Frequently asked questions about RapidGrowLED and the grow light industry.

What is PAR?

Plants are photoautotrophs – using a narrow band of light between 400nm -700nm as a source of energy to make food molecules from carbon dioxide and water.

This process is photosynthesis. The only type of light which enables photosynthesis is defined as Photosynthetically Active Radiation (PAR): the narrow band of light between 400nm – 700nm.

Light outside of this range is not photosynthetically active and will not generate greater biomass for those seeking improved yield. 

What is PPF?

A measurement of the total light (photons) emitted by a light source each second. 

PPF tells us how much PAR is emitted by a light source. Measured in “micromoles per second” 

and expressed as: µmoles/second

What is PPFD?

Before light can be effective in a photochemical reaction (whether photosynthetic or photomorphogenic), it must first be absorbed.

Photosynthetic Photon Flux Density (PPFD) is the measurement of PAR delivered to a specific area. It is expressed as micromole-per-meter squared-per second (µmol/m2/s).

This is the only measurement that informs us of the amount of light being delivered to a crop which enables photosynthesis.

What is DLI?

Day Light Integral (DLI). A cumulative measurement of the total amount of light (photons) that reach the target during the photoperiod. DLI is measured over a one meter square area in “moles per square meter per day” and expressed as mol/m2/d.


In photosynthesis (the light reactions) oxygen (O2), ATP, and NADPH are produced via cells called chloroplasts. These molecules are used in the Calvin cycle to convert carbon dioxide (CO2) and water (H2O) to glucose (C6H12O6). Glucose is a form of chemical energy and is used for growth, respiration (the opposite process of the calvin cycle, which produces a different form of energy to maintain cellular activities and keep the plant alive), or it can be stored as starch or similar compounds in fruit.


There are 4 growth stages in cannabis that have different spectrum requirements.

Vegetation – In Vegetation (VEG) stage, rapid, healthy overall plant and root growth is desired, and in general most growers desire maximum growth but with shorter compact plants with short inter-nodal spacing preferred.

Pre-flower – Pre-flower is the period from when the 12/12 flower cycle is first initiated, to roughly the end of the second week (in an 8-week flower), or until the small flowers are prevalent and the rapid growth stretch slows. Again, for most growers, the desire in this stage is to maximize SIZE, while limiting stretch.

Flower – The peak Flower period is generally from week 3-7 and is the time when the plant (stem / leaf) growth stops and all the plant energy focuses on flower production. Maximum flower matter size and good structure is generally the goal here.

Ripen or Finish – The Ripen period is generally from week 7 to finish (in an 8-week flower) where the Flower growth, (i.e. size) slows and plant energy refocuses on resin and terpene production. This is the period where the flower acquires a significant portion of it’s density, ie. resin content. This transition is not clearly defined, and some strains have big increases in resin production during this period, and others not as much.