<crossposted to sci.optics>

zach wrote:

Hi- I have recently picked up some LED flashlights, being sold on the
idea that the bulbs will last longer than the light, and they have
much lower power consumption than filament bulbs. I am kind of
dissappointed, however, in the intensity of the light. I have a little
portable light (one LED) that takes two 1.5V cells. It seems like it
is bright when I shine it into my eyes, but it does not seem very
bright when shining it on objects. I used it when I went into a pitch
black mine, for instance, and it was truly pathetic. Luckily, we had
brought along a large halogen mag. light... It does reflect nicely off
of the retinas of deer at night (and they didn't seem bothered by it,
stupid deer).

So, I bought a larger flashlight (with 2D cells) that has four LEDs.
It does seem _almost_ as bright as the standard mag. light (which has
4 Ds for power supply, but one bulb), and hurst to look at, but has
very poor reflectance off of objects. I looked up something on white
LEDs and saw that it is basically a blue LED, with a high peak at
around 460 nm, which excites a weaker, broad yellow peak... mixed
together the eye apprehends as white. The blue peak seems to be at
about 9% range of light sensitivity for the human eye. Therefore, I
would conclude (and this is my question) that the reason it seems to
bright to me is because of that high intensity peak near the lower end
of the wavelength detectability of my eyes. At a young age, I noticed
that black lights seem to "feel" brighter than they are when looking
directly at them (now and then, I didn't make a habit of it), but
didn't really seem "bright". I assumed this was because their peak
intensity was well below that of human eye detectability.

And this segues into the related question... that of emissivity. I
assume that the reason the "white" LED flashlight does not seem that
intense when illuminating other objects, especially where no other
light sources are present (like the mine), has something to do with
emissivity wrt the light source. The halogen white-yellow illuminates
external objects nicely, but the "white" LED is kind of weak. Can
someone give a brief explanation of this (if my assumption is correct)
comparing the two light sources? Thanks for any input or corrections.

Z.

Ah, a real-world application of phots and nits!

LED sources are nice because a lot of the electrical power (something
like 10%, IIRC) is converted into visible photons. LEDs are "efficient"
when compared to typical incandescent (halogen) bulbs (only something
like 0.1% of electrical power ends up as visible light). And, because
LEDs are solid state, they don't burn out- 100 khour lifetimes are
standard, I think, as compared with about 1 khour on an incandescent.

But... The total amount of light energy coming out of an LED is less
than an incandescent, generally. Industrial-strength LEDs are around
100 mW:

http://www.lumileds.com/products/family.cfm?familyId=9

Lumileds claims to sell the brightest LED in the world. Note they quote
brightness in terms of 'lumens', not Watts: hence the first line of my
reply. I'm not going to bore you with the specifics, let's just say
manufactuers like to use 'lumens' and 'candelas' when they want to
confuse the typical buyer. Not sure what the rating is on a flashlight
LED, but let's say 10 mW. Say the halogen bulb burns through 4 'D'
cells in 1 hour, (18 W*hr/D cell), means the halogen bulb is consuming
about 80 Watts, which means 0.8 W = 800 mW visible energy, considerably
more than the LED.

As for why the LED is bright when you look directly at it versus the
reflection, LEDs emit light in a highly directional way, so that can
explain why you notice the difference.

In any case, I suspect LEDs or their various bretheren (OLEDs, etc) will
eventually replace fluorescent lights- not sure about incandescents due
to source color issues, which you allude to in paragraph 2.


--

<crossposted to sci.optics>

zach wrote:

Hi- I have recently picked up some LED flashlights, being sold on the
idea that the bulbs will last longer than the light, and they have
much lower power consumption than filament bulbs. I am kind of
dissappointed, however, in the intensity of the light. I have a little
portable light (one LED) that takes two 1.5V cells. It seems like it
is bright when I shine it into my eyes, but it does not seem very
bright when shining it on objects. I used it when I went into a pitch
black mine, for instance, and it was truly pathetic. Luckily, we had
brought along a large halogen mag. light... It does reflect nicely off
of the retinas of deer at night (and they didn't seem bothered by it,
stupid deer).

So, I bought a larger flashlight (with 2D cells) that has four LEDs.
It does seem _almost_ as bright as the standard mag. light (which has
4 Ds for power supply, but one bulb), and hurst to look at, but has
very poor reflectance off of objects. I looked up something on white
LEDs and saw that it is basically a blue LED, with a high peak at
around 460 nm, which excites a weaker, broad yellow peak... mixed
together the eye apprehends as white. The blue peak seems to be at
about 9% range of light sensitivity for the human eye. Therefore, I
would conclude (and this is my question) that the reason it seems to
bright to me is because of that high intensity peak near the lower end
of the wavelength detectability of my eyes. At a young age, I noticed
that black lights seem to "feel" brighter than they are when looking
directly at them (now and then, I didn't make a habit of it), but
didn't really seem "bright". I assumed this was because their peak
intensity was well below that of human eye detectability.

And this segues into the related question... that of emissivity. I
assume that the reason the "white" LED flashlight does not seem that
intense when illuminating other objects, especially where no other
light sources are present (like the mine), has something to do with
emissivity wrt the light source. The halogen white-yellow illuminates
external objects nicely, but the "white" LED is kind of weak. Can
someone give a brief explanation of this (if my assumption is correct)
comparing the two light sources? Thanks for any input or corrections.

Z.

Ah, a real-world application of phots and nits!

LED sources are nice because a lot of the electrical power (something
like 10%, IIRC) is converted into visible photons. LEDs are "efficient"
when compared to typical incandescent (halogen) bulbs (only something
like 0.1% of electrical power ends up as visible light). And, because
LEDs are solid state, they don't burn out- 100 khour lifetimes are
standard, I think, as compared with about 1 khour on an incandescent.

But... The total amount of light energy coming out of an LED is less
than an incandescent, generally. Industrial-strength LEDs are around
100 mW:

http://www.lumileds.com/products/family.cfm?familyId=9

Lumileds claims to sell the brightest LED in the world. Note they quote
brightness in terms of 'lumens', not Watts: hence the first line of my
reply. I'm not going to bore you with the specifics, let's just say
manufactuers like to use 'lumens' and 'candelas' when they want to
confuse the typical buyer. Not sure what the rating is on a flashlight
LED, but let's say 10 mW. Say the halogen bulb burns through 4 'D'
cells in 1 hour, (18 W*hr/D cell), means the halogen bulb is consuming
about 80 Watts, which means 0.8 W = 800 mW visible energy, considerably
more than the LED.

As for why the LED is bright when you look directly at it versus the
reflection, LEDs emit light in a highly directional way, so that can
explain why you notice the difference.

In any case, I suspect LEDs or their various bretheren (OLEDs, etc) will
eventually replace fluorescent lights- not sure about incandescents due
to source color issues, which you allude to in paragraph 2.


--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
CWRU School of Medicine
tanspose 'op' for mail

victorthecleaner@netscape.net (zach) wrote in message news:<f7fbe28c.0409211301.3ad55fe@posting.google.com>...

Hi- I have recently picked up some LED flashlights, being sold on the
idea that the bulbs will last longer than the light, and they have
much lower power consumption than filament bulbs. I am kind of
dissappointed, however, in the intensity of the light.

I recently purchased a 12 LED flashlight from the CCrane Company: www.ccrane.com
They are selling them at half price now; a $100.00 flashlight for $50.00.

They have an Advance Control Circuit which makes them even more efficient.
They take 3 alkaline C cells and have peak brightness for over 20 hours.

I never thought I would own one because they have been so expensive.
I am totally impressed with its brightness. It would make a blackout tolerable.

This is a great deal. I understand there is only a limited quantity.
They delivered promptly with free shipping.

zach <victorthecleaner@netscape.net> wrote:

Hi- I have recently picked up some LED flashlights, being sold on the
idea that the bulbs will last longer than the light, and they have
much lower power consumption than filament bulbs. I am kind of

The first is true.
The second is marketing.
Compared with a minimaglite-solitare bulb, the best LEDs at the same
light output are about twice or a bit more efficient.
Compared with 1W or bigger halogen or krypton bulbs, they are about
the same.

They have advantages.
Instant-on, means that you can pulse them without any power loss at
the optimum light level for your application, with no colour shift.
Much more resistant to shock.
Thousands of times better life.

Mixing the best red-green-blue LEDs, to give white can get a fair bit
better.
Nowhere near the closing on 50% that some fluorescents can get.