Spectral Radiance and it's Units

In summary, the spectral radiance of a source is measured in W/cm^2 * sr * nm and is used to determine the spectral output of a monochromator in W. However, when multiplying the spectral radiance by other terms such as slit width, slit height, solid angle, and transmission optics factor, the resulting units may not cancel out to just W. It is important to consider the wavelength of the transmitted light from the monochromator, as the source and monochromator are both monochromatic. When multiplying the original equation by the transmitted wavelength, the nm units cancel out, but it is unclear if this is the intended interpretation of the units. It could potentially be a reminder that the spectral radiance
  • #1
Umabel
21
0
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?
 
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  • #2
What would be the wavelength of the transmitted light from the monochromator?
 
  • #3
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
So...what happens when you multiply your original equation by the transmitted wavelength??
 
  • #5
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?
 
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Related to Spectral Radiance and it's Units

1. What is spectral radiance and how is it measured?

Spectral radiance is the amount of electromagnetic radiation emitted, reflected, or transmitted per unit solid angle, per unit projected area, and per unit wavelength. It is measured using a spectroradiometer, which measures the intensity of light at different wavelengths.

2. What are the units used to measure spectral radiance?

The units used to measure spectral radiance are watts per steradian per square meter per micrometer (W·sr⁻¹·m⁻²·μm⁻¹) or watts per steradian per square centimeter per nanometer (W·sr⁻¹·cm⁻²·nm⁻¹).

3. How does spectral radiance differ from irradiance and radiance?

Spectral radiance is a measurement specific to a particular wavelength, while irradiance is the total amount of electromagnetic radiation at all wavelengths incident on a surface. Radiance, on the other hand, is the amount of radiation emitted, reflected, or transmitted by a surface at all wavelengths.

4. Why is spectral radiance an important concept in science?

Spectral radiance is important because it allows us to quantify the amount of electromagnetic radiation emitted or reflected by different objects. It is used in a variety of fields, including remote sensing, atmospheric science, and astronomy, to study the properties of objects and their surroundings.

5. How is spectral radiance used in practical applications?

Spectral radiance is used in a variety of practical applications, such as determining the composition of materials, measuring the temperature of objects, and studying the properties of light sources. It is also used in satellite imagery to gather information about the Earth's surface and atmosphere, and in medical imaging to diagnose and monitor diseases.

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