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Light: Difference between revisions
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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.
=== PPFD ===
Photosynthetic Photon Flux Density: its unit is μmol/m
=== 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 [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.
=== 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/m
Full light intensity from the sun in summer reaches around 60 DLI<ref>https://www.youtube.com/watch?v=ID9rE5JewVg&t=1170s&ab_channel=ApogeeInstrumentsInc. - Bruce Bugee Grow lighting myths</ref>
==Types of lighting ==
There are several light technologies that have differing efficiencies, spectrums and form factors.
===CFL - Compact fluorescent lamp===
===HID - High-density discharge===
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<ref>Karuna Chourey
https://www.youtube.com/watch?v=u8XNN4V7qsU</ref>. It is a good idea to match the light schedule with the cultivar's [[Plant_processes#Circadian_rhythm|circadian rhythm]]; Many growers recommend 20/4 or 22/2.
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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 the rate of [[Plant processes#Transpiration|transpiration]], meaning higher light intensity can raise the [[Temperature and Humidity|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===
=== 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 trichomes.<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.
==== UV A (315-400 nm) ====
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