For something as simple as bringing light to your plants, lighting and lighting systems can quickly become a technical rabbit hole of terms and nuanced descriptions that may leave you feeling confused and overwhelmed. The more you learn, the more complicated it can become, but the essentials of lighting are simple; you have to be able to see through the marketing language. A light is simply a photon delivery system. How efficient that light operates mechanically and the spectrum it delivers are the only variables; how many photons per watt or µmol per watt is the metric to measure the total output. The quality of the light can be determined by the quality of its spectrum.
Below you’ll find a few key definitions of commonly-used lighting terms, as well as guides for properly identifying the right lighting for you as well as gauging if you’re getting enough lighting at canopy.
Common Lighting Terms & Units of Measurement
These are some of the most common terms and descriptions used to classify and categorize grow lights and lighting systems:
- Watts (w) :
A Watt is a unit of power that measures the rate of energy transferred. In traditional lighting systems, this typically reflected the input which was usually equal to the output of the light.
In LED systems, you may see “equivalent” watt ratings, where a product claims to produce the same output with less input, but the wattage is based on the input; 100W is 100W.
- PAR, PPF, and Photons
Photons - Massless particles of light that move at the speed of light and are received by plants as energy used in photosynthesis. More photons will increase yields, but most photons is not always the most beneficial for alternative goals.
Photosynthetic Active Radiation (PAR) - Refers to the wavelengths of light within the visible range (400-700 nanometers) that power photosynthesis. PAR is not a unit of measure but rather a type of light on the spectrum that supports photosynthesis. The amount and quality of light are the most important factors.
Photosynthetic Photon Flux (PPF) - A measure of the total PAR in a system that is produced per second. The higher the number, the better.
- µmol/micromol - A measurement of photons within the visible spectrum of a plant that trigger photosynthesis. The higher the measurement, the more photons present in the system.
- Lumens (lm) - If a light is rated in lumens (not µmol), then it is not for growing. A Lumen is a measure of the amount of visible (to human eye) light that a light emits over a period of time. As Gavita likes to say, “Lumens are for humans.” Lumens are a measure of the light that our eyes perceive, not how plants see light.
Red, Blue & Green Lighting
Within the visible range (PAR) of the lighting spectrum are a variety of colors, ranging in wavelength and frequency. Colors with longer wavelengths, such as red and orange, will measure heavier readings than those with lower wavelengths, such as blue and purple. Photons in the red spectrum and invisible blue spectrums (<450nm) can affect morphology and resin production, and affect when a photo period plant flowers, but these spectrums don't create photosynthesis. Photons outside of PAR can affect the plant’s morphogenesis but not photosynthesis.
For example, growers often use green lights (500-550nm) in dimmed intensity to work in the garden at night. Green light is visible to humans as it falls within the human visual spectrum, but is lower than more visible light such as yellow and orange. Green light is just still visible to plants, just less so than other wavelengths within PAR. Think of it like a very dim red light, which would also work (less visible), but would be hard for us to see any sort of detail in.
Lighting Spectrum & Grow Lights
The most common grow lights commercially are High Pressure Sodium (HPS), LEDs and Ceramic Metal Halides (CMH) and plasma. HPS lights are commonly used in conjunction with MH lights to get a balanced spectrum.
While an HPS may provide a lower Kelvin (K) output (more red), metal halide and LEDs are on the middle-higher end of the spectrum, producing 3000-6000K, as plants prefer higher Kelvin output (daylight) in vegetative states. Blue lighting can encourage more lush growth but does not deliver photons as efficiently as HPS. The primary goal of an HPS light is to produce as many photons as possible, as efficiently as possible. It is a tried and true commercial light because of its high efficiency, but, if the end game is true boutique, a preferred spectrum is closer to 4000K.
During veg, you need a maximum of half as much µmol as you do during flower. MH are often autonomously used in veg, and often in combination with HPS in flower, to cover both stages of growth.
µmol (Photons per Watt) Output by Light Type:
• HPS (Old Generation): 1800 µmol/watt
• Double-Ended HPS (Modern Generation): 2100 µmol/watt
• MH (Old Generation): 1300 µmol/watt
• CMH (Modern Generation): 1800 µmol/watt
• LED: 2000+ µmol/watt -
- Different diodes allow for a broader and higher quality spectrum and the precision of diodes in LED allows for higher quality photons that are more visible to the plant.
- The quality of the spectrum makes LED more efficient; It can use less watts, which is why you see “equivalent” ratings such as an 650-800w LED for replacing a 1000W HPS.
- LED can minimize energy lost during the conversion of watts to photons and offers more control over quality of photon and where it lies within the spectrum.
How to Gauge Light
To accurately gauge the amount of light your plants are receiving, a PAR meter is necessary, and will give you mol and µmol readings of visible wavelengths (400-700nm) within PAR. These readers typically cost $200-350 for reliable units, but they’ll last forever and be your most valuable tool for taking PAR readings at canopy.
Ideally, you’re looking for 800-1000µmol at canopy. The further your light is from the canopy, and the reader, the less precise these measurements will become. Because light travels in waves, you are measuring waves, some of which (like red lights) travel faster than others, which may give an artificially high µmol rating.
Night Lighting: What happens if light leaks into the grow?
There is a general fear in the growing community of light leaks and how they affect the plant. Most commonly, there is the incorrect fear that light leaks will cause the plant to display intersex and hermaphroditic traits, but will likely not cause intersex traits unless there is significant exposure early on during bloom. It’s also believed that even a few seconds of exposure to light during the wrong time of the cycle can be detrimental to grows.
If you can keep unexpected light exposures to under 30 seconds, you should be fine. Additionally, light leaks that come through pin holes in your grow or tent generally cause indecisive, vegetative-state growth (where the plant can’t tell if it should be in vegetative or flower state).
From my experience, light exposure under 60 seconds, for a quick check in the grow or a snap of a picture, will not have a negative impact on the plant. The light exposure must be intense and prolonged (longer than a minute) for plants to “wake up”; during the night time, plants have turned their metabolisms off and are excreting glucose into the rhizosphere. These plants are in a completely different stage when they are asleep, and it can take plants up to an hour to fully wake up and change modes, firing all cylinders and beginning to process light and photons.
To play it safe, it is best to avoid exposing your plants to light during their night cycle when possible, and to choose lighting that is less visible to plants when absolutely necessary to work in the grow at night. This is why, as discussed, green is among the most common lighting for use at nighttime. Infrared also works, but enhanced infrared may affect your plant as it is more visible to the plant.
• Plants see uMole not Lumens.
• uMole is a measure of Photons within PAR.
• The most important qualities of a light are its efficiency (photons per Watt) and spectrum.
• The key to deciding what lights you need is knowing the amount of µmol for your space. Secondarily, knowing how much of the lighting spectrum is in use, at what times and why (photon blast for yields, full spectrum for additional qualities like resins and terpenes).
• Keep light exposure to a minute or less during “night time” to avoid mutant growth.