# How do color filters placed in front of a light source,change the color of light that passes through?

For instance, placing a blue transparent folder in front of a desk lamp, makes the light that shines through it appear blue.

From what I understand some red car lights do not have red light sources, but just that red cover on top.

How exactly does placing a color filter of sorts in the path of a light source affect the light that passes through it, other than changing the color that we perceive? I'm thinking the intensity drops, and there might be polarization but I am not sure. Is there anything else?

Also, how exactly can you determine the color of a light source, because from what I understand we may perceive a color different than it is?

I'd appreciate a link to a place where I could read more on this or an introduction to terms I missed out on.

Any help is appreciated! Thanks!

• To determine the true "color" of a light source you use a prism to separate it into its individual components. – Hot Licks Jul 22 '16 at 3:17

What we perceive as colour is basically the frequency, or combination of frequencies, of the light shone on our retinas.

The light that comes out of a light bulb (but, note, not of an LED) is a mixture of lights of many different frequencies, actually, of all frequencies in the visible range. This is what we call "white". Filters have the properties of absorbing certain frequencies: this is due to chemical properties of the molecules of the pigments within. A blue filter absorbs all colours except blue, etc. So yes, the intensity will drop. LED coloured lights do NOT work like that. An LED produces light of a single frequency, so we require different LEDs to make a white-looking lamp.

To determine the colour of a light source in an objective way, without relying on subjective feeling, there are instruments called spectrometres. These can be very precise and are used for astronomy and chemical analysis.

Polarisation is unaffected by a coloured filter, but polarising filters exist.

• I agree with what you're saying at the physics level. However, some people may be confused since "white" LEDs are available, and look just like and come in the same packages as mono-color LEDs. These have a LED inside that emits a single color, as you say, but also phosphors that absorb some of the light and re-emit it at different colors. Your physics is right, but what you can buy as a "LED" isn't always just a LED inside. – Olin Lathrop Jul 21 '16 at 11:50
• I see, I actually wasn't aware of that. – Bzazz Jul 21 '16 at 13:00

"Color" is how we sense visible light with different mix of wavelengths. Each wavelength of light in the visible range is a particular color. White light is all the wavelength mixed together at the right balance.

What filters do is attenuate selected wavelengths of light. A red filter blocks most blue light, for example. If you start with white light and take out the blue, green, and some of the yellow, the result looks red.

The reverse also works. You can create white light by combining light of multiple colors at the right proportions. This is how computer displays work. Look at a white area on your monitor with a jeweler's loupe, and you will see individual dots or red, green, and blue. These colors match the color receptors in your eye. This allows a wide range of perceived colors to be created by balancing the relative proportions of the red, green, and blue contributions.