Converting Between Light Intensity and Irradiance: How Do They Relate?

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SUMMARY

The discussion clarifies the relationship between light intensity and irradiance, establishing that irradiance is defined as power per unit area (W/m²) while intensity is defined as power per solid angle. It emphasizes that irradiance is typically measured at the target, whereas intensity originates from the source. The confusion arises because intensity is often incorrectly used in place of irradiance. The conversion between spectral irradiance and light intensity is contingent upon the bandwidth of the measurement.

PREREQUISITES
  • Understanding of the concepts of irradiance and intensity in photometry.
  • Familiarity with units of measurement such as W/m² and W/(m²·nm).
  • Knowledge of spectral irradiance and its implications in light measurement.
  • Basic principles of solid angles in physics.
NEXT STEPS
  • Research the differences between irradiance and intensity in photometric measurements.
  • Learn about spectral irradiance and its calculation over specific bandwidths.
  • Explore the implications of solid angles in light source measurements.
  • Investigate practical applications of irradiance in fields such as horticulture and solar energy.
USEFUL FOR

Physicists, optical engineers, and anyone involved in light measurement and analysis, particularly in fields like photometry and environmental science.

El Flea
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What is the relationship between light intensity and irradiance? How do I convert one to the other?
 
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Strictly speaking irradiance is power/area and intensity is power/solid angle.
Typically irradiance is at the target and intensity is from the source.
But intensity is often used when you should use irradiance - so often they are the same thing,
 
Thanks!
Isn't W(m^-2) the unit for light intensity?
So, if I have, let's say, 1W(m^-2)(nm^-1) spectral irradiance, in the 400nm wavelength range, can I say that I have 1x(10^-9)W(m^-2) light intensity over the same area?
 
El Flea said:
Isn't W(m^-2) the unit for light intensity?
Nope that the unit of irradiance. As I said - intensity is often losely used when you should say irradiance.

So, if I have, let's say, 1W(m^-2)(nm^-1) spectral irradiance, in the 400nm wavelength range, can I say that I have 1x(10^-9)W(m^-2) light intensity over the same area?
It depends on the bandwidth, if you have 1W(m^-2)(nm^-1) then you would have 100W/m^2 over a 100nm bandwidth
 

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