P3D Re: White LEDs hazardous?

[Tom Hubin <thubin@xxxxxxxxx>]

Hello Tom,
 
> I have done a bit of research into this, and I can't see why a white
> LED would be any more dangerous than any other light.  Just the
> opposite, in some ways they would be excellent, it they weren't so
> incredibly expensive.

<snip>

> Actually, in a way, white LEDs seem like the perfect slide
> illumination source, since they are ten times more efficient.  Think
> of getting ten times the life from your viewer batteries.

Actually, more than that. A 100 watt Halogen lamp uses 100 watts of
electrical power. It radiates less than 10 watts of visible light and
more than 90 watts of infrared. You need to add a fan and IR absorbing
glass to get rid of the heat caused by the excessive IR. The 10 watts of
light is radiated over a full sphere. With condenser optics you are
doing well to collect 10% of this or 1 watt to illuminate the slide. So
a good projector is less than 1% efficient.

If you used 2 watts of electrical power on some white LEDs I expect you
would get 50% efficiency or 1 watt of light. Since it is very
directional you get to use all of it. So the LED projector is about 50%
efficient or 50 times as efficient as the Halogen projector. You also do
not need IR absorbing glass or a fan. Also, because it is not hot you
can use light weight and thin Fresnel lenses.

Now scale this down to a 10 watt lamp for a viewer. You can skip the fan
and IR filter because there is not all that much heat. But it is still
1% efficient if you use condenser optics. If you don't use condenser
optics and just use a wide angle diffuser you probably only deliver 10%
to the eyepiece. The rest just lights up the inside of the box. So you
probably have the benefit of 10 miliwatts of light.

With this in mind, there is no reason not to put 20mw into a single LED
and get probably 10mw of light to the eye with condenser optics. This is
1/500 of the electrical power of the Halogen lamp without condenser
optics. Your battery lasts 500 times as long. The LED lasts 100,000
hours. The LED costs $2.50. The condenser optics is a Fresnel lens for
$30 (maybe 2 lenses due to low f/number). Seems like a good investment
to me. 

> A couple big problems, though.  One, they are incredibly *expensive*.
> White LEDs are fifty times more expensive than regular LEDs.  You
> could buy batteries for a regular viewer for years and still come out
> cheaper.

I completely disagree. But you do have to use the LEDs and optics
correctly or you are just throwing away light. Same as using any lamp
without proper optics to capture the light.

> Two, they aren't really white, just pretty close.  You've got to have
> other LEDs to balance the color, and it's tricky.  Together with the
> rest of the circuit, that drives the cost up further.

Not so sure about that. A filter to knock out the excess blue ought to
work.

As for the actual spectrum. I have seen 2 versions and I am aware of a
third. Each manufacturer has different stuff. All proprietary. Lots of
progress being made. I predict that many common lamps like household
bulbs and car headlights will soon be replaced with long life and very
efficient white LED equivalents. You'll see.

Tom Hubin
thubin@xxxxxxxxx