Matthew Russotto
OK - then convert lumens to watts and you're there. You do bring up a good point though - perhaps the accepted convention in technical circles is that the term 'efficacy' is allowed to compare dissimilar units (i.e., watts-lumen, and efficiency is required to be unitless (i.e., watts per watt). That could explain why the term 'efficacy' is used.
But I have to wonder why it is light-watt and not *useable* light-watt - but perhaps that is an issue of practicality in making the lab measurements (i.e., defining and measuring just the visibile light), or perhaps the application may change the definition of what *useable* would be, so the manufacturers don't even try to anticipate that in their data sheets since the applications for a praticular product may be numerous (and exploit different parts of the electromagnetic band - i.e. the UV band used to cure dental epoxy vs. near UV or blue visible from the same bulb used for some special visual application). I'm guessing that is the explanation.
If the above (that 'efficiency' is reserved only for unitless frations or percentages) is true, then yes - I agree it is sloppy - I'll go even further - it would be wrong in a technical discussion (but only sloppy in a layman's-Webster's dictionary level discussion). I concede on this point if that is the case, and it likely is.
Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')
