What are Radiance and Irradiance and how do they differ?

[JJ123]

Hey, I'm having trouble working out what Radiance and Irradiance are, all the definitions I get are along the lines of something really technical like:

"For a point of radiant energy, Radiant Intensity, in a specified direction, per unit projected area: Le=(dIe/dA)*Cos(theta) where A is the area and theta is the angle between the specified direction and the surface".

Would you be kind enough to give me a very simple wordy definition of what these are and the difference between the two? Thank you SO much.

 

[mgb_phys]

Radiance is the power from the source per area into a certain solid angle.
Irradiance is the power onto a surface per area

(hint, Irriadiance = I = input )

 

[sir-pinski]

Sure! Radiance and Irradiance are both terms used in the field of optics to describe different aspects of light. Radiance refers to the amount of light that is emitted or reflected from a surface in a specific direction. It is measured in watts per square meter per steradian (W/m²/sr), which basically means the amount of light energy per unit area and unit solid angle (a measure of direction).

On the other hand, Irradiance refers to the amount of light that is received by a surface from all directions. It is measured in watts per square meter (W/m²) and takes into account the total amount of light that falls on a surface, regardless of the direction it comes from.

To put it simply, radiance is the amount of light that is emitted or reflected in a specific direction, while irradiance is the total amount of light that falls on a surface from all directions.

Think of it this way - imagine standing in a dark room with a flashlight. The light from the flashlight would be the radiance, as it is directed in a specific direction. However, if you turned on all the lights in the room, the total amount of light that falls on your skin would be the irradiance.

I hope that helps clarify the difference between radiance and irradiance for you! Let me know if you have any other questions.