# mW/sr?

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#### Gandledorf

##### New Member
I've seen a number of IR led's giving their power in mW/sr, and I understand that this is an SI unit for radience over an angular area, but how does one compare this with mW/cm^2, or simply mW?

#### Nigel Goodwin

##### Super Moderator
Gandledorf said:
I've seen a number of IR led's giving their power in mW/sr, and I understand that this is an SI unit for radience over an angular area, but how does one compare this with mW/cm^2, or simply mW?

I've no idea, but looking through the RS Components catalogue I found these specs on two different LED's.

100mA max, 20 degrees, 120mW/SR

and

150mA max, 20 degrees, 44mW/CM^2

From that I would assume about a 4/1 ratio.

#### Optikon

##### New Member
Gandledorf said:
I've seen a number of IR led's giving their power in mW/sr, and I understand that this is an SI unit for radience over an angular area, but how does one compare this with mW/cm^2, or simply mW?

That specification is Milliwatts per Steradian.

A Steradian is a Solid angle of measure. You understand that a radian is a measure of an angle on a two dimensional surface based on a circle. Well, a steradian is a solid angle (cone shaped volume) in 3 dimensions based on a sphere.

There are 2*pi radians in one circle
There are 4*pi steradians in one sphere.

Now that spec is just telling you how much luminous output power is generated per steradian. It gives you an idea of the power density of the light emitted.

I think though, that they use a detector to measure output power and that would represent some cross-sectional area of the steradian and I think it is proportional to the power spec if measured closer or farther away from the device. - I think.

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