Luminous Intensity
5 Minutes
Luminous intensity and measuring light for students, covering lumens, candela, lux, and photometers. An English science lesson.
- Luminous flux and measuring bulb brightness in lumens
- Luminous intensity in a specific angle measured in candela
- Illuminance measured in lux and using a photometer in cricket
What this lesson teaches
Luminous intensity is the amount of light emitted by a light source in a specific direction within a particular angle. Science uses different measurements, such as luminous flux, luminous intensity, and illuminance, to describe how light travels from a source and lands on surrounding surfaces.
Luminous flux and lumens
Luminous flux measures the total power or brightness that a lamp produces in all directions. Just as distance is measured in meters and time is measured in seconds, luminous flux is measured in a unit called the lumen. For example, a standard forty-watt bulb produces about 450 lumens, while a tube light gives off around 2,400 lumens. Everyday bulb packaging displays these lumen numbers so shoppers know the brightness of the light.
Luminous intensity and candela
While light shines in all directions, luminous intensity focuses only on the light present along a chosen angle. The unit for luminous intensity is the candela, which is based on the brightness produced by an ordinary candle. If the light in a specific angle equals the brightness of ten candles, it measures ten candela. A person standing close to a bulb gets light across a smaller area, while someone standing farther away has light spread across a larger area, yet the candela value remains ten for both.
Illuminance and the lux unit
Illuminance describes how much light coming from a bulb actually falls on a specific surface, such as a person's face. The unit used to measure illuminance is the lux. This lux measurement changes depending on the power of the light source and the distance to the illuminated object. When an object stands closer to the light source, it receives more light and has a higher lux level than when it moves farther away.
Using a photometer
- A photometer is a device used to measure lux and show how much light is falling on a particular spot.
- Cricket umpires use a photometer to check if natural light is sufficient for players to see the ball clearly.
- In the 2013 Ashes Cup, a match was stopped and called off for bad light when only twenty-one runs were needed from twenty-four balls.
- Film crews use photometers to measure set lighting and will cancel an outdoor shoot if the light is insufficient.