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

Types of lighting

HID

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)

UV light rays can damage plants and stress them into creating more trichromes

Far range and infra-red

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