Light is a term referred to as the part of solar radiation visible to the human eye. This part is only a small part of the total solar radiant power. From the electromagnetic spectrum includes ultraviolet, infrared, X-ray, microwave and radio waves, some rays do not even reach the surface of the earth.
The light we see is the smallest part of electromagnetic radiation from 380 to 780 nanometers (nm). There are several methods of measuring light, such as lux and lumens etc.As we all know, the perception of light by plants is different from that of humans.So,photosynthetic active radiation(PAR) is the most important indicator in the industry.
Photosynthetic Active Radiation (PAR)
PAR is the part of light that is useful for plant growth. PAR refers to the light wavelength range of 400 to 700 nm. PAR is not a unit of measurement because it only represent the light between 400-700nm that plant photosynthesis. However, there is a flaw in the concept of photosynthetically active radiation: wavelengths below 400 nm (especially UV-A and UV-B) and above 700 nm (far red) are excluded from the PAR measurement. Photosynthetically active radiation assumes that all photons outside this area are not used for photosynthesis. However, plants also use light outside of PAR, such as infrared light.There are two units to measure PAR:
One is irradiance (w/m2), which is mainly used for the general photosynthesis of sunlight.
Other is photosynthetic photon flux density PPFD (umol/m2s), which is mainly used for the plant photosynthesis by artificial light sources and sunlight.
Photosynthetic photon flux (PPF)
PPF is a measure of the total PAR light emitted by the growth light,the unit is µmol/s. One micromole refers to 602,214,150,000,000,000 photons. PPF is a very important metric, which can tell us how many photons the light source emits per second. However, PPF is not represent for how many photons reach on the plant, it is only the total output.
Photosynthetic photon flux density (PPFD)
PPFD is a measure of the emission of photons on one square meter per second, and the unit is µmol/m 2/s.PPFD is the most important parameter for growers because it shows the average light level in your growing area. PPFD can show how many photons reach on the plant, but a single PPFD measurement will not provide you the number of entire growing areas.
The PPFD also will vary depending on the size of the growth light. Each growth light may have an incredible PPFD value directly below the light. Therefore, it is necessary to measure the PPFD in the growth area of different locations to calculate the average value, which will provide you reliable result. PPFD measurement also depends on the distance of the light source. In other word, the longer the distance of the growth light, the wider the coverage, but the lower the intensity. Therefore, it is important to know from where was measured. Measuring the PPFD over the entire illuminated area and at different distances above the canopy is a useful comparison between grow lights be made.
Relative quantum efficiency
Having a good PPFD does not mean that we have provided the best condition for plant growing. In fact, PPF/PPFD cannot provide the relative intensity of specific wavelengths. Two famous scientists (McCree/Inada) compiled the best growth spectra, which shows the effect of light quality on photosynthesis. The relative quantum efficiency curve is also called YPF curve or McCree curve, and is widely used to evaluate the effect of light quality on photosynthesis.
The curve shows that orange and red photons between 600-630 nm have 25% more photosynthesis than blue photons between 400-540 nm.
Yield Photon Flux(YPF)
YPF refers to the number of micromoles of photons radiated per second in the wavelength range emitted by the artificial light source, the unit is umol/s. It is considered wavelength range is 360-760nm. For LED light, the wavelength range is 380-800nm.
Photosynthetic Photon Flux Density(YPFD)
Corresponding to YPF is the number of micromoles of radiation per square meter per second, unit umol/m2s, which is the concept of density.
Daily Light Integral (DLI)
The Daily Light Integral (DLI) measures the total amount of light that is delivered to a plant every day.
DLI is a cumulative measurement of the total number of photons that reach the plants and algae during the daily photoperiod.
The DLI measures the number of “moles” of photons in the Par region per square meter per day and is expressed as mol/d.m².
The DLI is a good way to implement in the light strategy in greenhouse project with supplemental lighting.
For most crops you can define what is the ideal total light sum per day they can efficiently use.
The total light sum is the sum of the light perceived from the sun + the sum of the artificial light per day.
Of course your climate computer doesn’t tell you much about mols per day.
Therefore you have to calculate back from the solarimeter values in J/cm² to mols to make a total sum per day insight.
Keep in mind the solarimeter is on the roof, so you have to deduct the transmittance of the greenhouse glass from this value.
From solarimeter to DLI from the sun – J/cm² to mol/dm² conversion:
DLI from the sun = ((measured J/cm²)/100) x 2.15 x glass transmittance %
What the plants receive extra from the grow lights can also be calculated – therefore you convert your PPFD light level to DLI with the numbers of lighted hours
From PPFD to DLI conversion – μmol/sm² to mol/dm²:
DLI from the grow lights = (hours x PPFD x3600)/1.000.000
How to measure the PAR, PPF and PPFD of grow light?
You may have known these concepts such as lumens and lux, but can these parameters correctly evaluate the quality of grow lights? How to evaluate the quality of grow lights? Through the above introduction of PAR, PPF and PPFD, you may believe that these parameters are more important for grow lights,is there a easy tool to measure these parameters?Yes,that is spectral illuminance meter.
The spectral illuminance meter can measure PPF, PPFD etc ,which can display accurate measurement results on the display screen within a few seconds for rapid evaluation.
The spectral illuminance meter is used in high-end laboratories, research and development and quality control, providing lighting professionals with a simple and reliable spectral irradiance measuring instrument for the industry.