GIYGrowguru
Member
This is a very rough draft of a blog I plan on posting. I am hoping some new growers could read this over and give criticism. Is it easily digestible? Is it too complicated?
I would also like for more experienced growers to fact check me, If you notice something that is inaccurate please speak up! Thanks y'all.
Picking an LED light
What is is usable ppf?
PPF stands for Photosynthetic Photon flux density. This unit is used to measure the amount of light photons, Within the usable spectrum, that are emitted from a light source every second. This unit is expressed as umols/s. For obvious reasons, more photosynthetic radiation will result in faster growth and higher yield. (Usable PPF is just PPF - 30%) The general rule of thumb for lighting a grow space is to have at least 65 umols/s per sqft. This amount will help prevent the development of leafy, airy, flowers. For example, if I have a 4x4 tent, I know that there is an area of 16sqft within the tent. L x W =AREA. Now by using the rule of 65 umols per sqft, I will need a light that emits at least 1040 umols (usable).
65umols x sqft = desired PPF
How do you know the usable ppf of a light?
Now that you know how much light your tent needs, you can begin to look for lights that will be adequate. Most LED lights will include their efficacy in the description under PPE. PPE stands for Photosynthetic Photon efficacy and is the amount of photons emitted from the light per joule/watt. This unit is expressed as umols/joule.
(1 joule= 1 watt)
Now here's what gets most new growers, Chinese lights will often include a large number in their name. For example this light above is listed as a Spider Farmer SF-7000 and has a PPE of 2.9umol/J. Without knowing any better, many would assume that this is a 7000 watt light.. That unfortunately is not the case, this is a marketing tactic meant to lure in inexperienced growers.
This light above in fact only has a power draw of 650 watts, much less than than the 7000 included in the name. Always check the wattage before purchasing! To determine the Usable PPF of this light we use the equations below.
PPF = PPE x wattage
Usable PPF = PPF - (PPFx 0.30)
For example this light has a PPE of 2.9umols/j and a true wattage of 650. Now we plug these units into our equation.
PPF = 2.9umols x 650
PPF= 1885umols
To find the usable PPF we subtract 30%, this is to take in account light that is lost from reflecting off the walls of the tent.
Usable PPF = 1185 - (1185 x 0.30)
Usable PPF = 1319.5
Theoretically this light has the potential to adequately light a 5’ x 4’ tent, but PPF is only one of the things we look at. Next we will be determining the PPFD.
What is PPFD
PPFD stands for photosynthetic photon flux density and is a measurement of the amount of Photosynthetic radiation reaching the canopy. We use PPFD maps to determine the coverage of a light. Generally, the highest amount of photons will be reaching the center of the canopy, and will fade the farther you stray. When picking a grow light, we analyze this map and choose the light that will supply the most even coverage at the canopy level.
How much ppfd is needed
PPFD requirements will vary based on the maturity of your plant. Though, when picking a light we will be focusing on the amount needed for flowering as you can always either raise or dim light when needed. Generally we will be striving for around 800-1000 PPFD over as much of the canopy as possible. PPFD in this range will help maximize yield, anything under may produce more airy flowers and anything over will not likely produce any benefit that justifies the cost. (unless supplementing with C02)
Where to find PPFD map
Most LED grow lights will include a PPFD map in the description, If one is not included this is a huge red flag! These maps are normally grids that will include the dimensions of the tent it was tested in. Here is an example.
Here we have a light that was tested at different heights within the tent. The closer a light is closer to the canopy, the more concentrated the PPFD at the center will be. Raising the light will allow for a more even spread. As we can see this light will suffice in the 4x4 tent as the majority of the canopy will be receiving over 800 umols. Although, since we don't need over 1000 mols, we would want to raise this light a couple inches higher than 12” during the flower.
Now that you know how much light your space needs when flowering and how to analyze a grow light, you may begin searching for the one best fit for you.
