Talk:Optical filter

Stuff removed
Removed this from the ND filters section:
 * If such filters are dark enough, they can make looking at the sun safe (don't try this without expert advice!).

Since this is really awful advice. Most ND filters don't block mid-IR very well, so if you look at the sun with them, you're risking cataracts. (You need something like welder's goggles to block MIR). -- DrBob 18:29, 25 Oct 2004 (UTC)

Heh. On that note I found this really cool idea that involves the converse - taking a solar filter and using it as an ND filter (it just happened to be close to neutral density, I guess solar filters don't have to be in general):  Rawling  14:25, 11 December 2006 (UTC)

Definition of positive vs. negative filter?
I was looking for information about what a positive vs. negative filter would be. It surprised me that nothing touching on that aspect appeared in this article. I am pretty sure this applies to optics, in the way of filters that would either remove a certain color of light, or every other color. Mea 15:45, 2 November 2007 (UTC)

absorption/reflection
Two things I don't understand.

If filter is absorptive,then it "eats" up photons, but it should get hot soon? What happens with the energy absorbed?

If filter is reflective, let's say red filter, then it lets only red light through, while reflecting all other wavelengths. What is reflected at the side of the incoming light is everything but red color? So why is red filter then not of cyan color when you watch it from that side? If green and blue photons don't pass through the filter, do not get absorbed, and can't be seen as a reflection, where do they go? Mike, Europe. —Preceding unsigned comment added by 91.148.95.127 (talk) 08:58, 12 December 2010 (UTC)


 * Yes, the filter gets hot. you need to cool the filter. Yes, the reflected light is cyan-colored, if you look carefully.  (1) this is why the sky is red at sunset: the blue light has been filtered out (its why the sky is blue - the red light was not reflected).  A similar effect occurs with red wine in a wine-glass, if you look at it from the correct lighting conditions (strong light, dark room).  See also roman chalice jade cup, which looks red when illuminated from inside.


 * Yes, this article should be updated to provide answers to your questions.67.198.37.16 (talk) 16:28, 26 September 2015 (UTC)

Additional filter type
I know there is one type of bandpass filter not mentioned here (unfortunately I can't recall the name of it) which is based on scattering. A cuvette is filled with e.g. glass powder and liquid matching the refractive index of the glass at the desired passband. At the pass wavelength this mixture will be clear transmitting the light. At other wavelengths the mixture will be turbid due to the refractive index contrast and light will be scattered. Since only part of the light will be backscattered it is probably advisable to introduce dissipative mechanisms e.g. by making the edges black. In order to be efficient and get a narrow passband the refractive index contrast must grow significant outside the passband which it will if the dispersion is different for the two media as is often the case. The temperature coefficient of the two media will also often differ which means that the passband can be tuned by changing the temperature. Om the slopes of the passband the filter would be semiturbid which would reduce its applicability to certain purposes.150.227.15.253 (talk) 15:02, 28 September 2018 (UTC)