Defining growth light

In horticulture (greenhouses) light is measured in different ways. This can cause confusion, especially when comparing the different units.

Global radiation (natural daylight) is in most cases measured in terms of energy (J or W) with a solar meter. This meter is in general positioned on the top of the greenhouse.The value of global radiation in J or W is important for climate and humidity control in the greenhouse.A part of daylight is growth light (± 45%) and the rest mainly heat. Depending on the position of the sun and the glazing material of the greenhouse, only 60% to 70% enters the greenhouse.

Growth light is a measure of potential plant growth (photosynthesis or assimilation rate). Plant growth (Photosynthesis) is determined by the quantity of small light particles (= photons or quantum) from the blue to red (400-700 nm) part of the spectrum. In scientific terms this light is called ‘Photosynthetic Photon Flux (PPF). The more popular term is ‘Growth Light’. A photon or quantum meter measures growth light in µmol (micromole) of photons.

Photosynthesis is determined by the amount of growth light (in µmol/s) absorbed by the leaves.

A measurement of growth light can best be conducted on plant height