# Spectral Radiance and it's Units

1. Oct 30, 2005

### Umabel

I'm told the spectral radiance of a source is in the following units

W
----------------
cm^2 * sr * nm

And I try to use this to figure the spectral output of monochromator (in W), but I can't seem to cancel out the units. I multiply the spectral radiance by four other terms

slit width (cm)
slit height (cm)
solid angle, using a quick "f-number" equation (sr)
and then the transmission optics factor, which I assume could be unitless but I could be wrong.

Anyways, anybody know why I get W/nm instead of W? Or is it really just W?

2. Oct 30, 2005

### big man

What would be the wavelength of the transmitted light from the monochromator???

3. Oct 30, 2005

### Umabel

I assume the source is monochromatic here, so the wavelength coming out of monochromator is also monochromatic...let's just say that the radiation is at 400nm or something...

4. Oct 30, 2005

### big man

So...what happens when you multiply your original equation by the transmitted wavelength??

5. Oct 30, 2005

### Umabel

If I multiply the rest of the equation by let's say, 400nm, then the nm units cancel out...

edit: though I have to admit that I am not totally convinced that this is what the unit implies. Is it some invisible reminder that the spectral radiance depends on what wavelength region this source is emitting from?

Last edited: Oct 30, 2005