Light : units and measurement

Light – Notes on physical and subjective units of measure

Physical Units Subjective Units
  • Light is a form of energy, and can be measured in energy units (joules, calories) or in quantum units (quanta, einsteins). Conversion between these units is wavelength dependent.
  • 6 x 1023 quanta = 1 mole of light (or 1einstein in old terminology)
  • Light can be measured subjectively, based on the brightness seen by the human eye. Units include candles, lumens, footcandles and lux.
  • A source of light has a luminosity of one candle if its output appears to match that of a “standard candle”.
  • Power is the rate at which light is generated, transmitted or absorbed, and is measured in watts (1 watt = 1 joule sec-1) or einsteins sec-1
  • Luminous flux is equivalent to power. A source of one candle gives off a luminous flux of one candle power or 4π lumens.
Intensity of a beam of light is defined as the power per unit cross section, and is measured in watts rn-2 or einsteins m-2sec-1

  • 6 x 1017 quanta m-2sec-1 =1 microeinstein (mE) m-2 sec-1
  • Light intensity is measured for example with a LICOR light meter or a QSL (quantum scalar irradiance) meter. (the QSL type is considered more accurate because of its spherical rather than flat plate collector).
  • Luminous intensity

The intensity at a distance of:

-one foot from a standard candle is one footcandle

-one metre from a standard candle is one metre candle or one lux.

NB. 1 footcandle = 10.8 lux

  • Luminous intensity is measured with eg. a foot-candle meter, a type of photographic exposure meter.
  • These units are preferred for most purposes in photo-chemistry and photobiology. They are used for measuring environmental light intensities eg. in limnology and oceanography.

Useful range for micro-algal culture

20-200 μmol. photons m-2 s –1

Direct sunlight ( midday in tropics) is approximately 1700 μmol. photons m-2 s –1

  • These units are based on human vision and are therefore useful in specifying safe and comfortable levels of illumination eg. schools and offices.

NB. In other contexts, these units can be misleading. For example, certain photosynthetic bacteria, utilizing infrared light, will not grow under bright fluorescent lamps which are deficient in infrared; however, tungsten lamps of dimmer appearance, but rich in infrared emission, will support much better growth.

Note: Conversion of physical units to subjective units depends on the relative visibility of different wavelengths, ie certain wavelengths are “more visible” to the human eye than others. Therefore as the conversion between lux and μmol. photons m-2 s –1 is wavelength dependent conversions should be approximated only;

eg; X μmol. photons m-2 s –1 = Lux x ~0.0165 …or…1000 Lux = 16-20 μmol. photons m-2 s –1



Clayton, R. K. (1970). Light and Living Matter. Volume 1. The Physical Part. McGraw-Hill Book Company, New York .

Hershey, D. R. 1991. Plant Light Measurement & Calculations. The American Biology Teacher 53:351-53.

Morel, A. and Smith, R. C. 1974. Relation between total quanta and total energy for aquatic photosynthesis. Limnol. Occanogr 19:591-600.