Illumination based on the asymmetrical reflector and LEDs.pdf
- 1. 1 Illuminating system based on the asymmetrical reflector, Visible and SWIR LEDs. 1. Introduction. The purpose of this article is to describe the illuminating system for the research flow of gas with some substance inside a tube. The SWIR and Visible LEDs were positioned on PCB side by side. See Fig. 1. The light from the LEDs was reflected by asymmetrical reflectors to create homogenous illumination inside the thin volumes that cross a chamber. The volumes had dimensions of 2mm×80mm×1080mm. See Fig. 2. The walls of the chamber were made from glass. The volumes were imaged by the line scan cameras operating in both Visible range and SWIR. See Fig. 2. Figure 1: Positioning of LEDs on PCB. Figure 2: Configuration of the illuminating system with two cameras. Direction of view of the second camera Direction of view of the first camera Reflectors Flow of gas PCB with a row of LEDs (a) (b) 80mm Thin volumes imaged by line scan cameras Walls of chamber LED with diffuser
- 2. 2 The volume was imaged in the reflection and the transmission modes. Light reflected from the reflector was reflected again from particles located in the gas (reflection mode). Light from LEDs passes diffusers that create homogenous illumination for transmission mode. See Fig. 2. The chamber had a width of 80mm and a length of 1080mm. See Fig. 3. 2. Description of illuminating systems. Only SWIR illuminating system is described here. The visible system has a similar structure and performance. The optical configuration of the SWIR system is shown in Fig. 3. Seventy-three SWIR LEDs were positioned at a distance of 15mm each from the other. The distance between walls was 80mm. The system illuminated the surface with a width of 120mm and a length of 1080mm. The distance of 120mm is considered instead of the distance of 80mm to take into account tolerances to shifts/tilts of LEDs, shift/tilt of reflector, manufacturing errors in the production of the reflector, and shifts/tilts of cameras. Figure 3: Configuration of SWIR illumination. The spectral and angular emissions of every SWIR LED are shown in Fig. 4. Visible LEDs with similar angular emissions were used. Figure 4: Spectral and angular emissions of every SWIR LED. Detectors with width of 2mm 80mm LEDs Reflector
- 3. 3 To measure homogeneity in the thin volumes the detectors with a width of 2mm and a length of 1080mm were positioned every 10mm along the surface with a width of 80mm. See Fig. 3. The image that took the line scan camera had a width smaller than 2mm. But I took a width of 2mm because the camera can be shifted or tilted relative to an optical axis. The reflector was represented as a mirror with a polynomial surface. Its mathematical equitation is shown below. 𝑍 = 𝑎𝑥 + 𝑏𝑥2 + 𝑐𝑥3 + 𝑑𝑥4 + 𝑒𝑥5 The mirror was coated with an aluminum coating that had high reflectance in the visible range and SWIR. The parameters of the surface were considered as variables and the merit function shown in Fig. 5 was written. Optimization of the design was completed. Two detectors were considered. The first detector (detector 150) had a width of 120m and a length of 1080mm. See Fig. 3. The second detector (detector 151) had a width of 2mm and a length of 1080mm located in the middle of the chamber. See Fig. 3. Figure 5: Merit function. Irradiance on the surface with a width of 120mm and a length of 1080mm after the optimization is shown in Fig. 6. Figure 6: Irradiance across the surface with a width of 120mm and a length of 1080mm.
- 4. 4 The irradiance on the 2mm detector located in the middle of the chamber (see Fig.3) is shown in Fig. 7. The irradiance at the edges of the surface is smaller than in the central part because the central part is illuminated by a larger number of LEDs. This problem can be solved by image processing. Figure 7: The irradiance on the 2mm detector located in the middle of the chamber. 3. Tolerance analysis of the illumination in SWIR. The reflector with a length of 1080mm was manufactured in two parts. So, two CAD files of the reflector with a length of 540 mm were imported into the design. The list of tolerances is presented below. See also part of the full list of tolerances in Fig.8. 1. Tolerance to the shift of LED in the X, Y, and Z-axis is +/-0.5 mm. 2. Tolerance to the tilt of LED around the X, Y, or Z-axis is +/-0.2 degrees. 3. Tolerance to refractive index on N-BK7 glass (material of window) +/-0.0001. 4. Tolerance to shift of reflector in Y or Z axis is +/-0.2mm 5. Tolerance to the tilt of the reflector around the X, Y, or Z-axis is +/-0.4 degrees. Figure 8: Part of the full list of tolerances mentioned above.
- 5. 5 First, sensitivity analysis was completed. Mean homogeneity of irradiance on the three detectors with widths of 2mm and lengths of 1080mm were used as a criterion. The detectors are located at the center of the chamber, 250mm from the left edge of the chamber, and 250mm from the right edge of the chamber. The tolerances to shifts and tilts of the reflector affected the homogeneity most strongly. Effects of tolerances on the shifts and the tilts of LEDs on the homogeneity were weaker. Next, 20 files with random tolerances were created. ZEMAX provided the best and the worst cases of homogeneity. Let's consider the worst case. The distribution of irradiance on the surface with a length of 1080mm and a width of 80mm is shown in Fig. 9. Figure 9: The distribution of irradiance on the surface with a length of 1080mm and width of 80mm. The graph showing the change in irradiance across the center of the surface is shown in Fig. 10. The percent change is 6.43%. Figure 10: Irradiance across the center of the chamber.
- 6. 6 The graph showing the change of irradiance across the line located 250mm from the right edge of the chamber is shown in Fig. 11. The percent change is 10.76%. Figure 11: Irradiance across the line located 250mm from the right edge of the chamber. The graph showing the change of irradiance across the line located 250mm from the left edge is shown in Fig. 12. The percent change is 11%. Figure 12: Irradiance across the line located 250mm from the left edge. The irradiance on the line surface with a width of 2mm located at the center of the chamber (see Fig.3) is shown in Fig. 13.
- 7. 7 Figure 13: The irradiance on the line surface located at the center of the chamber. The irradiance on the line surface located at the upper wall of the chamber (see Fig.3) is shown in Fig. 14. Figure 14: The irradiance on the line surface located at the upper wall of the chamber. The irradiance on the line surface located at the lower wall of the chamber (see Fig.3) is shown in Fig. 15.
- 8. 8 Figure 15: The irradiance on the line surface located at the lower wall of the chamber. The illuminating system was successfully first prototyped and tested. 4. Acknowledgements. I would like to say thank you to my customer that allowed the publication of this article. The name of the customer and the exact application of the illuminating system cannot be disclosed. This article was written by Mark Gokhler Ph.D. He provides optical design and consulting services. See more information on the following website: http://www.mark-electro-optics.com. The design was completed on the Premium version of ZEMAX-OpticStudio. The illuminating system based on an asymmetrical reflector can be used as table illumination, illumination for memorial places, and illumination in theaters and exhibitions.