Light

In ideal conditions, Cannabis can absorb 1500 - 2000 umols/s. This means that the light needs to 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. Slowly increase the intensity of your light over several days by increasing power draw by 5-10 watts a day and observe the leaves. Any yellowing leaves can be a sign of light stress, this may be accompanied by curling leaves which is a sign of heat stress, often from 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 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 growing 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 a larger 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 come in 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 slowing being replaced full-spectrum lights. Throughout the plant's life cycle different light spectrums are more important

low frequency light (below visible spectrum e.g Blue)

UV light rays can damage plants and stress them into creating 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

High frequency light

Red light

Red light promotes stretching of the stem

Light schedules

Common light schedules are 24/0 (24 hours of light a day), 20/4 (20 hours light, 4 hours darkness), 18/6 (18 hours light, 6 hours 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 makes sense to give the plant some dark period to try and match its circadian rhythm, I recommend 20/4 or 22/2

More on circadian rhythm in plants

Plant responses to circadian rhythm