On Mon, 5 Sep 2005, Bill Putney
Erm...convert lumens to watts? How, pray tell, do you plan on accomplishing that? One might as well convert Buicks to kumquats, or inches to volts.
There are people over on sci.engr.lighting who can explain it more articulately than I have done.
Because "usability" is a moving target that occurs somewhere between the light source and the fixture in which it is used. Take a look at these two isocandela diagrams:
The top one is a low beam generated by a 5-3-4" round headlamp that takes an H1 bulb producing 1500 lumens. The beam contains 687 lumens. System efficiency = 45.8 percent.
Are your headlight lenses getting cloudy 2430On Thu, 1 Sep 2005, Rick (etc.) You're right. Lens haze sharply reduces seeing light and sharply increases glare light. Federal Motor Vehicle Safety Standard 108 -- Lamps and Reflective Devices -- specifies UV...
The bottom one is the matching high beam, 5-3-4" round headlamp that takes the same H1 bulb producing the same 1500 lumens. The technology and size are identical. The beam contains 856 lumens. System efficiency = 57 percent.
That doesn't mean, though, that the high beam is a better lamp, or that it would make a good subsbreastute for the low beam. Mostly it illustrates that system efficiency is certainly not the only measure of system effectiveness.
Remember also, my "usable light" definition of efficiency is only one kind of efficiency. There are numerous others (energy efficiency, thermodynamic efficiency, etc.)
DS
