How to measure light

There are many devices available to measure light. It’s important to use the correct device in order to be able to draw correct conclusions.

LUX METER

Most lamps are built for the human eye. One of the units to express the amount of light visible to the human eye is Lux. 1 lux=1 lumen/m² and lumen is the total amount of light coming from a light source. Lumen takes into account the sensitivity of the human eye by giving more weight to the frequencies most visible to the eye like green (550nm) and less for blue (shorter wavelength) and red (longer wavelength).

Plants have a completely different sensitivity to light than the human eye and they are most sensitive to the colors where the human eye is the least sensitive (blue and red). Therefore, it is not a good idea to use a Luxmeter for horticultural purposes.

PAR METER

A PAR meter measures the light within the 400-700nm range. This is the photosynthetic active radiation (=PAR) range. As opposed to a LUX meter, the PAR meter does not take into account the sensitivity of the human eye. The PAR meter measures the number of photons on a given surface during a given time. It is expressed in μmol/m²/s. This device measures thus the amount of light that a plant can use for photosynthesis and it’s the only relevant value regarding plant growth.

SPECTRORADIOMETER

A spectroradiometer measures the spectral distribution of a source of electromagnetic radiation (like light). It does not only give you information about the total amount of light (like the PAR meter), but also about the amount of light of each color. It’s range is much wider than the PAR meter and is typically between 200 and 1.000nm.

These devices are very expensive and are mostly used for R&D purposes.