Light: Difference between revisions

In ideal conditions, Cannabiscannabis can absorb 1500 - 2000 umolsμmol/s. For context, in California, the sun can emit up to 2000 μmol/s around midday. This means that the light needsshould toideally 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. SlowlyTo avoid this, slowly increase the intensity of your light over several days, by increasing power draw by 5-10 watts aper day, and observeobserving the leaves regularly. Any yellowing leaves can be a sign of light stress,. thisThis may be accompanied by curling leaves which isare a sign of heat stress, often fromcaused by lights being too intense. Note also that seedlings require less light than flowering plants.

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) [https://en.wikipedia.org/wiki/Mole_(unit) 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.

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 [https://www.samsung.com/led/lighting/mid-power-leds/3030-leds/lm301h/ 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.

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 growinggrow light,. forFor 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 afour largertimes the area, so it can support a larger plant. In this case, light B has a greater PPFD.

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.

LED drivers comecan inbe 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.

The colour of light is dependant on the frequency of the light. see also [https://en.wikipedia.org/wiki/Electromagnetic_spectrum Electromagnetic spectrum]. Blue/purple light used to be standard in Cannabis growing but are slowingslowly being replaced by full-spectrum lights. Throughout the plant's life cycle, it can be helpful to apply different lightranges spectrumsof arethe morelight importantspectrum.

=== lowHigh frequency light (below visible spectrum e.g Blue) ===

UV light rays can damage plants and stress them, intotriggering creatingthe creation of more trichromes<ref>What light spectrum does weed need to grow? - Growing Cannabis 201: Advanced Grow Tips | Indica Institute - https://www.youtube.com/watch?v=NY0bB_Jmt5E</ref><ref>Pate, David W. (1983). "Possible role of ultraviolet radiation in evolution of Cannabis chemotypes" doi:10.1007/BF02904200</ref> UV lights specifically for growing are available but it is also possible to re-purpose a UV light intended for a reptile enclosure.

=== HighLow frequency light ===

Common light schedules areinclude 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 makesis sensea good idea to givematch the plantlight someschedule darkwith period to try and matchthe itscultivar's circadian rhythm,; I recommend 20/4 or 22/2.