> For example, it gives the LI of a particular Red/Yellow 
> bi-colour LED as
> 150/60 mcd. So that means the red LED is 150 mcd, and the 
> yellow one is
> 60 mcd. Is that not a bit odd? I'm not sure if I 
> understand candelas
> properly, but does this not mean that the red one is more 
> than twice as
> bright as the yellow one? If so I think it'd be a bit 
> weird if my
> Connect4 game had brighter chips for one of the players.
>
> I've tried using Wikipedia to learn a bit about candelas 
> and also
> lumens, but it seems to me that these two units already 
> take into
> account the way in which the human eye perceives the LI of 
> different
> colours of light, which would make me believe that if a 
> Red and Green
> LED had the same brightness, that they'd have the same LI 
> rating.

Wikipedia is a marvellous tool.
I think of it as a subset of Google :-).

As noted by Nick, LEDs are liable to have different 
efficiencies at different wavelengths. To further complicate 
things, different colours may utilise different basic 
materials (GaAs, InGaN, ...) and to even further complicate 
things, there are two basic technologies - either direct 
production of the wavelength of interest or excitation of a 
phosphor with a blue or near UV LED with reradiation at the 
desired wavelength(s).

Also, while, as you note, lumen and candella include human 
eye response in their definitions, this still has an effect 
on the result. eg imagine a bi-colour red and yellow LED 
which has identical energy conversion efficiency for each 
LED. As the response of the human eye peaks at a 
yellow-green colour the yellow LED would probably have an 
apparently brighter response to the human eye and so would 
produce more lumens than the equally efficient red LED. This 
effect is so pronounced at the blue and near UV end of the 
spectrum that manufacturers such as Luxeon / Lumileds rate 
their deep blue products in mW of light output rather than 
lumens. Have a look at data sheets for nominally identical 
Luxeon products of different colours and note the lumen 
efficiency reductions as colours get further away from 
yellow-green. (The effect of changed LED basic technologies 
also has an effect).

The eye-response related lumen-efficiency changes with 
colour are also noticeable with other lighting technologies. 
Look at the packets or the side of the CFL base for Philips 
"Tornado" spiral CFL's (or other models of CFL from Philips 
or others who write the luminous efficiency on the packets. 
Philips do, but few others do so).  You will see that the 
lumens per Watt varies between 'daylight white' and "warm 
white" bulbs. Also note that the lumens/Watt ratings tend to 
be higher for larger bulbs - about 50% more for 20 Watt 
tornado tubes compared to 8W straight Philips tubes.

I have no commercial relationship with Philips - I'm just a 
satisfied user of their lighting products. In a Consumer 
Institute test a year+ ago of CFL tubes sold on the 
Australian retail market the Philips tornado was most 
efficient and produced up to twice the lumens per Watt 
compared to some other tubes. Whether the spiral design has 
any effect on efficiency is unknown to me.



        Russell McMahon




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