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

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## 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?

**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.

**Next, determine the frequency of the light.**For this problem, the frequency is calculated to be 40 m^-1

**Next, determine the incident area.**The total area the light is hitting is found to be 50m^2.

**Next, determine the change in time.**The light emission is measured for 10 seconds.

**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).

## FAQ

**What units are light intensity measured in?**

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