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Light quantity

PAR Sensors:

The most light sensors use the photoelectric effect where electrons are emmited when they absorn energy from light producing electrical current. Several types of photodiode are suitable for measurement of the photosynthtically active radiation (PAR, 400-700 nm) and a Si photodiode is the most commonly used type for the measurement of PAR. To construct your own low-cost PAR Sensors see (Fielder P and Comeau P 2000).

Radiation Sensors

Out of total incident solar irradiance only less than 50% is PAR and often the entire incoming radiation is measured e.g. in meteorology.

- Pyranometers measure specifically the broad solar irradiance. These sensors are composed of a thermopile (a sensor with black coating that absorbs the entire radiation (300-50000nm)) and a glass dome with field of view of 180° and spectral response between 300 and 2800 nm. Depending on the light source

- Pyrgeometers measure the infrared radiation beyond 4.5 μm, as a pyranometer they are composed of a thermopile and silicon dome that transmits only thermal (long wave) radiation beyond 4.5 μm.

Approximate conversion factors for various light sources (from Principles of Radiation Measurements, www.licor.com).


Light quality


Spectroradiometers measure the irradiance at different wavelengths (spectral radiance: radiance as a function of wave length) which is distinct from the measurements of quanta such as “photon counting” with PAR sensors (unit of spectral radiance: W sr-1 m-2 mm-1).

Red/Far-Red Light Meter

These sensors are light sensors usually use filters around 660 nm (red) and 730 nm (far-red) the irradiance

Further reading

Fielder P and Comeau P (2000) Construction and Testing of an Inexpensive PAR Sensor. Crown Publications, Victoria