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
-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 ( |
· 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. |
eg; X μmol. photons m-2 s –1
= Lux x ~0.0165 …or...1000 Lux = 16-20 μmol. photons m-2 s –1
References
Clayton, R. K. (1970). Light and Living Matter. Volume 1. The Physical
Part. McGraw-Hill Book Company,
Hershey, D. R. 1991. Plant Light Measurement & Calculations. The American Biology Teacher 53:351-53.