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what surface do you use for evaluating lights?

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I have been playing with 740nm LED (it almost qualifies as thermal for this forum) and I was wondering since its the defacto optics forum,

what surface should you use for evaluating the color of light. Do they have special surfaces for this?

Kodak Gray card, white card used in photography


so like if you compare two lights this is a good way? I noticed when I look at red and deep red, what I used to associate with being red is now orange lol

Reflectance standards (or on the cheap side, white balance cards) are available for the optical band, but they do get somewhat trickier to find as you get farther out of band.  For 740nm, I'd expect basically any option that's good in visible to be doable, but you're going to want reflectance graphs for your material of choice (and always as matte finish as possible) if it needs to be accurately representing intensity between bands.

If you're trying to capture all the light from your source and accurately pass it to a spectrometer or similar for measurement, look at integrating spheres - not cheap, but basically a matte reflective material all over a spherical interior surface with some mechanism to hold/power a device in the middle.

You can buy characterized white surfaces from most suppliers that deal with optics. Another option is also a ground glass difusor.

But for something quick and easy, white wall paint will do. They are made of mostly titanium oxide as the white pigment and they dry into a mat finish. Titanium oxide is really good at looking white in the visible range. It is even reflective quite a way into the IR. It just won't work for UV since it suddenly becomes very absorbent in those wavelengths (this is why it is also used in sunscreen)


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