Below a picture of an 8mm ball bearing taken with a blue LED back light (447.5nm). I assume what we see here is a diffraction pattern, and the blue/yellow banding is due to chromatic aberration.
I didn't expect to see these blue/yellowish fringes because the LED is monochromatic.
Is this normal? I notice a lot of red pixel noise in the picture, maybe it's more a software thing? Would a blue filter in front of the camera lens help?
Fyi, this question is related to this question
Thanks,
If the light is monochromatic, there are no yellow fringes. Likewise, there is no red light. So a filter would not help.
I would look to your camera. If the blue light is too bright, does it respond by showing yellow?
I presume you are getting a numeric measure of intensity as well as a visual display. You might calibrate your camera with a uniform blue light. Then you can get correct values for the intensity of the diffraction pattern. This should help you find the location of the edge as discussed in the link.
Based on suggestions from mmesser above, I solved the problem. Here some pictures for information.
-First I checked the spectrum (looks good)
-The yellowish colour is indeed saturated blue. For some reason the gain of the camera is set too high when there is only blue light (this is also the cause of the red artefacts I assume)
A little diffuse white light from the front solved this for me. I would prefer a software solution, but that's not possible. I might also try a different colour LED later on.
As Comparison, a picture with only blue back light, then blue + a little diffuse white light from the front, and finally a lot of front light. Backlight was the same for the 3 pictures.
edit; an extra picture taken with very little back light, quite a lot of front light, and a piece of paper held at a distance of maybe 1mm next to the object. I need to check what is more important; the reflection of a white surface into the camera, or the blocking of a lot of blue light. But anyway, that's more like it.
