White LED Flashlight vs. Halogen

From: zach (victorthecleaner_at_netscape.net)
Date: 09/21/04 

Date: 21 Sep 2004 14:01:45 -0700

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.

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