Enter the frequency, the number of photons, incident area, and time into the calculator to determine the light intensity.

Light Intensity Formula

The following formula is used to calculate the intensity of light shining on an area.

I = [n*f*h] / [ A * t ] 
  • Where I is the light intensity (candela, cd)
  • n is the number of photons
  • h is Planck’s constant
  • f is the frequency
  • A is the incident area
  • t is the time

Light Intensity Definition

Light intensity, or luminous intensity as it’s often referred to as is a measure of the power emitted by a light source in a certain direction.

This light emission is both wavelength-weighted and per unit of solid angle. Light intensity is studied in the field of photometry.

Example Problem

How to calculate light intensity?

  1. First, determine the number of photons being emitted.

    For this example, to be simple, the number of photons emitted in the specific direction is found to be 2,000.

  2. Next, determine the frequency of the light.

    For this problem, the frequency is calculated to be 40 m^-1

  3. Next, determine the incident area.

    The total area the light is hitting is found to be 50m^2.

  4. Next, determine the change in time.

    The light emission is measured for 10 seconds.

  5. Finally, calculate the light intensity using the formula above.

    I = [n*f*h] / [ A * t ] = [2,000*40*(6.62607004 × 10-34 ] / [50*10] = 1.0601712e-31 Candelas (cd).


What units are light intensity measured in?

Light intensity (luminous intensity), is measured using the units candela which is abbreviated (cd)

light intensity calculator