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Question regarding Light Answered

Hi guys!

I had this strange question in my mind from a long time, but i have to yet get a satisfactory answer:

What is the difference between a naturally emitted colour and a colour which is generated by covering the light source with a translucent cover of a particular colour?

For example: I have a Blue LED whose body/case is actually transparent but on supplying power emits the colour blue naturally. On the other hand, i have a LED which emits a white colour on powering up. Suppose i put some kind of blue translucent film on it, or a translucent paint. I will still get a blue-colour. What are the differences of colour emitted in these two cases?

7 Replies

Orngrimm (author)2013-01-24

Only thing i want to add to the superiour answers already posted here:

Be aware that if you filter light to get blue, you end up with only a fraction of the light-ammount you generated (in white) at the start. The rest of the light (all but blue) gets converted to waste-heat in the filter.
So if you use high-power light be sure your filter can take the heat generated.

Also the contrary is true:
If you had a white LED with lets say 10W and you filtered it to get blue and the blue light was perfect in terms of light-ammount then if you swap the white 10W-LED to a blue 10W-LED, the blue would overwhelm you in terms of ammount. It will be a heck of a lot brighter! So be sure you would dial it down to lets say maybe 0.5W or so to get the same ammount of blue light as from the filtered 10W white LED.

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charmquark (author)Orngrimm2013-01-25

Yes i tried it out today and it was disappointingly dull, not to mention the problem you pointed out turned to be true, hot. I'd rather buy few blue ones for now. Thanks! :)

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Orngrimm (author)charmquark2013-01-26

No problem :)

I posted those hints because i made the same mistake in my childhood... Somehow i had the impression a colored piece of plastics magically turned all the light to the color of the plastic...
A melted piece of plastic proofed me otherwise... ;)

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charmquark (author)2013-01-20

Oh, Now i understand. Thanks guys! Been rattling my mind for this since a long time!

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lemonie (author)2013-01-20

A blue LED will emit blue in a rather narrow band (i think).
I also think that a "white" LED is a RGB mix, so the filter will reduce the intensity to ~ a third by blocking Red and Green. Yes you will get blue, but you'll be less energy efficient (or dimmer).


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Josehf Murchison (author)2013-01-20

One just generates blue light the other filters out all light but blue.
there is no difference when the light reaches your eye.

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Kiteman (author)2013-01-20

White light is a mixture of colours - that's why you can split it into a rainbow with a colourless glass prism.

Coloured light is missing part of the spectrum, and if it is generated by something like an LED, it is missing almost all the spectrum, except for a single, pure colour.

If you put a piece of clear, but coloured, material in the path of a beam of white light, the light is filtered. Like a coffee filter only lets through dissolved coffee, a colour filter only lets through part of the spectrum, blocking and absorbing the rest.

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