P3D White LED efficiency.

[Greg Erker <erker@xxxxxxxxxxxxxx>]

>I think I started this by suggesting how to more efficiently use a light
>source with condenser optics, even in a viewer. I claim that it is so
>much more efficient that you could even use one 3.6v, 20ma, white LED
>per slide. That is 72mw per slide or 144mw total electrical power.

  I'm a bit puzzled. Is the efficiency coming
from the LED or from using condensor optics?

  From this LED page:
http://www.misty.com/~don/led.html

the most efficient white LED he
tested is 15-16 lumens/watt

---quote---
Most Efficient White LEDs - 15-16 lumens/watt
by Nichia, maybe 9-plus by Toyoda Gosei,
probably Hewlett Packard.
[snip]
UPDATE 10/31-99 - Nichia NSPW-500BS models ordered mid-october 1999 test as
15-16 lumens/watt. Claims that I have heard of 15 lumens/watt
are credible. The current NSPW-500BS supposedly has a 5.6 candela 20 degree
beam.
---end---

  And from this page:
http://www.misty.com/~don/bulb1.html

---quote---
In a 120 volt, 100 watt "standard" bulb
with a rated light output of 1750 lumens,
the efficiency is 17.5 lumens per watt.
[later on the page]
A halogen bulb is often 10 to 20 percent
more efficient than an ordinary incandescent
bulb of similar voltage, wattage, and life
expectancy.
---end---

  So it seems like the best white LED
is less efficient than a household 100W
bulb. And I hear that low votage halogens
are more efficient than 120V ones. So I'd
guess that the LED is less efficient than
the 2.5V 0.8A halogens many of us use in
our viewers.

  So that seems to imply that all the efficiency
improvment in your proposed LED lit viewer
comes from the condensor optics, and that the
LED emits its light in one direction instead
of into a sphere (which benefits a condensor
optical system). Is this correct or am I
wrong somewhere?

  BTW slide projector bulbs with built in
reflectors probably emit close to 50% of
their light in one direction. Unfortunately
these are 100W and up bulbs. Not too useful
for handheld viewers or battery power (and
blindingly bright to boot).

Greg E.