Radiant Intensity vs. Irradiance

In summary, the conversation discusses the difference between radiant intensity and irradiance. Radiant intensity is the power from a source per solid angle, while irradiance is the power onto a surface per unit area. The difference lies in the measurement of a steradian, which is a 3-dimensional angular measurement that cuts out an area of a sphere equal to r^2. The conversation concludes with an example demonstrating how the irradiance changes with distance from a light source based on the subtended solid angle.
  • #1
JJBladester
Gold Member
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2

Homework Statement



I am having a hard time discerning the difference between radiant intensity and irradiance. I searched PF and found this distinction, but am still confused.

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

Homework Equations



Radiant intensity (Ie) is the output power per steradian, specified in mW/sr.

Irradiance (E) is the power per unit area at a given distance from a light source, specified in mW/cm2.

The Attempt at a Solution



Perhaps my confusion lies in the definition of a steradian.

I know that a radian is an 2-dimensional angular measurement representing the angle "cut out" of a circle when a length of its subtended arc is equal to the radius.

A steradian cuts out an area of a sphere equal to r2.

So, it appears that radiant intensity and irradiance both measure an amount of electromagnetic power per given area. Where is the difference in the two terms?
 
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  • #2
Let a source of light give 1mW/sr.
The Irradiance at 1cm away is 1mW/cm2
At 2 cm away it is 0.25mW/cm2
At 10cm away it is 0.01mW/cm2

This is because area subtended by solid angle = solid angle in steradians x distance squared.
 
  • #3
apelling said:
Let a source of light give 1mW/sr.
The Irradiance at 1cm away is 1mW/cm2
At 2 cm away it is 0.25mW/cm2
At 10cm away it is 0.01mW/cm2

This is because area subtended by solid angle = solid angle in steradians x distance squared.

Apelling,

Your example helps make perfect sense of the subject. Thank you!
 

1. What is the difference between radiant intensity and irradiance?

Radiant intensity refers to the amount of electromagnetic radiation (light) emitted from a source in a particular direction, while irradiance is the measure of the total power per unit area received on a surface. In simpler terms, radiant intensity measures the brightness of a light source, while irradiance measures the amount of light energy reaching a surface.

2. How are radiant intensity and irradiance related?

Radiant intensity and irradiance are closely related, as they are both measures of light energy. The radiant intensity of a light source can be used to calculate the irradiance received on a surface by dividing the radiant intensity by the distance squared between the source and the surface.

3. What units are used to measure radiant intensity and irradiance?

Radiant intensity is typically measured in watts per steradian (W/sr), while irradiance is measured in watts per square meter (W/m²). These units reflect the amount of light energy emitted or received per unit solid angle (for radiant intensity) or per unit area (for irradiance).

4. How are radiant intensity and irradiance important in scientific research?

Radiant intensity and irradiance are important measures in various fields of science, such as optics, astronomy, and environmental science. They help scientists understand the properties of light sources and how light energy is distributed in different environments. These measures are also used in the development of technologies, such as solar panels and LED lighting.

5. Are radiant intensity and irradiance the only measures of light energy?

No, there are other measures of light energy, such as radiant flux, luminous intensity, and illuminance. These measures may be used in different contexts and have different units, but they all provide valuable information about the properties and effects of light. Radiant intensity and irradiance are just two important measures among many others.

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