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?

The typical approach is to use a NIST (e.g.) traceable light meter 
and calibrate your meter to it. But how does NIST calibrate the meter 
in the first place?

Are there standard light sources that provide standard radiant powers 
for known current inputs? Or are there detectors that can read 
absoluted received power?

Some googling did not reveal answers to these questions.

In the olden days, at least we had a standard candle to use!
================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral@lcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
Virginia Beach, VA 23464-3239            Fax: 757-467-2947

"Vere scire est per causas scire"
================================================================

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