# LED intensity/angle mcd?

Discussion in 'Electronic Design' started by [email protected], May 18, 2006.

1. ### Guest

Looking at the specs for 3 red LEDs, I'm wondering which would be the
better choice for maximum brightness.

Allied part number 670-1257 which claims to be a 400mcd device with a
viewing angle of 60 degrees at 2x theta. Cost is 72 cents.

A second part 670-1224 claims to be a 1800mcd device with a viewing
angle of 30 degrees at 2x theta. Cost is 50 cents.

A third device is 670-1226 claims to be a 1200mcd device with viewing
angle of 30 degrees at 2x theta. Cost is 31 cents.

I'm not sure what 2x theta means, but I think if the angle is twice as
wide, the intensity should be 1/4 as much, so that a 1800mcd device at
30 degrees would be the same as a 450mcd device at 60 degrees?

Why would the the 400mcd device sell for 50% more than the 1800 mcd
device which has more total output?

Would the more expensive device be the better choice?

-Bill

2. ### Pooh BearGuest

It means 2x the 'view angle' theta e.g. 30 degrees, so it's talking about a cone
of light with a sixty degree included angle ( for theta = 30 ).
For the same led 'chip' inside the led you got it.
No idea.
What do you actually want ? A narrow angle led or a wider one ? Whichever's more
suitable for your application is better.

Graham

3. ### Spehro PefhanyGuest

I suggest you view all the data sheets side by side and try to
understand all the differences in detail.

The first one (670-1257) is a 10mm diameter LED with a wide viewing
angle. The wide angle means that the light is spread over a wider area
so it appears not to be as bright on-axis, but can be seen over a
wider angle. Too far off-axis and the others may be difficult to tell
whether they are on or off, depending on ambient lighting conditions.
It also has a red *diffused* lens, which means that the light appears
to come from the surface of the dome rather than through the 'water
clear' lens that the others have. It's also tinted red, which affects
how ambient light is reflected or absorbed. Chances are good it has
the exact same type of 660nm color die in it as 670-1226, just a
different molding.

The main difference between the second two is the color of the emitted
light. They are both 5mm diameter LEDs. The 670-1224 emits 636nm
wavelength light, which is more of an orange-red. The 670-1226 emits
longer wavelength 660nm light. Both are water-clear epoxy lens. They
are narrow angle (30°), which means they look much brighter when you
look almost directly into them, as opposed to much off the axis. Most
manufacturers provide a graph of brightness versus viewing angle. The
different dies have slightly different forward voltages as well
(higher typical forward voltage by 300mV for the 670-1224, which emits
shorter wavelength light).

BTW, please get into the habit of referring to such parts by the
manufacturer's (Lumex, in this case) part number rather than the
distributor catalog number. You will find it infinitely easier to deal
with the manufacturer's data sheets and when you change distributors
or shop around you won't have to translate everything if you stay with
the same manufacturer.

That said, why not buy all three, power them up at a sensible current
(in series is good for side-by-side comparisons) and look at the
differences for yourself?

Best regards,
Spehro Pefhany

4. ### RHRRCGuest

The ratio of the intesities for a given led die with different theta's
is the inverse of the ratio of the square of the tan of each theta.
Since tan15=0.268 and tan30=0.577 we would expect the narrow angled
device to be
(0.577*0.577)/(0.268*0.268) =4.64 times 'brighter' than the wider
angled device.

5. ### Pooh BearGuest

Which indeed seems to be the case here.

Graham

6. ### Spehro PefhanyGuest

It's diffused too, which should make it even worse.

Best regards,
Spehro Pefhany

7. ### Guest

Spehro,

Thanks for the reply. As I understand it, the (670-1257) part with the
red diffused lens emits a closer pure red light than the other options,
but also has less total power output due to the red colored lens which
filters out some of the light?

The 670-1224 emits more of an orange-red color, but also appears
brighter compared to the 670-1226 which is closer to pure red but rated
at a lower mcd?

How does one compare mcd rating to color and apparent brightness?
I'm guessing orange LEDs of the same power output will appear brighter
than pure red LEDs of the same power output?

-Bill

8. ### Spehro PefhanyGuest

It's the same color as 670-1226, just with a huge 10mm diameter lens
that is also tinted and diffuse (like a regular frosted incandescent
bulb as opposed to a clear glass candelabra bulb). It's less bright
looking right on axis because the light goes all over the place, and
is emitted from a large surface.
Yes, although the difference in brightness is not much.
The number is already corrected for eye response AFAIUI, so compare
directly. But the eye has a log response so a 2:1 difference in
brightness doesn't look huge.

Best regards,
Spehro Pefhany

9. ### James WaldbyGuest

All else being equal (eg illumination pattern and viewing angle, besides
power output, LED efficiency, "ordinary" eye) that might be so, since
as mentioned before and as shown in some of the following webpages,
the eye's sensitivity is dropping off as wavelength increases from
orange to red. http://www.yorku.ca/eye/specsens.htm
http://www.yorku.ca/eye/lambdas.htm
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/light/u12l2b.html

However, I don't recall that you've said yet what it is you are
trying to do (other than having blue and red LED's with about the
same apparent brightness). Will the LED's be panel indicators, or
do they illuminate something? From how far are they being viewed?
Under what lighting conditions? Proper answers depend on knowing

I don't know what you are trying to do, and don't know your
requirements, but will mention a few things anyway. First, based on
a nonscientific look at some ebay prices, it's substantially easier
to get red/green/white LED's that are bright, vs. yellow/orange, so
(See ebay examples below.) Second, most of these LED's are bright
enough that they are dangerous to look at, making them unsuitable
for use as panel indicators if driven at full power. Third, several
of these will produce visible (ie, noticeable but not very bright)
light at currents of a few hundred microamps, and could be used as
indicators with currents of a milliamp or two.

Auction# \$Each Item and pertinent specs from auction pages
7619669955 .23 100x 5mm Red LED 25ha 15000 mcd typ. at 20mA
7620754821 .32 50x 5mm Red LED 20pa 20000 mcd min at 20mA
7619480649 .60 20x 10mm Red Led 30va 80000 mcd claimed at 60mA
7620924300 .17 30x 3mm Yellow 20va 4000 mcd typ. 5000mcd claimed
7620916574 .12 100x 5mm Yellow 20va 4000 mcd typ. 5000mcd claimed
7619668700 .26 50x 10mm Green 13ta 65000 mcd typ. at 20mA
7619668531 .36 50X 10MM White 13ta 110000mcd typ. at 20mA
7619670585 .26 50x 10mm Orange 13ta 45000 mcd typ. at 20mA
7620929777 .24 50x 10mm Yellow 12va 60000 mcd typ. at 20mA

\$Each= (Price and shipping total dollars) divided by (number of LED's).
The angle (in degrees) of the LED light is listed 4 different ways in
the above auctions, which I've indicated by ha, pa, va, and ta, as
follows: ha, half angle; va, viewing angle; pa, "50% Power Angle";
ta, "1/2 Viewing Angle". Last auction also has a "Radiation Diagram".
Note, where typical mcd ratings were given I listed those rather
than the max values, or if typ not given, min instead of max.

-jiw

10. ### RHRRCGuest

Just to clear up any misunderstanding about photopic units:

An Xmcd led of any colour has *exactly* the same apparent 'rightness'
as a led with the same mcd of any other colour.
That is what a Candela (milli candela or whatever) is by definition.
It is not a measure of the power emitted by a light source it is a
measure of the human response to the radiated power.
(An infra-red led obviously emits power (in the infra-red region) else
my TV remote control would not work but it emits none in the visible
region ad thus emits zero milli candela's.)

11. ### RHRRCGuest

could even be the same 'brightness' - iff eye kood spel propurly