piclist 2008\07\06\144501a >
Thread: What's a "bright" LED?
www.piclist.com/techref/index.htm?key=whats+bright+led
BY : Sean Breheny email (remove spam text)

Tomas,

You are right to put a lot of emphasis on figuring out things
beforehand using a theoretical basis, but designs always require
iteration. In other words, you work out a basic scheme, then you test
it out, then you refine your calculations based on the results of the
test, and then you try it out again.

When it comes to LED brightness, there are many factors involved in
determining how bright an LED needs to be:

Candelas are a unit of light intensity, as in watts per solid angle,
except that they are also adjusted for the spectral response of the
human eye.

Lumens are a unit of total light output, similar to watts, but also
adjusted for the eye's response to color.

A certain LED will have a range of total output at a given current
(due to manufacturing differences). This range can be specified in
Lumens. The output will vary linearly with the current through the
LED.

This same LED will output this light in a non-uniform way (that is, it
has a radiation pattern due to the lensing effect of the case). The
Candela rating is the light intensity at the peak of the LED's pattern
(usually looking straight into it). The points where the light
intensity is cut in half (I think) are referred to as the LED's
viewing angle.

The human eye's response to brightness is close to logarhythmic so
unless you are going to put several LEDs next to each other and want
them to look identical, you normally do not care about changes in
intensity which are less than, say, 50%.

You stated that you want these LEDs to be easily visible in daylight.
You need to specify the range of angles for which you want this to be
true. I think that the packaging around the LEDs will also greatly
affect this.

I suspect that designing your LED light to look bright in full
sunlight will make it painfully bright indoors. You may even want to
be able to vary the drive and have a photosensor which detects the
ambient light and adjusts the LED drive accordingly.

Roughly, here's what you would need to do to try this mathematically
(but be prepared for the results to be wrong enough to require
iteration):

1) Determine the maximum background illumination level you need to
operate in (full sun you said)
2) Determine the range of angles you want someone to be able to see the LED from
3) Determine how uniform you want the illumination to be (i.e., the
original Connect4, I think, had large plastic buttons which lit up
rather than point sources like LEDs)
4) Take the entire surface area which the LED will light (i.e., the
"button") and compute the number of lumens of sunlight which might, at
max, fall on this area.
5) Determine how much of the light incident on that surface will be
reflected. Now call this Lc, as in light you have to compete with.
6) Assume that this light reflects back over the same range of angles
which your LED will. Under this assumption, your LED needs to emit Lc
lumens.
7) You can then convert the LED's radiation pattern to solid angle,
divide Lc by this solid angle in steradians, to get the required
intensity in Candela.

Sean

On Sun, Jul 6, 2008 at 8:48 AM, Olin Lathrop <olin_piclistembedinc.com> wrote:
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