Once I had to measure light output power, so I shone the light onto the 
surface a measured volume of water colored with ink, and measured the 
temperature rise with a thermistor.  The container has to be more or 
less insulated, and it is a good idea to sprinkle graphite on the water 
surface to discourage reflectivity.  One cal/sec is equal to about 4.2 
watt 
(http://www.convert-me.com/en/convert/units/power/power.calories_sec.en.html).  
As you recall, it takes one calorie to raise a gram of water one degree C.

You can check your results versus the published power output of the LED. 

Okay, I know it's not real accurate, but it's a start

Herbert Graf wrote:
> On Sun, 2009-08-09 at 13:24 -0400, Robert A. LaBudde wrote:
>   
>> I realized recently that I have no idea how an absolute measure of 
>> radiant light power at even a single fixed wavelength can be made.
>>
>> E.g., suppose you have an LED at 620 nm center frequency and you want 
>> to measure power output. How do you do this?
>>
>> It is easy enough to construct a detector system that gives a result 
>> proportional to power, but how do you scale that measurement to 
>> absolute radiance?
>>     
>
> A while ago it was suggested that using the sun is a quick and dirty
> way, assuming a clear sky it's supposedly pretty easy to get radiance
> numbers. Only issue is how much brighter the sun is then most sources,
> it might saturate your detector.
>
> TTYL
>
>   

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