Background
I am an electronics novice/hobbyist who is recently using an Arduino connected to a breadboard to prototype a set of navigation lights for a remote controlled hexacopter that I am building.
Similar to nav lights on airplanes, I have four strobing white lights that go on the tips of the back four rotor arms, and this alternates with strobing orange lights (beacon) on the top center and bottom center of the craft. This is all fine.
In addition to the strobing lights, I have two solid green lights to go on the two right rear rotor arm tips, and two solid red LED lights to go on the left rear rotor arm tips. All of this is fine too, but the red LEDs are really dim.
Question
No mater if I run the red LEDs through a battery or the Arduino, with a resistor or without and no matter the brand or MCD rating of the LED, they are always more dim than their counterparts of other colors. How do I brighten the darn red LEDs?
Update
Here are the specs for the LEDs. I got them off of eBay from a Chinese manufacturer. There is not a brand name, but all are from the same source:
Emitted color: red, blue, green, yellow, white
20pcs for one color
Size: 5mm
Forward voltage: 1.9 – 3.6V DC
Luminous intensity: 500-20000 mcd
Best Answer
It is highly likely that your red LEDs are of lower luminous efficiency than your white LEDs.
Also, driving an LED incorrectly can destroy it or reduce it's output from then on. Such damage can occur almost instantaneously. Red LEDs have lower operating voltages than White LEDs so are more likely to be damaged by the application of a constant voltage that is too high and not current limited.
The efficiency of LEDs varies widely. It can be expressed in lumens/Watt = l/W.
The best white and the best red LEDs have similar enough l/W ratings that both should appear about equally bright when driven with the same power input. White LEDs typically have operating voltages = Vf in the 3.0 - 3.6V range and red LEDs have Vf = 2.0 to 2.5 Volts. So a red LED would need about 50% more current to achieve the same power as a white LED as Power = V x I.
You state the LED output range as 500 - 20,000 mCd. That's a 40:1 brightness range.
Note that candela are a measure both of energy out and area of illumination. Less area = more brightness for the same light energy.