On Mon, 31 Jan 2005 11:53:00 -0800, "Bob May"
wrote:

One thing to remember is that with LEDs, the light is directional to a fair
degree so they don't need reflectors to reflect the light to the area of
interest. This will magnify the lumens that are viewed by some ratio.
In the relativey early days of using LEDs for indicators, the problem with
the LEDs as that they didn't have the wide angle light production so there
had to be some piece of material to widen the light so that it could be seen
brightly way off-axis from the direct on viewing position. The LED
indicator was brighter on-axis than the incadescent bulbs were but the
off-axis light output caused the LEDs to not be used so much as they were
dimmer in those directions. Eventually light output got high enough in LEDs
that they could use frosted plastic in the construction and thus be used for
that application.

Luxeons are not very directional. The radiation pattern of the
Lambertian model approximates a cosine curve, with luminosity down to
50% at about +/- 50 degrees off axis. The batwing and sideshooter
models emit more of their light sideways, less on-axis. The
sideshooter is visible over 360 degrees, is designed for use in a
parabolic reflector.

A bare Luxeon acts more like a "bare bulb" in front of a flat
reflector. A reflector or collimator is necessary if directionality
is desired. Good molded acrylic collimators are available for
about $3.50 in unit qty. Beam divergence angles are available from
"flashlight" (10 degrees) up to about 45 degrees.

The lumen is a measure of total light flux; lenses or reflectors
don't magnifiy lumens, but just govern where the lumens go. You did
say "lumens that are viewed", so I mention this merely for clarity.

They do magnify luminous intensity, measured in candela (lumens per
steradian) or lux (lumens per square meter). The luminous
intensity of a 1-watt Luxeon collimated to a beam with 10 degree
divergence is 342 cd, as compared to an "ultrabright" LED of perhaps
12 cd. A Luxeon III running at 1 amp in the same situation would be
1368 cd. Do not view directly with remaining good eye....