…PAR, PPF, PPFD and DLI. Plants use light energy in the photosynthesis process, which, in a nutshell, converts the light energy into chemical energy, consumed by the plants for growth or fruiting. This process is made possible by two types of chlorophyll, present in A and B plants. The chart below shows that chlorophyll uses two PAR ranges: blue (435-450nm) and red (640-665nm).
The key issue, in addition to selecting the appropriate wavelength, is a right proportion of red and blue light. Of course, other lengths spectrum of PAR are also used by plants, but the photosynthesis process occurs most intensively in the blue or red light.
PAR (Photosynthetically Active Radiation) designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more or less with the range of light visible to the human eye. Photons at shorter wavelengths tend to be so energetic that they can be damaging to cells and tissues, but are mostly filtered out by the ozone layer in the stratosphere. Photons at longer wavelengths do not carry enough energy to allow photosynthesis to take place.
PPF (Photosynthetic Photon Flux) – measurement specifying the total amount of light produced by the light source within each second, in other words PPF tells us how much of photosynthetically active radiation is emitted by the light source in a second, measured in umol/s (micromoles per second). It is the second most important factor in determining the effectiveness of the lighting system for the plants. Most manufacturers of the LED panels do not mention this value in the technical specifications of their products.
PPFD (Photosynthetic Photon Flux Density) – measurement specifying the amount of light that actually reaches our plant. PPFD determines the number of photosynthetically active photons presence on the defined surface per second, measured in mol/m2/s (micromoles per square meter per second). PPFD is the measurement made on specific points on the surface of the plant (or any other area).
DLI (Daily Light Integral) is the amount of PAR received each day as a function of light intensity (instantaneous light: μmol·m−2·s−1) and duration (day). It is expressed as moles of light (mol) per square meter (m−2) per day (d−1), or: mol·m−2·d−1 (moles per day). The daily light integral (DLI) is a measure of the amount of light received in a single day in a particular area. For example, the number of moles of Photosynthetically Active Radiation (PAR) photons received during a single day in a square meter, or: mol·m−2·d−1. Another example is the total solar radiation received during a single day in a square meter, or W·m−2. Depending on the application or research question the time integral of incident light may be a more useful tool than the instantaneous incident light. DLI varies depending on latitude, time of year, and cloud cover, and ranges from 5-60 mol·m−2·d−1 in the United States for PAR. In greenhouses or growth chambers, DLI values are typically much lower, which can affect the shoot/root ratio, morphology, and the timing of flowering.