How many mcd's = 1 Lumen??

[pike]

Well i'm stuck between choosing a bunch of one type of LED and another type of LED.

I can choose either:

3 x 20000mcd white LED (5mm @ 20 degrees) for $0.59 *each*
1 x 12 lumens White LED (8mm @ 55 degrees) for $2.70 *each*

Which one has the better output for light output.

Put simply how many mcd's = 1 lumen??

 

[zevon8]

(a) candella = 1000mcd

(b) lumen divided by 12.57 = 1 candella

answer from (b) x 1000 gives mcd

this will give you a rough comparison.

Also, alot depends on the lens used, a brighter source that is wider difused will "look" dimmer than a closely focused source that is weaker.

Strictly speaking, there is no direct conversion. The units are measured in diferent ways, and involve a bit of "fudging" of numbers due to historical reasons and SI unit conventions, colour, etc. Long boring story. We use a light meter calibrated to human eye sensitivity when comparing light sources at work ( we use LED, incandescent, and halogen lamps at work on emergency vehicles and their output must match a government specification for light output. )

 

[gerty]

The narrow angle ,20 degrees, also means you'll have to be almost directly in front of it to get the total effect. So a lot depends on your use of the led to determine which is best...

 

[Oznog]

There are many oddities to the way these are used.
For one, the lumen scale is supposed to already be corrected for human eye response. Yet Lamina Ceramics devices have a 270 lm red and 78 lm green that appear of similar intensity to the human eye with each at the specified max rating so there's no compensation here.

Candela ratings (mcd) are only the average over the specified output, not the total emitted power. Looking at the Mouser catalog, LEDs of a particular mfg and line, same test current and die, with a tight focus report much higher mcd rating than the wide angle versions.

The difference in angle can be approximated for small angles with just the ratio of the squares. So 55^2/20^2 is about 7.56 times the area. The 12 lumen is half the intensity at any point in front of it but due to the greater area puts out about 3.61x more power.