Alright, get ready for one of my wall-of-text posts
:wavey: I am going to post this so far and then expand on it.
First point: Red behind clear will be brighter.
Here's why:
With white behind red, you are filtering out a huge chunk of the spectrum just to get the red. This reduces your efficiency. Combine this with the fact that white LEDs tend to be blue biased as is, you have a problem. White LEDs are blue biased because they are actually a yellow phosphor on top of a blue LED die. The blue LED has a wavelength high enough in energy to excite the phosphor and make it glow. The combination of the blue and yellow produces a roughly white light. You can also do red behind a red lens.
This is obvious in this spectrograph of the output of a white LED done by The LED Museum over at candlepowerforums:
As you can see a MINIMAL amount of light produced is in the red region. Personally, I would go with some Lumiled Superflux because those are HEAVILY used and easy to find information on.
Second point: most tail lights you see actually use what are called red/orange LEDs. True red in LEDs is a DEEP red and the luminous efficiency is REALLY dismal (usually 3/4 to half of red-orange).
According to
this document. A Red-orange outputs 5 lumens at 70ma while true red only puts out 3.8.
Next point: mcd ratings are GARBAGE. They are often HIGHLY inflated by using the HIGHEST luminescent portion of a beam, since they are flux per unit area this method gets high numbers. Lumens output is much better, because this is the total number of lumens put out by the device. Combine this with the viewing angle and you can get the true luminous flux. Since this is going to need ot be HIGH brightness and reliable, I honestly advise skipping the cheap ass low reliability LEDs (think BHK and other ebay sellers) and going for some real-deal OEM quality LEDs from companies like OSRAM, Lumileds, etc. You will get more light output in a unit area than the cheapie ones and if properly regulated, MUCH MUCH MUCH more reliability. On top of that, when properly regulated, you can maximize the energy that goes into light production (important for controlling heat).
Alright, how to chose a Lumiled Superflux LED.
Typically you will see something like this:HPWT-DH00-G4000 Bin G32
This tells you everything you could ever want to know about the LEDs.
HPWT is the Die technology. In this case it means TS AlInGaP (not really important other than there are only Red, Red/Orange and AMber offered in this technology
The D in DH00 refers to the typical luminous output range for this model: Min Lumens=3.5 Max Lumens=4.
HPWT-x
H00 refers to the forward voltage: min = 2.19 typ=2.6 max=3.03
Now these are broad ranges, which is where the Bin Code comes in. If you know what you are looking at is an HPWT red-orange LED then you can IGNORE the rest of the above and focus on the bin code.
In our case the Bin Code is G32.
G is Luminius output rating: Minimum Lumens = 3.5 Maximum Lumens = 4.8 @ 70mA (the best is J at min 5 max 7.3)
3 is the wavelength range for the LED: min = 619nm max = 629nm (this is the most red color option, while 1 is 611 to 617)
2 is the forward voltage (the voltage to run the LEDs at): min Vf = 2.31V max Vf = 2.55 (this is the second lowest Vf, with the lowest being min 2.19V max 2.43V and the highest being code 7 with 2.91 to 3.15).
This person sells 60 G32 binned Red-orange leds for $14 shipped.
There is another seller on there selling 120count J16 bins for around 30ish.
Now ideally you want to run these in a series parallel because that gives you the most efficiency. THat being said you MUST run a series parallel array at constant voltage. This means using a Vreg like a sharp PQ12RD21 or a PQ12RD11, I prefer these ebcause they only require an extra 0.5V as opposed to the LM317 and others like that which require an extra 3V above the regulated value. These extra volts are lost as heat. Also these Sharp regulators have an isolated tab, meaning you can mount them to anything and not worry about a short circuit.
How to make a series parallel array:
This website will design a layout for you including specifying resistors.
http://led.linear1.org/led.wiz
Just use your Vf (for the LED), a current of 70mA (or less), the number of LEDs and your voltage as 12 (if regulated), 15 if not regulated (VERY unadvised).
Here is a sample 12 LED array:
The resistors are just normal 1/4w 27ohm resistors.
The capacitor is a filtering cap used to take out transient spikes etc. which can cause problems for the regulator.
