Re: Measurement units of light

["John S. Denker" <jsd@MONMOUTH.COM>]

At 07:45 PM 2/28/01 -0500, Herbert H Gottlieb wrote:
> John Roeder asks.........
>
> What is the relationship between the cd and W/m^2  lighting ?

The difference IMHO is that W/m^2 makes sense and candela does not.

> The difference seems to me to be that that
> the cd refers to energy intensity at a given wavelength
> and that this is of
> significance because the eye is not equally sensitive to all frequencies,

The candela _per se_ doesn't account for the variation in eye
sensitivity.  It is, by definition, inapplicable and meaningless for light
at any frequency other than 540 THz.

  http://physics.nist.gov/cuu/Units/candela.html

There may be some unit lying around that measures perceived brightness, but
if so I don't know anything about it.

There are some cases where perceptual effects are noticeable;  for
instance, a 1mW green laser looks a whole lot brighter than a 1mW red laser.

OTOH for ordinary illumination, there are good reasons for preferring a
broad, more-or-less flat spectrum.  If we assume such a spectrum, knowing
the W/m^2 is all you need to know.

>  Is the distinction between cd and W/m^2  too  subtle for ninth
> graders.

Subtlety isn't the problem.  Forget candelas.