Equal brightness on RGB LED?

[William At MyBlueRoom]

I've attached a web page with specs on a RGB LED. I would like to know if there is a simple way to get the brightness of each color to appear of similar magnatude. In my tests with (not optimal for sure)

1200mcd RED = 220 ohm
3700mcd GREEN = 220 ohm
700mcd BLUE = 100 ohm

The Blue is the dimmest, the Red is bright, and Green very bright.

Trial and error work to some degree but not very accurate, is there a math formula for mcd, VCC, color & current?

http://www.superbrightleds.com/TriColor LED.htm

 

[audioguru]

Each tri-colour LED is different.
Each colour of LED in a tri-colour LED has a different voltage drop which causes a different current in the current-limiting resistor.
Your eyes are most sensistive to yellowish-green, less sensitive to red and have very poor sensitivity to blue. The mcd spec's don't account for the response of your eyes.

So there are too many variables for a formula that works.

 

[Nigel Goodwin]

There's also the problem of deciding what 'white' is, it's not as simple as it sounds!.

 

[justDIY]

The mcd spec's don't account for the response of your eyes.

actually, the candela or millicandela does account for human eye sensitivity ... same goes for the lumen, although the measurement of lumens and candela are different, they're both based on human eye sensitivity, or "visible light". so, if you find three diodes, red, green and blue, with nearly equal mcd ratings and viewing angles, they will appear equal in brightness to the naked eye

there are formula that help you mix led output based on the CIE chart, but it's a whole lot of math and you have to rely on spectral emissions data from the manuf, which may or may not be accurate for the exact LED you have.

depending on your application, you could use PWM to set the levels of the led, reflect it off something white, and adjust the levels until you get a satisfactory color.

if you want a good bright "cool" white, you'll want two blue emitters for every red + green pair. if you want a good bright "warm / neutral" white, you'll want two green emitters for every red + blue pair ... LEDs of these type normally come in the 10mm (round) package, the piranha (superflux) package and the barracuda (luxeon) package.

if you have access to a technical library or a university nearby, these publications might interest you:

CIE 013.3-1995 Method of measuring and specifying colour rendering properties of light sources
CIE x013-1997 Proceedings of the CIE LED Symposium '97 on Standard Methods for Specifying and Measuring LED Characteristics
CIE 127-1997 Measurement of LEDs
CIE x022:2001 Proceedings of the 2nd CIE Expert Symposium on LED Measurement "Standard methods for specifying and measuring LED and LED cluster characteristics"
CIE 167:2005 Recommended practice for tabulating spectral data for use in colour