I bought some of the 1157 and 1156 replacement bulb arrays, probably 8 years ago. I experimented with them and like Tom, found that the brightness was not what I’d hoped for. While the individual LEDs look bright from the right angles, the overall effect is not great.
Since then, however, the LED industry has made some huge strides and you can now get seriously powerful LEDs. As Tom notes, you need significant side illumination. LEDs are specified by their half-angle, which is the angle off the center axis where the brightness reaches 1/2 the peak value. Most of the old ‘superbright’ LEDs had half-angles of 15 degrees or less. Good if you’re illuminating a lightpipe but not so good otherwise. What we need is something with between 60 and 120 degrees half-angle.
The next thing you need is good old-fashioned power. The LED has higher efficiency than incandescent, and it’s nearly monochromatic, but an 1156 or 1157 has a 25 watt rating on the signal filament and 8 watts on the running light filament. If you want to outshine that then you’re still going to have to expect to put in at least a couple of watts of electrical power, if not five or six. So the old generation of LEDS where an array of 10 LEDs dissipates maybe one watt, is not going to impress that much in bright daylight. The current generation of LEDs that are designed for automotive signaling, can be had in 1 and 3-watt versions, and a few of those will outshine an 1156.
The last thing that you need to look at is the LED color. It is important to use a red LED behind a red lens, and a yellow/amber LED behind an amber lens; otherwise a great deal of your illumination is going to just warm up the lens material or get reflected back inside. White LEDs, specifically, radiate only a very small portion of their spectrum in the red/yellow color zone, and these should definitely not be used to replace an incandescent bulb in the taillights, in spite of their visual brightness when they’re uncovered.
Hypothetically speaking, if I was to design a PCB to take, let’s say, 3 to 5 of these in each of the upper and lower tailllight assemblies, would there be takers?
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=475-1317-1-ND
You can see that the price isn’t insignificant. You’re paying for the PCB (in low quantities, that will be on the order of $10 – $15 per side) and then the LEDS, plus some regulation components so they don’t blow up when your electrical system burps. Figure a 12 LED (3×4) taillight set for $80, or a 20 LED set for $120. That would be assembled and tested…