5.2 Units of radiation and light
The radiation in the range from 400 to 700 nm is called photosynthetically active
radiation(PAR)for plants. The photo reactive part of photosynthesis is considered directly
proportional the amount of photons in the light, which has been absorbed by the plant
in accordance with the chlorophyll absorptance curve. This is the relationship of photons
absorbed as a function of wavelength, and it has two peaks in the blue and red regions. A
mole of photons is 6.02x10‘-0’ photons, with each photon having energy proportional to its
frequency. The photon energy of green light whose wavelength is 500 nm is approximately
2.35x105 J/mol photon.
Light, the visible art of radiation perceived by the human eye, is measured by an
instrument which has a filter of the range from 400 to 700 nm and a sharp peak in the green
region. The amount of illumination is expressed in a unit called lux. Plants have
different absorption characteristics than the human eye, and lux cannot be correctly used as
a measure of the light for a plant. Light measured by lux meters will exaggerate the green
light available. Radiometers which have a linear filter whose transmissivity is slightly larger
in red than the blue are also used to measure PAR. If radiation is measured beyond the visible region, an energy unit such as used. If an