# Thread: A few optics questions

1. To start, how do color filters work? As in the ones that are used in theaters to create blue light (any color) from white light. If I look through one at a room, everything will appear to be blue; I think people may take for granted what is going on here. I mean, even if you had a pure yellow object in front of you and viewed it through a good blue filter, it would appear blue, and you would see the object just as well as any other object regardless of its color, including blue. What this leads me to believe is that a color filter does not absorb everything but blue, and allow blue through; this would mean that the yellow object would appear black. So what I think happens is that the light is converted to blue. Now, blue is of a higher energy than yellow, meaning that if yellow light were just converted to blue, you would be violating the laws of thermodynamics. I think that while blue light is emitted from the yellow, some of the yellow light is selectively DESTROYED in order to compensate for the new higher energy blue light (either destroyed or fused together to get the higher energy blue photons). Is this correct at all?

2.

3. There are two types of filters ones that scatter/absorb and one that work by way of interference.

First scattering types because they are cheep and all over the place. If you look at a blue filter it looks blue. if you look through a blue filter things look blue. The non-blue colors are being absorbed and the blue light is being transmitted and scatted. It is this transmitted and scatted that makes things look blue and the filter itself look blue.

Interference type filters are made by layering different layers of alternating index-or-refractions on a substrate (a piece of glass). These filters will transmit a band of wavelengths and reflect the others. So if you have a band pass filter where only 460nm +/- 20nm (a nice blue) is passed then if you look at the filter it would look like a mirror with a reddish-orange tint because of the missing blue. But if you were to look through it you would just see a wonderfully clean blue. (This is how I know my favorite color is 520nm a real bright green.) A band reject filter on the other hand would look just like a piece of glass. If the band reject filter rejects the blue at 460nm+/-20nm then if you were to look a very blue object it would look less blue.

The reason the object would just look less blue is because our eye's receptors are broad in wavelength and most reflections spectra have only a little variation across the visible spectra and most interference filters are narrow. To really see the effect of a rejection band filter one would have to look at a discharge lamb that emits in the wavelength that is blocked by the filter.

The eye's response to color:

4. The reason the object would just look less blue is because our eye's receptors are broad in wavelength and most reflections spectra have only a little variation across the visible spectra and most interference filters are narrow. To really see the effect of a rejection band filter one would have to look at a discharge lamb that emits in the wavelength that is blocked by the filter.
I'm not quite getting this. I do not understand that if something does not contain blue, that it could appear to be blue. With a filter that does not scatter blue light, at all, would everything that is not blue appear black? Does the scattering act as a sort of artificial color overlay for the rest of the image?

Thanks.

5. It's unlikely that any real world objects only reflect yellow, for instance. There's probably some blue light being reflected, even if it's not as much.

If an object were purely yellow, with no specular component, and you used a blue filter on it, it would indeed appear entirely black.

6. I was scanning the internet and found this: http://blog.wired.com/wiredscience/2...-build-wo.html

I started thinking: what kind of optics do they use to deal with the resulting 1 peta watt beam? Whenever I read through optical documentation I come across a power limit, usually under 100 miliwatts. Why do optics have a power limit anyways? What happens when you exceed it?

7. I think the lenses/mirrors end up overheating, since they can't be 100% transparent. Which causes a cascade reaction where the warmer optics either change shape (thus wrecking the beam) or start to change their optical properties (become more opaque, causing a cascade reaction leading to part of the optic in a gas or plasma state )

8. Ahhh...if its only heat, I can just cool the components with liquid nitrogen! No problem, just drill a bunch of holes through the prism, run some LN through it, and I'm done.

9. Sure, I think that's even what most high power lasers do. But there's still going to be an upper limit where the laser puts more energy in per second than your cooling system can take out. If nothing else, the thermal conductivity of the optic is going to come in to play (there'll be a point where the heat can't travel through the optic fast enough not to build up and heat itself critically, even if you had the world's most awesome cooling system).

10. Doing more research and came across something called a Bayer filter. http://en.wikipedia.org/wiki/Bayer_filter

Are the color filters standard ones like you see in concert lighting? It amazes me a little that they use standard color filters like that in such small areas and actually create a really good color accurate image in the end.

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