Measuring light in horticulture: PAR, PPF, and PPFD.
To choose your horticultural lighting, it is essential to understand and know the metrics of the subject.
14 November 2023
How is light measured in horticulture?
To choose your horticultural lighting, it is essential to understand and know the metrics of the subject. These will help you to achieve your production and growth objectives. You will cross paths with certain measures such as: Watt, Lumens, LUX, PAR, PPF and PPFD. They are related to lighting, but not all of them are useful for horticultural lighting.
The PAR is the Photosynthetic Active Radiation. The wavelengths that make up the PAR are between 400 and 700nm used for photosynthesis. Since the PAR is not a metric but an absorption model, several units are used to characterize light for plant production. Like lumens and lux, the greenhouse industry uses a set of units to best define lighting in its environment.
PPF: How many photons in the PAR band does my lighting deliver?
The PPF is the photosynthetic photon flux. It is a direct analogy to Lumen. The PPF is given in micromole per second (µmol/s). It is the amount of PAR photons emitted per second in the light beam. Although it does not tell us how much light reaches the plant, the PPF is valuable information for calculating the energy efficiency of horticultural lighting.
PPFD: How many PAR photons on a surface?
The PPFD is the flux density of photosynthetic photons. Directly analogous to lux, the PPFD is given in micromole per second per m² (µmol/s/m²). It allows to quantify on a given surface, at a given distance from the lighting, the number of PAR photons incidents per second. It is a vital measurement because it indicates the quantity of light available to the plant for its photosynthesis.
PPFD measurements taken at several locations on the illuminated surface are mapped. The objective is to have homogeneity over the entire growing surface to avoid disparities in plant growth depending on their placement.
It is crucial to obtain this PPFD mapping because unique PPFD measurements are misleading. This is because most lighting has a very high concentration of light at the centre of the surface and this intensity decreases rapidly with increasing distance from the surface.
Radiometric Efficiency – How many PAR photons are delivered per Watt consumed?
It is expressed in µmol/J and tells us about the ability of a lighting system to create light photons from the electrical energy supplied (PPF/Watt). Be careful, this efficiency does not take into account the action on the plant. Therefore an unsuitable spectrum with a high efficiency will give lower results than a balanced but less efficient spectrum.
As a guarantee of quality, Rouge Engineered Designs tests and publishes all its PPFD measurements through an independent and COFRAQ accredited photonics laboratory.