Two Easy Brightness Meters for brightness comparisons

MrAl

Flashlight Enthusiast
I have posted an easy to make light
http://members.aol.com/xaxo/

Scroll down to the bottom of the page and
click on "NEXT", that will bring you to the
new page.

The 'meter' is very handy to use for comparing
the light brightness of one LED source to another.

The 'meter' is constructed out of two slices of
white translucent candle with two squares of
aluminum foil sandwiched in between. The two
aluminum foil squares are glued together
back to back and the two slices are glued
on opposite sides of the foil. The two
slices should be about the same width and
should be a white translucent type round candle.

This meter is viewed from the side as the two light
sources are directed at the meter on opposite faces,
adjusting the distance of one source from the
meter untill both lights appear to be the same
brightness.

Then measure the distance of each source to the
center of the meter. Compute the comparative
brightness of the #2 source to the #1 source
by using a hand calculator to square the
division of distance #2 divided by distance #1.
The brightness of source #2 is then apparant
as it relates to the brightness of source #1.

For example, say you set one LED flashlight
pointing at the meter from a distance of
5 inches. This light creates a certain
brightness on one side of the meter. Now
directing the second LED source to the
other side of the meter and varying the
distance untill it looks about equal in
brightness as the first source. If this
condition occurs when the second light is
10 inches away from the other side of the
meter, then the brightness of the second
source as compared to the first source
can easily be calculated like this:

10 divided by 5 equals 2, then 2 'squared' is
2 times 2, which equals 4.
This means the second light is four times
as bright as the first light. That's it.

For long term comparisons, choose a 'reference'
LED that you can compare to other LED's.
Keep that LED aside and dont use it for anything
else. When you want to compare another LED,
fire it up with the same current each time
and compare it to another LED with whatever
current you wish.

For example, if you wish to know the brightness
comparison of an LED running at 20ma to the
same type of LED running at 30ma, run both
first at 20ma and point them at opposite sides
of the meter. If they both appear to give the
same brightness when both distances are the same,
then they both have the same brightness. Now
increase the current to the second LED to 30ma
and adjust it's distance from the meter untill
again they appear to be the same brightness on
the meter.
Measure the second distance and divide by
the first distance and square the result.
This will give you the comparative brighness
of two similar LED's run at two different
currents.

There is a simpler type of meter you can make
a little easier then the two candle slice meter,
but the comparison is a little harder to interpret.
I like the candle wax meter better, but this
one is easier to make.
Take a bright white sheet of paper and
put a tiny drop of oil in the center so that it
forms about an 1/8 inch diameter spot.

Shine one source on the front side of the paper and
note the oil spot appears darker then the other
areas of the paper surrounding the spot.
Now shine the second source on the back side of the
paper very close to the paper and note that the spot
seems to get brighter, while at other distances
the spot seems to get darker, and at one
particular distance the spot seems to be almost the
same brightness as the paper immediately surrounding
the spot (as lit from the front source). This is
the point at which both brighnesses are equal on
the paper, and now you can measure the distances
from each source to the paper and compute the
relative brightness with the same method as
above: divide the distances and then square
the result.

Have fun

--Al