There are a few terms that are important to know when considering what grow light to buy: PAR, PPF, PPFD and DLI. They are used often by grow light sellers and manufacturers, with some even describing their products using lumens. So what do they all mean and why are they important?
PAR refers to the spectral range of radiation that is utilised by plants to drive photosynthesis. This range is 400-700nm however recent research suggests that plant utillise light outside this range causing many companies to use an extended PAR range of 380-780nm. When grow lights are advertised as “full spectrum”, it means they emit wavelengths from the entire PAR range. PAR is not a measurement itself but is something you measure and the the amount of PAR that reaches your crops from your grow light is what you want to know before buying.
PPF is a way of quantifying PAR, it measures the total number of photons in the 400-700nm range a light source emits per second and is measured in micromoles per second (μmol/s). PPF is measured using a piece of equipment called an integrating sphere and is usually tested by a third party due to the high cost of integrating spheres.
Knowing the PPF allows you to find out how efficient a light is, ie. how many photons in the PAR range are emitted per unit of energy the light draws. This is done by dividing the PPF by the wattage of the light to get a value for PPF efficiency measured in μmol/joule (since watts are measured in joules per second) and the higher the value, the better the efficiency.
PPF and efficiency are really important metrics that you should know before buying a grow light. A light may look impressive with a high wattage but this doesn’t always equate to better growth. You could end up throwing money away on an expensive lighting system and all the power it consumes for results that could be achieved with a less powerful but more efficient system that costs less.
PPFD is the total number of photons in the PAR range that hit a square meter per second from a light source measured in μmol/m2s (alternatively written μmol m-2 s-1). It determines the amount of PAR light reaching the crop canopy from a light source by using a quantum sensor that measures PPFD such as the Apogee SQ-520. These sensors measure the amount of PAR light reaching at a certain point each second and use this reading to approximate for a square meter. This approximation works well for a fully homogenous light souce like sunlight however for artificial lights, many readings should be taken over a square meter to get an accurate average PPFD measurement as results will be at a maximum directly under the light then will decrease as the sensor is moved away. Of course these sensors can be costly to consumers, so companies often do their own tests on their lights and produce a PPFD map.
A PPFD map is a grid that shows the PPFD value measured at multiple points over a certain surface area at a specified distance from the light source. For example a company may hang their light 60cm (2’’) from a 90cm x 90cm (3’’ x 3’’) surface area and take 100 measurements (10cm apart). These maps can be useful when buying a grow light however they can also be misleading as it is very easy to fabricate good results, for instance, by only measuring over a small area directly underneath the light or measuring in a reflective environment. Because there is no standard for producing PPFD maps, they are not the best way to compare the light output of two different branded grow lights.
DLI is the number of photons in PAR range that are delivered to a square meter over the course of a day. It is often calculated by measuring the PPFD throughout the day then using this data to estimate the DLI. This is an important metric to know as different types of plants require varied DLI for optimum growth. Growers in greenhouses often calculate the DLI they receive from the sun (which varies due to many factors including, latitude, seasons, weather conditions and greenhouse transmission efficiency) so that they can provide the correct amount of DLI from their grow lights to meet their crops needs. Indoor growers however rely solely on grow lights to provide the required DLI, which is why the lights are placed much closer to the canopy than they are in greenhouses. It’s worth noting that some plant species require a certain length of darkness over a day to trigger stages such vegetative growth and flowering.
Yield photon flux is another way of quantifying PAR. It differs from PPF in that PPF weights all wavelengths equally, whereas YPF is weighted to reflect how photons absorbed by plants yield different amounts of photosynthesis depending on their wavelength (350-750nm). This weighting is based on a research paper written by K. J. McCree who produced a graph that demonstrated how comparably effective different wavelengths were at driving photosynthesis once they had been absorbed (Photosynthetic Response Graph*). There are criticisms on how this graph is used by lighting companies (see more) and of how accurately YPF can be measured, so many companies do not calculate the YPF.
*graph made with data from McCree's 1972 paper: "The action spectrum, absorptance and quantum yield of photosynthesis in crop plants"
Luminous flux measures the total quantity of visible light emitted by a source, lumens are the unit of luminous flux and lux is the unit of luminous flux per unit area. These measurements are used for indoor and human centric lighting as they essentially measure how bright a light is, but should never be used for horticultural lighting. The reason being is that the human eye perceives certain wavelengths in the visible spectrum to be brighter than others and luminous flux is weighted to reflect this using a luminosity function, causing a bias toward green light emissions (see graph below). Since plants are known to also use red and blue light for photosynthesis, luminous flux is not a good representation of how well a grow light will work.