P3D Re: White LED efficiency.

[Tom Hubin <thubin@xxxxxxxxx>]

Greg Erker wrote:
> 
> >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?

It should be from both. A filament generates over 90% IR which becomes
heat. Less than 10% of the electrical energy is visible light. I do not
know what the electrical to visible conversion efficiency is for a LED
but it is much better than 10% since there is no non-visible light and
they do not get very hot.

The directionality is good for another factor of 5 or so. Filaments
radiate in all directions but most optics collect less than 20% of that
light and direct it toward the slide and the eye. Practically all of the
LED light can be collected and used.

> 
>   From this LED page:
> http://www.misty.com/~don/led.html
> 
> the most efficient white LED he
> tested is 15-16 lumens/watt

Is this optical Lumens out per electrical watt in or optical lumens out
per optical watt out?
  
> ---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.

The CZX is a 500 electrical watt bulb used in TDC116 projectors. It is
rated as 12500 lumens out at 3200K when new. That is 25 lumens per watt.
So the number above is reasonable for a non projection bulb with a lower
color temperature.

> [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?

This is possible but I believe that LEDs generate more light per
electrical watt than do filament bulbs. But I have no numbers to support
this. I would like to see published data on colored and white LEDs.

I have no doubt that the white LEDs are less efficient than colored
LEDs. They are newer and the technology is still immature. They can and
will get better and cheaper over the next couple of years.
 
>   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).

I don't think so. Think about a cube around the bulb. The condenser can
usually collect light from much of one face. A good afocal reflector
could reverse the light from the opposite face. But nobody uses these.
They use a simple spherical mirror which is too close to the filament.
As a result, much of the reflected light misses the collection optics.
Some of the light hits the filament itself and never makes it through to
the collection optics. The mirror is usually less than 85% reflective.
All together I figure you are lucky if you can capture and control 20%.
I have no references for this but simple geometric optics is where I
come up with this.