Subject: Re: Luminance/Illuminance measurements

From: [email-masked] (Benjamin Lonske/Tabatha Yeatts)
Date: Wed, 10 May 1995 08:23:44 -0400

In article <[email-masked]> [email-masked] (John D. Griffith) writes:
>From: [email-masked] (John D. Griffith)
>Subject: Re: Luminance/Illuminance measurements
>Date: 5 May 1995 18:08:32 GMT

>The light hitting the page can be described by an illuminance measurement;
>the units might be lumens/m^2 (lux) or lumens/m^2 (foot-candles).  You set the 
>detector on the page and point it toward the source.  The little diffuser on 
>the detector is an attempt to integrate the power from multiple sources.

>The light hitting their eyes is a little trickier. If there were no ambient
>light I think that you could use the luminance of the page as a measure of
>visual stimulation...

OK.  Thanks to this and several other very useful responses, I now know 
that measuring the light *hitting the page* or the light *hitting the eye* are 
both illuminance measurements. If I wanted to measure the light *reflecting 
off the page* this would be an luminance measurement.  I now also know that 
if I want to use a silicon sensor to do the illuminance measurements that  I 
need to use a diffusing disc to provide cosine correction.

Luckily (for me, the engineer on the project) we are not concerned about the 
amount of  light that *hits the retina of the eye* or the amount of light 
that is *perceived by the person as reaching their eye* (brightness) - only 
the light that is incident on the plane the surface of the eye is on.  So, I 
plan to measure the light hitting the eye just by putting a small sensor 
behind their glasses directly over the eyelid for a second and taking 
the measurement.  

The trouble I am having now is finding a illuminance sensor (with a diffusing 
disk) small enough and thin enough to do this measurement.  Does anyone know 
of such a sensor that is made to work with a commercial light meter?

Thanks again!

---

From: [email-masked] (Howard A. Bushouse)
Date: 10 May 1995 12:26:11 -0400

>the units might be lumens/m^2 (lux) or lumens/m^2 (foot-candles).

Can anyone tell me what physical measurement the lumen (or lux or foot-candle)
is based on or referenced against, i.e. is it some number of photons per
second of a given wavelength, or a given amount of light/photon energy
(e.g. ergs/sec, watts, etc.), or what?

Thanks,
Howard

---

From: Ben Wooley <[email-masked]>
Date: 11 May 1995 11:55:48 GMT

>(e.g. ergs/sec, watts, etc.), or what?
 
A candela is 1/60 the luminous intensity of a 1-cm^2 blackbody radiator 
at the temperature of the freezing point of platinum (about 2045 K).  
Consistent with this, a lumen is the luminous flux of  monochromatic 
radiation at 555 nm whose radiant flux is equal to 1/683 W.  I hope this 
helps.

---

From: [email-masked] (Jonathan E. Hardis)
Date: Fri, 12 May 1995 14:24:35 -0500

In article <[email-masked]>, [email-masked] (Howard A.

And a previous respondant said:

> radiation at 555 nm whose radiant flux is equal to 1/683 W.

Well, this is one area in which I can answer with authority.  I refer you
to Applied Optics, 32 (16), 2936-2948 (1993), "National Institute of
Standards and Technology detector-based photometric scale."  (I wrote
it.)  There's an even more detailed paper (which I also wrote) that is
going through the review process now for J. Res. NIST. -- expect it out
around the end of the year.

The platinum-point black body served in the definition for the candela
(and other photometric units, such as the lumen and lux) from 1948-1979. 
The answer above is out-of-date.

Since 1979, the definition reads, "The candela is the luminous intensity,
in a given direction, of a source that emits monochromatic radiation of
frequency 540x10**12 hertz and that has a radiant intensity in that
direction of (1/683) watt per steradian."

There are a lot of subtiltiles that this definition leaves out, like
converting frequency to wavelength, and luminous intensity at other
wavelengths.  These are mentioned and referenced in the Applied Optics
paper.

Okay.  What you *really* asked is what physical measurement the lumen is
based on or referenced against.  Since 1979, the answer has been:  any
that a national measurement laboratory wants.  The definition relates
photometric units to radiometric units (watts of light, perhaps with
geometric factors), but the watt is not an SI base unit (no wordy
definition).

At NIST, there are various calibrations and deliverables that we offer,
involving lamps, detectors, etc, for US (and other) laboratories that want
to reference their lumen (etc.) to the national standards.  One way or
another, each method relates back to our High-Accuracy Cryogenic
Radiometer (never let a computer expert choose an acronym).  This is a
device for measuring optical power (watts) by comparing the power to
electrical watts.  Both light and electricity are used to heat a receiving
cavity to the same temperature.  It follows that this happens when the
applied powers are the same.

So, it ends up that the standard lumen is based on the standard watt,
which is based on the standard ohm and the standard volt (P=V**2/R).  The
standard ohm is based on the quantum Hall effect, and the standard volt is
based on the Josephson junction.  And so it goes.

(In case anyone is wondering, yes, we compare our candela and lumen to the
candela and lumen realized by other national laboratories around the
world.  While we have the freedom to choose our methods, there are a
continual stream of reality checks.)

I also sense in your question a more basic one:  what is a "lumen" in the
first place?  The lumen is the photometric analog to the watt.  If you
have light with a certain spectral density (watts of power per
wavelength), you integrate this function against a pre-defined weighting
function (the CIE "Visability" function) to compute the lumens of the
light.

I hope this helped.

  - Jonathan