White LEDs and colored lenses

I am curious about using white LED and red lenses. If I understand correctly, white LEDs should only be emitting blue and yellow light. "Real" white light is made up of a mixture of all colors.

The way I understand it, a red lens or filter works on white light by blocking all the colors except red. Consequently you see red light through the filter.

This leads me to my question. If a white LED contains only blue and yellow light (no red) shouldn't a red filter prevent any light at all from passing through? I have tried shining white LEDs through red filters and the light seems to pass through just fine.

Does the LED contain some red light? Perhaps the filters are not capable of stopping all non-red light, but if this were the case, shouldn't I see the white and yellow rather than just a red glow from the filter?

I suppose the thing to do would be to measure a white led and a white incandescent of equal intensity through the red filter to determine if the light that passes through is lower for the LED. I don't have the capability to make this measurement.

Any thoughts?

Hmmmm.....good question!

Welcome to CPF.

White LEDs have a blue die which gives the blue) and a wide spectrum phospher which gives green yellow orange and red. The phospher looks yellow in room light but emits the other colours. So there is still some red.

I have a 1 watt headlamp that uses red/blue filters. With the red filter on the light output is roughly the same as a headlamp with a 5mm red LED. I lost a lot of light but there is still red.

The spectrum of white LEDs is more continuous than you think:

Note the different x-scale (wavelength) on both graphs.

White LEDs give off quite a bit of red light.

Utik said:

This leads me to my question. If a white LED contains only blue and yellow light (no red) shouldn't a red filter prevent any light at all from passing through? I have tried shining white LEDs through red filters and the light seems to pass through just fine.

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There is a difference between a colored lens and a true color filter. Are you sure you're using a true filter and not a colored lens?

Thanks everyone for the responses. This has cleared it up well for me.

LEDninja. Thanks for the welcome. Actually I have been lurking around this board since 2001. Obviously I am not a very active poster but have learned a great deal here. What you are saying about the dramatic reduction in output is consistent with my hunch.

evan9162. These graphs are exactly what I needed to see to really understand what is going on. If I am reading this correctly, there appears to be quite a bit of light in the lower graph in the 600 - 675 nm range (corresponding to red in the top chart). This must be the light which can pass through the red lens. I wonder what that spike around 610 nm is? Perhaps a flaw in the phospher?

elgarak. Thanks for pointing out the scale differences. I might have eventually noticed that, but had not until I read your post. Clearly you are right about there being quite a bit of red in white LEDs.

greenLED. I was using the words too loosely. I was simply shining the light through a red taillight lens. I was not using a proper colored filter. Based on what I learned above though, I am not sure it would have made much difference in my observation. The filter may have stopped non red light more completely than the lens, but since there is quite a bit of red light anyway that would have passed throught the filter. Right?

Thanks again everyone!

At the risk of taking my own post off topic, let me explain what I am thinking about. I was considering a project to make some LED lighting for a boat. I was trying to decide if I would be better using white LEDs shined through existing red and green lenses, or using red and green LED with white lenses.

I suspect the red and green LEDs would be better but would complicate my project a bit due to the different electrical characteristics of the different colored LEDs.

I also thought that I might be able to use PWM in my driver. My idea for this was not so much to dim the light as it was to intentionally cause a strobe effect when the light was in motion. I think this might be effective in drawing attention to the light and improving my visibility. I would have to make sure this was legal, but doubt it would be an issue.

Again, any thoughts or comments are appreciated.

Utik said:

I was trying to decide if I would be better using white LEDs shined through existing red and green lenses, or using red and green LED with white lenses.

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While at this point in time the best white LEDs are more efficient than the best red and green LEDs, using filtered white LED light to get red or green would be markedly less efficient. Look at the white LED spectrum shown a few posts back. Now look at the area under the curve between 500 and 550 nm. At best it's only about 20% of the total area. That means you would lose 80% of the light using a green filter in order to end up with a spectrum similar to 100% of what a green LED gives. Do the math yourself. Let's say you use a white LED with an efficiency of 80 lm/W (about the best currently available). You'll end up generating green with an efficiency of at most 16 lm/W. The better green LEDs offer efficiencies of about 2.5 times that. Doing the same thing for red-orange will give about the same results. And I'll add that for power LEDs you'll be hard pressed to get anything better than 60 lm/W so figure a green or red-orange LED will be at least 3 times more efficient than filtered white. If you want deeper red then the results favor using colored LEDs even more since the white LED spectrum has very little above 650 nm.

I suspect the red and green LEDs would be better but would complicate my project a bit due to the different electrical characteristics of the different colored LEDs.

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Not really. A constant current driver, which is the accepted way to drive LEDs, will handle LEDs with different characteristics just fine.

Utik said:

Again, any thoughts or comments are appreciated.

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I've used a piece of red plastic (lens? filter?) in front of a "white" Luxeon. Although you do get some red light out the front, I've found the amount of light to be underwhelming. If you intend to use this as a marker light on a boat, I'd strongly suggest you considering using a red Lux instead of a white Lux with a filter. The difference in output is considerable.