How can I maximize contrast for a grayscale machine-vision camera tracking reflective strips?
Asked 8/15/2017
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2 answers
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I’m using a grayscale machine-vision camera to track reflective strips inside a tubular structure. The strips are lit by bright LEDs mounted coaxially with the camera. I can adjust the light intensity, exposure time, and gain, and the lens aperture is already set as needed for focus.
After capture, I threshold the image to isolate the strips, so I want the raw image to be as close as possible to black everywhere except bright white on the strips. Some parts of the tube and internal brackets reflect light differently, which reduces contrast.
What is the best way to increase contrast before thresholding? Is adding more light preferable to relying on longer exposure and gain, or is there a better optical approach for suppressing unwanted reflections?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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Yes, you can polarize the light.
Use a polarization filter to maximize the contrast of the light coming into the camera.
You can also polarize the light going out of the illuminator.
By doing this you can limit the light entering the camera to a particular angle which can emphasize or de-emphasize the capture of the reflected light depending on the polarization settings.
Originally by user56382. Source · Licensed CC BY-SA 4.0
user56382
8y ago
0
Generated from our catalog & community — verify before relying on it.
A good optical fix is polarization. Add a polarizing filter on the camera, and if possible also polarize the LED illumination. By rotating the polarizers, you can emphasize or suppress certain reflections and often reduce glare from the tube walls while keeping the reflective strips bright.
That usually improves raw contrast more effectively than just changing exposure or gain. Longer exposure mainly increases brightness if nothing is moving, and gain mostly amplifies the signal along with noise. More light can help, but if the problem is unwanted reflections rather than insufficient brightness, polarization is often the better tool.
So the practical approach is:
- keep gain as low as possible
- use exposure/light level to avoid clipping the strips
- add a polarizer to the camera
- if possible, polarize the lights too and rotate for best separation
Test the polarizer orientation while viewing the scene and choose the setting that makes the background darkest relative to the strips.
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