PAR is a measurement of Photo-synthetically Active Radiation within the 400nm-700nm range, this is the range of light plants use to photosynthesize. Plants utilise this range of light, the light reacts with Chlorophyll a and b and converts light into energy to grow and thrive. 

Photosynthetic Photon Flux Density is measured to determine the amount of micromoles per meter per second µmol/m²/s

Using this figure and some further information we can determine information like par maps and DLI 

 

PPF - (Photsynthetic photon flux) is the amount of light which is in the range of 400nm -700nm that is being produced by a lighting source per second. measured in the umol / second. We use this figure and the power of the light to determine the Umol/J rating. You need an advanced spectrometer called and integral sphere to measure PPF, the light source is fixed into place in a perfect sphere, sealed and tested, the reflection from inside the sphere gives a accurate reading of the photosynthetic photon flux. (ppf) 

PPFD - (Photo synthetic photon flux density) PPFD is a measurement of how many photo synthetically active photons reach your canopy. PPFD is important to understand so that you are giving the right amount of light to your plants. It is measure in μmol/s.m². Between 700 - 900 μmol/s.m² is the ideal amount of light for crops which require high light intensity such as flowering crops.

YPF - (Yield Photon Flux) is measurement where photons are weighted in the 360nm-760nm range, it goes deeper than PAR by extrapolating the photons to the plant sensitivity curve per crop. If you have an exact spectrum produced by your grow light you can alter the umol / second values by adding additional colors such as 450nm, 660nm or even 730nm

DLI - (Daily Light Integral) This is the amount of light which a plant can receive and process in a day, it is a combined total of the the amount of photons (umol/s) which reach the plant in a day.  The total number of umol is measured as a mole.( mol/d.m² )

Utilizing DLI can be beneficial in making sure your plants get the right amount of light each day, working out exactly what ppfd you need at canopy level will give you the information you need to determine what your daily DLI is. 

Using the calculation

PPFD x Hours of light x 60 x 60 / 1000,000

For example for a DLI for light intense flowering crop you should be aiming for the region of a DLI of 40. Using the calculation above we can determine the DLI. 

Lets say we have a ppfd measurement off 950 umol/m²/s

950 x 12 hours x 60 minutes x 60 seconds / 1000,000 = DLI 41.04

So you can see by the calculation above that being in the area of 900-950 is going to be the perfect par for flowering crops. 

Another example would be for vegetative growth stages, in this stage a lower DLI is required of around 15 -20  If we start with a PPFD of 400 umol/s.m².

400 x 16 hours x 60 minutes x 60 seconds / 1000,000 = DLI  23.04

By using DLI it give you an effective way to measure the exact amount of light your plants will receive daily. 

As a guide, Flowering Crops require 20-40 mol/m²/d and vegetative stage required 15- 20 mol/m²/d