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**To**:**aquatic-plants at actwin_com****Subject**:**Re: watts vs. lumens****From**:**busko at stsci_edu (Ivo Busko)**- Date: Tue, 16 Mar 1999 11:06:47 -0500 (EST)

Regarding the discussion about lumens/Watts/PAR/etc. you techies on the list can look at http://www.cs.indiana.edu/hyplan/kuzimmer/IES/section3.2.html Lumens are units of luminous flux, and this quantity is defined based on an integral over wavelength or frequency. Thus the assertion that "For any given freqency lumens can can be converted to either watts or PAR values" is meaningless. Lumens cannot be computed in any given infinitesimal frequency/wavelength interval, but only in a finite range of frequency/wavelength. Thus a spectral plot can never have lumens in the vertical axis. To translate the relative units used by lamp manufacturers into Watts one first have to compute the integral under the spectral curve. One could copy the curve onto graph paper and estimate the area by counting squares. This total area S (in units such as inches X nanometers) corresponds to the lamp output in Watts P thus one just have to multiply the vertical axis by P/S to get the lamp spectrum in Watts/nm. Of course, this assumes 100% conversion efficiency. Following the same principles one could also multiply this spectrum by the photopic curve (e.g. in http://www.reefnet.on.ca/gearbag/wwwlux.html) and compute the integral to get the lamp output in lumens. Comparison of this figure with the manufacturer's published lumen rating for the lamp should give an idea of the conversion efficiency. The same procedure could be used with a photosynthesis action spectrum in place of the photopic curve. This would give the lamp output in PAR units instead of lumens. -Ivo Busko Baltimore, MD

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