I want to design tail-lights for onboard use in vehicles (mostly bicycles), but I wonder how should I configure the system so as to get the best visual perception from a distance.

Each lighting element of the system would be composed of a number of LEDs, and the design parameters would be:

  1. Number of LEDs;
  2. Brightness of each individual LED;
  3. How far the LEDs are from each other (or alternatively, the area "covered" by each led array);

So if I had 3 Watts available, I wonder if it would be best to use a single, point-like high power led, versus using 3 x 1 W LED, and in the later case if they sould be as close as possible or a bit far from each other, or even yet to use a wider area with, say, ten or twelve 5mm "bright" leds similar to those used in traffic signs.

Is there well-established way to anticipate how perceptually bright any light source would be, as seen from a distance, regarding those three parameters in the context of LED lights?

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    \$\begingroup\$ Consider law.cornell.edu/cfr/text/49/571.108 . The requirements for lights on motorcycles are a reasonable start for the design of lights for bikes. (I was also looking in to the design of such lights.) \$\endgroup\$
    – Dan D.
    Apr 29, 2015 at 7:04
  • \$\begingroup\$ Very interesting question. I think looking at the available bike lights, there must be some advantage to spreading them out. It's technically feasible to put any amount of light out of a 20 mm lens, and the front lights do that, but for the red rear ones they all seem to spread out the light. Perhaps it allows some situated movement?Gets past small obstacles better? Easier to judge distance? \$\endgroup\$
    – tomnexus
    Apr 29, 2015 at 16:29
  • \$\begingroup\$ I suspect the shape/area of a light being non-point is to enable recognition of the type of light it is, rather than affect apparent brightness. As a small aside, I suspect also larger area would prevent streaks/temporary blindness. I agree with Michael Karas' suggestion that quantifiable experimentation is important. The human visual system is very complex: given the multitude of visual "tricks" so far established, no doubt there are more to be found. However, for bicycles, the trend is flashing lights. I'd stick to that because drivers will recognize it. \$\endgroup\$
    – Jodes
    Apr 29, 2015 at 17:18

1 Answer 1


One of the best ways to evaluate the various configurations that you have raised in your question is to actually build them up as prototypes and then take a look at the results.

This method also allows you to get multiple evaluators into the game and you can test in as many sets of external sets of parameter variations as you can make time for.

The experimental method is a powerful engineering tool and often leads to discoveries that you never anticipated when you first started out.

  • \$\begingroup\$ Yeah, that is what I have been doing so far. This and deliberately analyzing my fellows' tail-lights during overnight long-distance events in open roads. One thing that I have noticed is that size and amount of emitters appear to have less effect than sheer power output, even if the more powerful light source is tiny. Also, I notice that it is much better, for the human eye (specially indoors), to compare lights by projecting them against a wall or a sheet of paper: that way you take away the glare and can compare relative brightness much better. \$\endgroup\$ Apr 29, 2015 at 21:40
  • \$\begingroup\$ But experimentation aside, the core of my question is exactely to know what is the state-of-the-art regarding the theory behind all of this, so I'll keep waiting for more answers. Thank you very much for now! \$\endgroup\$ Apr 29, 2015 at 21:41
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    \$\begingroup\$ I suspect that in the product deployment arena that you are talking about here that almost all implementations are based in perceptions derived from experimentation. The science to advance the state of the art from a theoretical standpoint may very well come later when someone in a research role happens to notice what folks are doing and decide that it would be interesting to study!! \$\endgroup\$ Apr 30, 2015 at 13:21
  • \$\begingroup\$ I couldn't agree more. Add to that the intentional misconceptions fostered by the industries themselves, when they intermix "lumens", "candles", and "lux" so that they can bias the advertised intensity of their product to any convenient "maximum" without outright lying. What I know is that there is already an ample scientific background coming from human factors in traffic safety, car-following strategies, brake-detection, etc., but I'm not from that field, so it is difficult to find specific knowledge like this one I asked. Interesting discussion, thanks! \$\endgroup\$ Apr 30, 2015 at 13:30
  • \$\begingroup\$ It is well known that perceived light intensity (luminosity) varies inversely as the square of the distance, and directly proportional to the area. This is for "normal" light. If you use lasers or LED's, you provably will get better performance. So I recommend that as a "first approximation," you use all the formulas that have been developed for "regular" (non-coherent) light. \$\endgroup\$
    – Guill
    May 1, 2015 at 6:47

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