PPF is a way of quantifying PAR, measuring the total number of photons in the 400-700nm range a light source emits per second, known as micromoles per second (μmol/s). PPF is measured using a piece of equipment called an integrating sphere. However, this is usually tested by a third party due to the expense of integrating spheres.
Knowing the PPF allows growers to find out how efficient a grow light is, i.e. 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. This is 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 important metrics that growers 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. Growers risk wasting money 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.
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 a specific point each second and use this reading to approximate for a square meter.
This approximation works well for a fully homogenous light source 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 and 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 the 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 from the sun, which varies due to factors including latitude, seasons, weather conditions, and greenhouse transmission efficiency. Greenhouse growers use this calculation to then 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 is worth noting that some plant species require a certain period of darkness over a day to trigger specific 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 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 grow 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.
