I’m sure this is a simple question for those that understand these things.
Let’s say I have a source of light - a lightbulb in this case. In front of the light I place a colored filter - a red piece of glass, a blue piece of cellophane, whatever. The filter only lets the light with the color of the filter go through. What happens to the rest of the light? Is it absorbed into the filter? How? As heat? It doesn’t seem too hot.
Is it reflected back? I don’t think you see a big source of light, like a mirror, if you’re standing behind the filter.
For ‘ordinary’ filters, such as the lighting gels used in the motion picture industry, absorbtion is the key. The filter will get hotter (how hot were you expecting?).
Some filters - called Dichroic I think - use reflection. If this is the case then a filter which transmits red light would reflect cyan. I once had some sunglasses which worked this way. From the front the lenses looked red but the transmitted light had a definite bluish hue.
Depends on your definition of “detectably.” Take blacktop, for example. We all know that black absorbs most of what you throw at it. It does get detectibly hotter than, say, your clear windows.
Measuring the difference in something small could affect the outcome – Heisenburg – so that’s why I say your definition of “detectably.” What’s the difference between measuring a pool of water and a single drop of water with a normal, mercury, medical-style thermometer?
Of course, the gel is also getting heated by the air & the body of the lamp itself. I’d still wager that they do get burned more by radiation than by other means of heating.
The filament of an incandescent may be around 3000 K, giving a peak in the infrared range (~1000 nm). Most filters work over a narrow range in the visible part of the spectrum, which means they’ll be absorbing the majority of energy from the bulb. Given that stage lighting is around 500-1000W, I think that’d be enough to give an easily measurable temperature change in a rather brief amount of time.
The sun is an even better source of power to work with. It’s around 6000K; though its peak is actually in the visible range (meaning a filter is going to pass proportionally more light), power goes up as the fourth power of temperature, so you’re getting 16 times as much energy as you did from that stage lamp. If you could hang the filter in a vacuum chamber for a few hours in sunlight, it’d get quite warm.
Define “clear”. An incandescent lightbulb emits most of its light in the infrared. (That’s one reason that incandescents are terribly inefficient.) If your filter is clear in visible light but absorbs infrared it’ll surely heat up.
But, yeah, if you had a ‘filter’ (unfilter?) that was perfectly clear at all wavelengths and a perfect filter that only let red light through, a light that emitted any non-red light would heat the red filter much faster than the clear medium.
Pointless factoid: Solar filters for telescopes are quite reflective. They have a lot of light to stop, so they’d probably get awful hot if they had to absorb it all.
But they do get quite hot, or at least the one I had did. After a few minutes it would be almost too hot to touch. Maybe mine was just poorly designed, or maybe the technology has improved since then.
