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Re: Measurement units of light

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.

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

Subtlety isn't the problem. Forget candelas.