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Light Measurement


Light is an energy that gives us the sense to visualize the things around us! To know how much light falls on anything or to justify which part is brighter you need to measure it. There are different units used to measure light intensity namely Candela (cd), Foot Candle (fc or ftc), Lumen (lm) and Lux (lx) and there are many parameters in the light. Let's study about these.

What is a Light Meter?

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A light meter is an instrument to measure the intensity of light, used to know how much proper the exposure is while taking a photograph. To achieve correct film or sensor exposure we just need to know how much light falls on the camera lens. To do so we use this meter. Basically light meters are of two types :
  • Reflected light meter
  • Incident light meter
Reflected Light Meter
Reflected Light Meter

How to Measure Light?

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A light source is that what makes us to see. There are basically so many things that are while doing the light measurement. Some of them are
  • Luminous Flux
  • Luminous intensity
  • Illuminance
The power radiated in watts is the luminous flux. It is expressed in Lumens. The amount of light produced by any source of light is called the luminous intensity of that source. The SI unit for the luminous intensity is called the Candelas (cd). Illuminance or Intensity of illumination (E) of a surface is the luminous flux incident normally on unit area of the surface. The unit used here is a foot candle (fc).

Light Measurement Units

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There are several units in light each measuring a specific term related to light. some standard units are
  • Candela
  • Foot candle
  • Lumen
  • Lux
To measure the luminous intensity we go for candela and to measure the light intensity we go for foot candle. A foot candle is the amount of light falling on a l-square-foot surface that is located 1 foot away from one candle. The direct sunlight entering a window on a clear summer day may be as high as 8000 foot candles.To measure the luminous flux we go for Lumens. Illuminance is measured in Lux.

Light Measurement Device

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There are two light measurement devices basically used
  • Photometer
  • Radiometer
Photometer is a device that measures only Visible wavelengths but Radiometer measures all the wavelengths that carry both visible and invisible spectrum of the light.