[sdiy] calculating resistance to match brighness across different colours of leds?

[Tom Wiltshire]

Julian,

Unfortunately I think you're right - this is difficult to work out  
simply on paper. Not only does the human eye have an uneven response  
to different wavelengths of light at the same luminosity, but LEDs  
have a non-linear response to current too. This is one reason why LED  
brightness is often controlled using PWM not current. It gives a more  
linear response and better control.

Consequently, the best way to match the brightness is probably just  
to try it. Get a load of different resistors and the LEDs and a few  
experimental subjects (family members are always good if you don't  
live on a busy thoroughfare...) and try it out on them!

HTH,
Tom

On 25 Mar 2009, at 01:04, Julian wrote:

> Hello,
>
> Rather than doing it 'by eye', i thought it was about time i should  
> do this properly -
>
> I have two leds, a red and a green.  The specs are:
>
> Red:
> Wavelength: 700
> Lum. int. (mcd) @ IF (20mA): 10
> IF max. (mA): 15
> VF typ. (V): 2.2
> VF max. (V): 2.8
>
> Green:
> Wavelength: 570
> Lum. int. (mcd) @ IF (20mA): 30
> IF max. (mA): 30
> VF typ. (V): 2.1
> VF max. (V): 2.8
>
>
> If i use a 10k resistor on the red led, and i want the green one  
> next to it to be, to the observer, the same brighness is this just
> a simple cacluation?
>
> Im concerned that it may not be simply a case of matching the  
> luminosity - i dont know, but does the human eye (assuming a 'normal'
> individual) have equal sensativity across the visable spectrum?
>
> Im thinking of how the ears are more sensative to treble than  
> bass... (?)
>
>
> Julian
>
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