Colored light through a colored lens vs white light

I was on a website that sells LED replacements for auto bulbs. Their suggestion was to order the bulb to match the lens color (red LEDs for red brake light lens, etc). My thinking is that I’d want to use white LEDs and the colored LEDs are for only if you have clear lenses.

I guess part of me is thinking that I want to throw the full color spectrum, ie “white”, at a colored lens and have it filter the desired color through rather than risk throwing a limited range of color at a lens that might pass a slightly different range and risk having, effectively, nothing pass through.

However, Physics 101 was a million years ago and I know somebody on here knows optics better than I. If I want maximum brightness, what color bulb should I buy for my red lens?

Well if you have x white lumens and you filter out y% of the non-red light, then you are worse off than having x red lumens and filtering out < y% percent.

LEDs are remarkably good at producing single-color light. It’s what they do best. Making “white” LEDs was something of a challenge. Some LEDs did this by using multiple LEDs (red, green, and yellow, say), but the commonest way was to add a fluorescent material that absorbed some of the lower-wavelength light (almost invariably blue) and re-emitted it in a broad band across the visible. It works sorta well, although the light has a perceptible blue tinge. And the fluorescent material tends to degrade with time, leaving you with a mostly blue LED.
When you put a white LED behind a colored filter, then, assuming the filter peaks in the red and doesn’t have a high transmission elsewhere, you’re taking blue light from the LED, converting it to a host of other colors (with some loss, inevitably), and letting only the red portion through.

Whereas if you use a red LED, you’re pretty much transmitting almost all of the light that comes out of the LED.
No contest – using a red LED behind a red filter is WAYYYYYY more efficient than putting a white LED behind it.
This is true even if you use multiple LEDs – the light from the green and the yellow LED will mostly get filtered out, and only the light from the red one comes through. It’s as if you only had the red one back there to begin with, anyway.

On top of all this, as a rule of thumb the longer the wavelength of the LED, the more efficient it is. Red LEDs are more efficient than green, yellow, or blue at converting the electricity into light. There have been improvemernts in LEDs through the years, but that’s still pretty true.

It is also worth pointing out that the fluorescent phosphor used to make a white LED does not actually emit white light - in that it isn’t a true broadband of wavelengths like a black body radiator - but rather is an ensemble of various discontinuous bands and line emissions. And the red end is peculiarly nasty being mostly made up of a few line emissions. You eye sees something roughly white because the balance of colours averages to white - but it isn’t white light. So if anything your issue of a mismatch between filter and light from the LED has actually a greater chance of being a problem with a white LED.

By the way – most colored filters are quite broad (if you want a narrow filter you have to go to expensive things like interference filters, or exotic things like diffraction gratings), so don’t worry about not transmitting the light. If you have a green filter, just about any green LED will get mostly passed by the filter, so your worries about not getting enough of your wanted color through are pretty groundless.

Whereas you know you’ll be losing a lot of light by using a white LED back there. And it’ll work less well as the phosphor degrades through time.

LEDs have REALLY long working lives, by the way – typically 100,000 hours. That’s a lot longer than incandescent bulbs.