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 source 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.
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.
Photosynthetic Photon Flux Density: its unit is μmol/m2/s. It is a measure of light intensity within the PAR range over an area per second. The maximum PPFD a grow light can produce is important to know before buying as well as the size of the area it can deliver that PPFD.
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.
DLI (Daily light Intergral)
The amount of light energy (photons) a plant is receiving every 24 hours. For example, if you used a light with a PPFD of 500μmol/m2/s on a 20/4 lighting schedule that would result in a DLI of 43.20 mols. DLI is generally the most useful metric of light intensity the plant is exposed to as it takes the light schedule into account.
Full light intensity from the sun in summer reaches around 60 DLI
Horticulural light terms
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.
CFL - Compact fluorescent lamp
HID - High-density discharge
LED - Light emitting diode
LEDs are the most compact and efficient light source but can be more costly than other means and require a driver.
LED drivers can be categorized 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.
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. The absence of a dark period can also stave the plant's rhizosphere, negatively affecting growth. It is a good idea to match the light schedule with the cultivar's circadian rhythm; Many growers recommend 20/4 or 22/2.
Ideal light conditions
In the initial stages of growth when the plant is a seedling, it should be exposed to much less light than when in flowering. This is for photoacclimation.
|Seedling||100 - 400 μmol/s|
|Flowering||600 - 2000 μmol/s|
Once a plant moves into the vegetative stage the maximum light intensity it can tolerate will be limited by other conditions such as CO2 level or nutrient deficiency. Note that light intensity is proportional to the rate of transpiration, meaning higher light intensity can raise the humidity of the environment as more water exits the plant. This creates another potential limitation to the maximum light that can be applied in the grower's ability to control the environment.
Optimum light intensity
In this instance, "optimum light intensity" refers to the amount of light that provides the maximum yield from the cultivar.
The threshold for optimal growth and photosynthesis, from the vegetative stage, is DLI of 65 moles.
Yield appears to be linear with ppfd to around 1500 - 2000 μmol/s 
Temperature and light optima for photosynthesis were concluded to be at 25–30 °C and ∼1500 μmol m2/s respectively
Upper limit of light intensity
The theoretical upper limit that a plant can tolerate before experiencing symptoms of poor health.
The Maximum ppfd cannabis can tolerate is around 2000 - 2500 μmol/s in perfect conditions and a DLI of around 40 - 60 moles. Above 500 μmol/s supplemental co2 is required, without it more intense light level will harm the plant.
Meta-analysis of plant morphology to light intensity.
Measuring light intensity
The amount of photosynthetically reactive light (PAR) present can be measured using a dedicated device known as a Quantum sensor. They are very accurate, however, as most smartphones have a lux meter, a much cheaper alternative is to use a "PPFD meter" app. A lux value can be combined with the grow light's spectral power distribution (SPD) to calculate the amount of PAR radiation. This is necessary because lux meters measure light in the frequency relevant to human vision and not photosynthesis.
Simply put, these apps use the phone's brightness sensor to find a lux value and ask the user to input the frequency of the grow light they are using. Often 3500K or 4000K for "full-spectrum lights", Samsung produces a number of LEDs in this range.
Some examples are PPFD Meter on Android or Photone on iPhone (Better implementations may exist).
A competant online implementation exists at waveformlighting.
The color of light is dependent on the frequency of the light. see also Electromagnetic spectrum. Blue/purple light used to be standard in Cannabis growing but is slowly being replaced by "full-spectrum" lights. The light spectrum applied to a plant can affect its morphology and cannabinoid content. 
High frequency light
Approximately 3% of the light radiation from the sun is ultraviolet; in the range of 100-400 nm. The light is the ratio of 1:30 UVB to UVA, and all UVC is absorbed by the atmosphere, particularly the Ozone layer. UV light rays can damage plants and stress them, triggering the creation of more cannabinoids such as THC, CBD, and CBG . It is common to find LED boards that include UV LEDs.
UV A (315-400 nm)
UV B (280-315 nm)
UV C (100-280 nm)
Low frequency light
Low-frequency light is constituted of red light in the visible spectrum, "far range" and infra-red.
Red light promotes cell elongation in plants, in cannabis, this manifests as stretching of the stem. While not visible to the human eye, far-red light is also photosynthetically active
Adding far-red light can increase cannabis yield by an average of 20%
see also: How Lighting Affects Cannabis Cultivation - Cannabis Grow Lights
- ↑ https://www.youtube.com/watch?v=ID9rE5JewVg&t=1170s&ab_channel=ApogeeInstrumentsInc. - Bruce Bugee Grow lighting myths
- ↑ Karuna Chourey Ph.D. - Shango Los 2019 https://www.youtube.com/watch?v=u8XNN4V7qsU
- ↑ Daily Light Integral DLI Relation To Cannabis Yield (Meta-analysis, Matthew Debacco, 2021) https://www.youtube.com/watch?v=au7G-oVDeKg
- ↑ Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment (Rodriguez-Morrison, 2021) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8144505/
- ↑ High Light Intensities Can Be Used to Grow Healthy and Robust Cannabis Plants During the Vegetative Stage of Indoor Production (Moher, 2021) https://www.preprints.org/manuscript/202104.0417/v1
- ↑ Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions (Chandra, 2008) https://dx.doi.org/10.1007%2Fs12298-008-0027-x
- ↑ Light dependence of photosynthesis and water vapor exchange... (Chandra, 2015)- https://doi.org/10.1016/j.jarmap.2015.03.002
- ↑ Cannabis business times Oct, 2021 https://www.cannabisbusinesstimes.com/article/growing-under-high-light-intensities-lighting-report/
- ↑ Fluence Cannabis Cultivation Guide, 2020 - https://fluence.science/guides/cannabis-cultivation-guide/
- ↑ A meta-analysis of plant responses to light intensity for 70 traits ranging from molecules to whole plant performance (Poorter, 2019) https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.15754
- ↑ 11.0 11.1 Magagnini 2018, The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L -http://dx.doi.org/10.1159/000489030
- ↑ What light spectrum does weed need to grow? - Growing Cannabis 201: Advanced Grow Tips | Indica Institute - https://www.youtube.com/watch?v=NY0bB_Jmt5E
- ↑ Pate, David W. (1983). "Possible role of ultraviolet radiation in evolution of Cannabis chemotypes" doi:10.1007/BF02904200
- ↑ Dr. Bruce Bugbee (2019) - https://www.youtube.com/watch?v=sS7aAcacfgk&ab_channel=ApogeeInstrumentsInc.
- ↑ Fluence science - https://fluence.science/broad-spectrum-leds-cannabis/
- ↑ https://www.youtube.com/watch?v=tKzmx6XDOkE&t=235s&ab_channel=CannabisTech