One interesting variant to be aware of on the LED discussion - if you're putting an LED behind a colored lens (I was considering marker lamp applications), the LED at the point of focus needs to be the _same_color_ as the lens! Putting a white LED behind, say a red lens will look considerably dimmer than you might expect, as the lens acts as a filter on the non-red light spectrum.

LEDs proper are very voltage sensitive and will dim quite a bit more sharply than incandescents. For instance a typical white LED, at 3.6v is freakishly bright, much weaker at 3.0v and by 2.4v is extinguished. That's why driver circuits are often used, which have the effect you describe within their operating limits.


Incandescents are already this way, so it's something we've dealt with for 100 years. An incandescent gives you 400-950nm, and the red filter blocks everything but 620-680nm (red). The rest turns into heat inside the lens.* Whereas an LED gives you the opportunity to spend your hard dollar on only the color range you actually want, e.g. 655nm, and the filter will let all that through.

incandescent - 100w - 1300 lumens - filter lets 200 lumens through.
white LED - 16w - 1300 lumens - filter lets 207 lumens through**.
red LED - 4w - 210 lumens - filter lets 205 lumens through.

White LED is still very good.

Red is more efficient, not only because it's making red only, but because it's not using phosphors, which are inherently inefficient*. White LEDs make blue light and use phosphors to turn it full spectrum. You may recall that for 40 years, LEDs have been used as indicators and they were always cold to the touch. The "hot white LED" is due to the phosphors. Also due to the emitters being overdriven to thermal limits* to save money.

You know, in 1981, you could buy 44mw red LEDs in bulk for about a penny. Yellow and green were a bit more. Railroad signal lights going LED is not novel to me, in fact I wonder why they waited so long?

* All three of these mean that a non-overdriven, red LED signal light will make essentially no heat, which could create a new snow/ice problem.

** the white LED spectral map has humps and it'll depend on whether the desired color is in a peak or valley. For instance most white LEDs have a local peak in red and valley in green`