Light

In ideal conditions, cannabis can absorb 1500 - 2000 μmol/s. For context, in California, the sun can emit up to 2000 μmol/s around midday. This means that the light should ideally be powerful enough to deliver this intensity over the area the canopy will occupy. Equipped with a powerful and efficient grow light, you may be tempted to use as much power as you can afford, to increase yield - but this is often not the best strategy; leaves can be damaged by overexposure to light (and heat) which will decrease yield. To avoid this, slowly increase the intensity of your light over several days, by increasing power draw by 5-10 watts per day, observing the leaves regularly. Any yellowing leaves can be a sign of light stress. This may be accompanied by curling leaves which are a sign of heat stress, often caused by lights being too intense. Note also that seedlings require less light than flowering plants.

Horticultural light metrics

There are many different metrics that can be used to measure the intensity and quality of light for photosynthesis in the cultivar.

μmol/s

The measure of photons per second produced by a light. Photons are measured in micro (10^-6) moles there are a huge amount of photons coming from any light source. One µmol is actually 602 quadrillion photons. Simply put, this metric is the intensity per second from a light.

μmol/J

The light intensity a horticultural light produces per Joule of power it uses. This is a measure of its efficiency. At the time of writing, the most efficient LED package available is the LM301H by Samsung. It achieves 3.10 μmol/J @ 65 mA, 25°C. Note the efficiency will change depending on the temperature and current supplied.

PPFD

Photosynthetic Photon Flux Density: its unit is μmol/m^2/s. It is a measure of light intensity over an area per second. This is important to note when buying a grow light. For example, light A may provide an intensity of 800 μmol/s in a 1m^2 area, while light B provides the same intensity in a 4m^2 area. Light B provides the same intensity but over four times the area, so it can support a larger plant. In this case, light B has a greater PPFD.

Horticulural light terms

PAR

PAR stands for Photosynthetic Active Radiation: light within the range of 400 to 700 nanometers that drives photosynthesis. For reference, the human eye can see light between 380 to 750 nm.

Types of lighting

There are several light technologies that have differing efficiencies, spectrums and form factors

LED - Light emitting diode

LED Drivers

LED drivers can be categorised into constant voltage and constant current, both with dimmable options. If your panel has a recommended voltage, it's best to select a constant voltage driver. Dimmable drivers are more expensive but worth it because your plant requires more light at the flowering stage than at the seedling stage, so you may wish to use less power initially to conserve power. Meanwell drivers are the industry standard for efficiency and lifespan.

Light spectrum

The colour of light is dependant on the frequency of the light. see also Electromagnetic spectrum. Blue/purple light used to be standard in Cannabis growing but are slowly being replaced by full-spectrum lights. Throughout the plant's life cycle, it can be helpful to apply different ranges of the light spectrum.

High frequency light (e.g Blue)

UV light rays can damage plants and stress them, triggering the creation of more trichromes UV lights specifically for growing are available but it is also possible to re-purpose a UV light intended for a reptile enclosure.

Low frequency light

Red light

Red light promotes stretching of the stem

Light schedules

Common light schedules include 24/0 (24 hours of light a day), 20/4 (20 hours light, 4 hours darkness), 18/6 (18 hours light, 6 hours darkness) and 12/12 (12 hours of light, 12 hours of darkness).

Auto flowers can be grown with lights on 24 hours a day (24/0), but constant light can leave the plant with no time to recover from deficiencies, or stress in general. It is a good idea to match the light schedule with the cultivar's circadian rhythm; I recommend 20/4 or 22/2.

More on circadian rhythm in plants

Plant responses to circadian rhythm