Try labsphere's website they have a nice article on it.
Here is there home page
http://www.labsphere.com/catdetail.aspx?id=3D227

This article appears to be one you might want to look at

http://www.labsphere.com/data/userFiles/LED%20Measurement%20Guide.pdf

--- On Sun, 8/9/09, Robert A. LaBudde <ral@lcfltd.com> wrote:

> From: Robert A. LaBudde <ral@lcfltd.com>
> Subject: [EE]: How do you calibrate a light meter?
> To: piclist@mit.edu
> Date: Sunday, August 9, 2009, 1:24 PM
> 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!
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> Robert A. LaBudde, PhD, PAS, Dpl. ACAFS=A0 e-mail: ral@lcfltd.com
> Least Cost Formulations, Ltd.=A0 =A0 =A0 =A0
> =A0 =A0 URL: http://lcfltd.com/
> 824 Timberlake Drive=A0 =A0 =A0 =A0 =A0
> =A0 =A0 =A0 =A0 =A0=A0=A0Tel:
> 757-467-0954
> Virginia Beach, VA 23464-3239=A0 =A0 =A0 =A0
> =A0 =A0 Fax: 757-467-2947
> =

> "Vere scire est per causas scire"
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> =

> -- =

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