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re: comment on lumens

Robert Miehle wrote:

> > Assuming you have the same ballast and are
> > comparing two lamps with the same wattage rating, the light energy that
> > leaves the system has to be equal for both lamps, right?
> Sorry, no, that's not right.


> So the light energy leaving the system has not to be equal for different
> kind of lamps, because more or less UV energy is lost as heat, dependent on
> the efficiency of the flourescent substance.

This might be a relatively small factor compared to the spectral effect
that Mr. Shimoda mentioned.

According to Allen and Rowe in the CRC Handbook (Weast, 1974) the
theoretical maximum "efficiency" of any light is 680 lumens/watt; that
would be if all the energy is emitted as green light (555 nm).  A white
light with completely even output across the visible spectrum would be
less than 1/3 that "efficient"  - only 200 lumens/watt.  Fairly small
differences in the output spectrum can give large differences in the lumen
output without any changes in the amount of heat generated by the

This difference is because the lumen measure is weighted toward green
light - the light to which our eyes are most sensitive.  Light of that
color is near the minimum in the photosynthetic action spectrum.  That
makes lumens nearly irrelevant to whether or not a light is good for
growing plants.  It is of course a very relevant measure when you are
talking about how bright you want the tank to look, but that's a
different consideration.

Roger Miller