Hello,
I am currently designing an optical setup to perform measurements of reflected light (BRDF actually) and I am asking myself if I should consider a "focused on-sample" light beam or a collimated one. What are the influence of collimated and/or focused light on the signal after reflection ?
PS : I am considering diffuse reflection.
Thank you very much
Kind regards,
You definitely want collimated light. A focused beam with its range of input angles hides features of the BRDF. For example suppose you have a mirror. A collimated input goes to a collimated output. These are essentially delta functions in angle space. However, if you put in a focused cone of light, you always at least get out that cone of light, a large range of angles in angle space regardless of the surface. Now say the mirror is slightly scattering. A collimated beam goes to a small diffuse cone. However the cone of focused light goes to a slightly fuzzier cone. The details of the BRDF of the surface are hidden in the large cone of the focused beam. In short, the angle measurement will be the convolution of the BRDF of the surface and the angular profile of the incident beam. If you want to measure the BRDF you want to convolve with as close to a delta function as possible.
- "A collimated input goes to a collimated output." This works in the case of specular reflection only isn't it ? I am considering diffuse reflection so the output light is not collimated.
- What are you calling a delta function ?
I think in agreement with Mike Albert, the simplest method of measurement is with a collimated beam. Although, perhaps, it could also be done with a focused beam, although with more elaborated measurement technique, and a more complex calculation method.
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