Tuesday, September 25, 2012

What color is a red filter?

Here's a thing that surprised me.  It makes a lot of sense when you think about how sunblock most likely works: by absorbing UV light.  I had never considered how sunscreen would appear in a UV photograph, however.  That said, I think this is the first UV photograph of a person I have seen.  While I admit UV is not a flattering light source, I see plenty of photographs of people taking with X-Ray or green-spectrum sensitive film which is also less than flattering.

It does make me wonder if one could make a sunscreen that reflected UV light instead of absorbing it.

Thinking about light reminded me about a question I had wondered about on and off for a year or so.  I ended up pondering for several minutes what color a red filter is.  I admit this sounds like a silly question.

Let me explain a bit.  First of all, there's a question of whether a red filter is a filter that filters out red or a filter that filters out non-red wavelengths.  Usually we mean the latter, and those red filters look red.  Since it looks red, it is red right?

I guess the problem is that for opaque objects, we say they are a color if they reflect that color and for translucent objects, we say they are a color if they transmit that color.  What color(s) does a red filter reflect?

I had to grab a red filter and try it.  I used an uncoated filter since the coatings will probably affect the answer.  I held the filter against a black fabric so no light was passing through the filter and tried to judge the color cast of the reflected light.

Admittedly I am doing this in a fairly dark apartment under pretty weird fluorescent CRT light, and the filter doesn't reflect much.  However, it seems to reflect all colors the same amount.  In some very limited sense, the red filter seems colorless.

Is it possible to make a material which transmits one wavelength but reflects another?  What would that look like?

Edit: one of my photography teachers pointed out these things do exist!  They are called dichroic filters.  Nifty.  I want to play with some.

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