# Is there a light distance chart for LED light based on the MCD rating? ?

I'm trying to light a desk surface from the ceiling about 6 ft above the desk.  Not sure if light type makes a difference either - trying to use a natural light LED.

Thanks!
Kory

steveastrouk2 years ago
Ah, but what lands on the surface is "lux", which is one lumen/m^2

Using the example quoted: 12000mcd, 25 deg and 1 metre, I reckon you only get 3.7 lux at your surface - and you probably want >>3000.

Steve
kelseymh2 years ago
The discussion of illumination measurement in Wikipedia might help.

You need to know the size of the cone (emission angle as Steve wrote). From that, you can calculate the area illuminated (= pi * angle * dist2), and therefrom derive the luminance (cd/m2).

That last link has a nice table of "typical values," which you can use to work out how many LEDs you'll need, for example to emulate a 60W incandescent bulb. You can also use the keywords fro the discussion above to do your own search for more detailed resources.
sk888smooth (author)  kelseymh2 years ago
Thanks for the help! Most LED specs list viewing angle. Can I assume that is the same as the emissions angle?

Here is an example. The distance is 6ft. Viewing angle is 25 degrees. Mcd is 12000. Can you calculate the luminance with that info?

Thanks again!
K

2 years ago
According to Wikipedia (http://en.wikipedia.org/wiki/Viewing_angle), yes, viewing angle is the same as emission angle. It's the half-angle of the cone where most of the light comes out.

And unfortunately, I didn't write the definition of luminance properly above; what I did write is quite misleading. The luminance is defined as the light output per unit area of the source, not of the observer.

If you know how big your LED is, just divide the power by the area, and make sure you take care of the units correctly (e.g. 1 m2 is 1,000,000 mm2).

A typical LED is about 5 mm in diameter, or just under 20 mm2. Your 12 cd (12,000 mcd) example would then have a luminance of 12 cd / 20x10-6 m2 = 600,000 cd/m2, compared to 120,000 for a 60W bulb.
steveastrouk2 years ago
Depends on the emission angle of the LED.

Steve