HORTICULTURE LIGHTING METRICS PAR PPF PPFD DLI PPF/W

July 18, 2021
Posted by Harry T

Lux and Lumens are misleading for Horticulture. These metrics for General Lighting are weighted according to human eye's sensitivity to various wavelengths. Human eyes are most sensitive to 555nm green light and not as sensitive to Blue and Red light. Lux/Lumens fail to correctly account for Blue and Red wavelengths, which are important light wavelengths for plants. These metrics can be used to compare two Horticulture Lamps only if the light spectrum is identical.

Horticulture specific metrics PPF PPFD are appropriate metrics when comparing different Horticulture Lamps. These cover the entire range of light that is used by plants for photosynthesis.

Metric for General Lighting	Metric for Horticulture
Lumens	PPF
LUX	PPFD
Lumens/Watt	PPF/W

What is PAR or PPFD

PAR (PHOTOSYNTHETICALLY ACTIVE RADIATION)

Photosynthetically Active Radiation is the spectral range (wave band) of light radiation from 400nm to 700nm wavelength. Photosynthetic organisms use light within this range for the process of photosynthesis. PAR is a range of wavelengths and is not a metric with any units.

PPF (PHOTOSYNTHETIC PHOTON FLUX)

PPF is the total amount of photons within the PAR range that are is produced by a light source each second. Most common unit is μmol/second. Each μmol (Micro Moles) means 6.02214076×10¹⁷ photons. PPF can be measured by use of a Integrating Sphere, which encloses the entire lamp in a reflective sphere or a Goniophotometer, which moves around the lamp to measure light at different angles. PPF is a metric gives total Photosynthetic Photon Flux output of a lamp.

PPFD (PHOTOSYNTHETIC PHOTON FLUX DENSITY)

PPFD is a measurement of the amount of Photosynthetic Photon Flux per unit area. It number of photosynthetically active photons that fall on a given surface each second”. PPFD is measured at a specific position of plant canopy and is measured in micro moles per square meter per second or μmol/m2/s. This metric is commonly used in PPFD Charts like below. HLG 550 V2 PPFD Chart

DLI (DAILY LIGHT INTEGRAL)

Daily light integral (DLI) describes the number of photosynthetically active photons that are delivered to a specific area over a day. PPFD gives us the intensity your plants are receiving per square meter per second. DLI gives us the intensity your plants are receiving per square meter per day. DLI with a lamp running 12 hours a day is exactly half of lamp running 24 hours a day. Similarly DLI with a lamp running 12 hours a day is 50% less than a lamp running 18 hours a day.

PPF/W (PHOTOSYNTHETIC PHOTON FLUX PER WATT)

PPF/W is one of the most important metrics for efficiency of a light source. It is the total PPF produced by each watt of electrical power consumed. It is also same as micro moles per joules of energy used. While comparing PPF/W for different lamps it is important to ensure PPF/W number is calculated at system level since some electrical power is lost in power supply losses and electrical wires/traces.

PPF/Watt is same as μmol/j

1 Watts =1.00 J/s ( joule per second )

PPF = μmol/s ( micro moles per second )

LIMITATIONS OF PHOTOSYNTHETIC PHOTON METRICS

PPF, PPFD, PPF/W metrics measure only photons within the PAR range. These metrics are limited to PAR range, however there are well known benefits of light outside PAR range. Also these metrics do not account for quality of light spectrum. It is important to look at spectrum along with these metrics for a complete picture.

A full spectrum lamp at 2.5 PPF/W will greatly outperform a 3 PPF/W lamp that has only only Red + Blue photons