffraction efficiency is 0.62, and energy utilization rate of the optical illumination system is 0.122. When the incident azimuth and altitude angles of the illumination beam are 45° and 24°, respectively, the diffracted light cannot be imaged because of its departure direction from the projection system.
5. Experimental verification
5.1 Experimental verification of image surface uniformity of the illuminating system
The system was experimentally verified to explore the illumination uniformity and energy efficiency of the LWIR scene projector. An LWIR scene projection system was established based on the overall design results. Figure 14(a) shows a photograph of the optical illumination system of the LWIR scene projector. The LWIR scene projector after system installation and adjustment is shown in Fig. 14(b). Its overall structure size is 26.5 cm × 24.0 cm × 9.3 cm.
Fig. 14. Infrared scene projector: (a) Optical illumination system of infrared scene projector; (b) Overall physical picture of infrared scene projector
In the experiment, the equipment used to detect the output image of the scene projector typically includes an LWIR thermal imager and an LWIR camera. The LWIR thermal imager can measure the temperature of the simulated image, whereas the LWIR camera has a higher resolution. An LWIR camera with a higher resolution was used to detect the uniformity of the output image of the infrared scene projector. To verify the effect of the Scheimpflug principle on improving the uniformity of the optical illumination system, the design results before and after the Scheimpflug principle were replaced by adjusting the angle of the incident DMD of the illumination system in the experiment. The image obtained from the experiment was calculated by substituting the detected gray value into Eq. (18). The red and black curves represent the gray value distribution of the horizontal and vertical central axes of the image, respectively. The results are shown in Fig. 15.
Fig. 15. Uniformity analysis of output image surface of infrared scene projector: (a) Before applying the Scheimpflug principle; (b) After applying the Scheimpflug principle.
Through analysis and calculation, the uniformities of the simulated output images before and after applying the Scheimpflug principle were found to be 95.17% and 97.85%, respectively. As the projection system affects the uniformity of the image surface, the experimental results are consistent with the simulation results.
5.2 Experimental verification of the energy utilization rate of the illuminating system
An LWIR thermal imager with variable focal length was used to measure the output temperature of the illumination system at the projection system, namely, the TIR exit end. The temperature measurement range of the thermal imager was 0–150 °C. The temperature range of the light source is +20 °C−700 °C. To avoid deviations caused by external environmental factors, the temperature of the light source was measured after it became stable. However, to avoid measurement errors, the output temperature of the illumination system was repeatedly measured under different blackbody temperatures, and the average energy utilization rate was calculated. Black and white stripes were used as the input images to determine whether imaging could be performed at different angles, as shown in Fig. 16 (a). When the black body simulation temperature was 249.3 °C, the output temperature of the illumination system was 18.6 °C. Combined with Planck's formula [11], the incident azimuth of the illumination beam of the illumination system was determined to be 0°, and the average energy utilization rate was 0.117 when the incident height angle was 47°. We verify the experimental phenomenon when the diffraction condition is not considered, i.e., the illumination beam provided by DMD manufacturer has an incident azimuth angle of 45°, and an incident height angle of 24° is the reference angle. By rotating the angle of the DMD, the azimuth angle of the beam incident 0° can be changed 45°, and incident altitude angle 47° can be changed 24°. Although this process causes a certain degree of defocusing, which blurs the image, the energy utilization and imaging performance of the system remain unaffected. Figure 16(b) reveals that the direction of the projection system cannot be imaged, and the energy output is zero.
Fig. 16. Output temperature diagram of an illumination system with a black body temperature of 450 K: (a) Incident azimuth angle of 0° and altitude angle of 47°; (b) Incidence azimuth angle of 45° and altitude angle of 24°.
The simulation and experimental results for the energy utilization rate and lighting uniformity of the illumination system are listed in Table 3. Figure 17 shows the black and white grid output images of the infrared scene projector when the incident azimuth and altitude angles of the illumination beam are 0° and 47°, respectively. The experimental measurement results show that the developed LWIR target scene projector based on diffraction characteristics has a high imaging quality and can provide high-quality LWIR dynamic scene conditions for semi-physical projector systems.
Table 3. Experimental and simulation results
6. Conclusion
This study proposed an optical illumination system of an infrared scene projector based on vector diffraction characteristics and the Scheimpflug principle to balance the energy utilization rate, illumination uniformity, and system size of the infrared scene projectoroptical illumination system. The influence of the incident angle of the illumination beam on the diffraction efficiency and energy utilization rate was investigated, and the imaging principle of the tilted object plane was studied. An optical illumination system with a Kohler lighting structure was designed to solve the problems of low energy utilization rate and low lighting uniformity in the optical illumination system of the LWIR scene projector. The experimental results showed that the overall structure size of the infrared scene projector was 26.5 cm × 24.0 cm × 9.3 cm. When the beam incident angle of the DMD meets the azimuth and altitude angles of 0° and 47°, respectively, the energy utilization rate of the illumination system during the simulation and experiment reaches 0.122 and 0.117, respectively. Satisfies the requirements for the use of LWIR scene projection. The simulation and experimental uniformities of the illumination system using the Scheimpflug principle were 98.97% and 97.85%, respectively. The design results are superior to comparable visible or infrared light scene projectors [4,5] and American National Standards Institute standards [21]. The analysis results are expected to contribute to improving the energy efficiency and illumination uniformity of the infrared scene projector and provide a new concept for designing a scene projector affected by diffraction.
Funding
National Natural Science Foundation of China (61703057); Science and Technology Development Program of Jilin Province (20210201034GX); Science and Technology Research Project of Education Department of Jilin Province (JJKH20220754KJ).
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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