Does the LED grow light terminology confuse you? Below is a glossary to the most important terminology around LED grow lights. Système international d’unités (SI) units is the internationally recognized standard of measurement units for the different terms.
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Table of Contents
- Light emitting diode (LED)
- LED driver
- Electromagnetic radiation
- Electromagnetic spectrum
- Radiant flux
- Luminous flux
- Integrating sphere
- Photosynthetically active radiation (PAR)
- PAR chart
- Photosynthetic photon flux (PPF)
- Photosynthetic photon flux density (PPFD)
- Photon efficacy (PE)
- LED light spectrum
- White LEDs vs blurple
- Diode vs luminaire efficiency
Light emitting diode (LED): a semiconductor light source that emits light when an electrical current passes through it. They need direct current (DC) to operate, usually via an LED driver. [source]
LED driver: an electrical device that changes alternating current (AC) power from the electrical grid/your wall outlet into a regulated DC current that the LED’s can use. It can often include a dimmer too. [source]
Electromagnetic radiation: waves of the electromagnetic field. It includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays . Can be defined by energy, wavelength and frequency. Energy is measured in electron volts (eV), wavelength is measured in meters (m) and frequency is measured in cycles per second, aka Hertz (Hz). A cycle is the time it takes a periodic signal to complete the period (i.e. from one max point to the next max point). [source]
Electromagnetic spectrum: a range of frequencies of electromagnetic radiation with respective wavelengths. Different types of electromagnetic radiation (e.g. radio waves, microwaves, gamma rays, etc) have different wavelengths and frequencies. Radio waves travel much slower, thus have longer wavelengths and travel at lower frequencies. This is because wavelength and frequency is inversely proportional (i.e. f = c/wavelength). If wavelength is a big value, the frequency must be a small value. One radio wave from peak to peak is roughly 20% taller than the tallest skyscraper in the world. For LED grow lights, the important spectrum is between infrared and ultraviolet on the image below. 
Radiant flux: Also known as radiant power, it is a measure of the perceived energy transfer per unit time (i.e. power) of a light source. Basically the amount of energy a light produces per second. E.g. for the sun, this would also include UV and IR light, which is outside the PAR range. SI Units: Watt [W]
Luminous flux: similar to radiant flux, luminous flux focuses on the visible light spectrum (i.e. the light our eyes can see). Integrating spheres are used to measure luminous flux.  SI units: Lumen
Integrating sphere (i.e. Ulbricht sphere): A hollow sphere with white reflective coating on the inside. Small commercial spheres can cost thousands of dollars. This is why many grow light manufacturers do not have one for measuring luminous flux of their grow lights. [source]
Photosynthetically active radiation (PAR): is the spectral range from 400-700 nm that photosynthetic organisms (e.g. plants) use during photosynthesis. The light intensity within this range is measured. PAR is not a measurement unit. It is just a definition for the type of light required for photosynthesis. PPFD is the measurement for PAR (defined below). [source1] SI Units: Watts/meter squared [W/m2] (irradiance (i.e. radiant flux/power) of PAR)
A PAR chart gives the PPF values for a specified grow area. This can often be 2×2 ft (61×61 cm), 3×3 ft (91×91 cm) or 4×4 ft (122×122 cm), standard grow tent measurements. As can be seen from the first image above, the recommendation is for a 4×4 ft (120×120 cm) grow tent. The light is located dead center in the chart, either 18 or 24 inches (46 or 61 cm) from the lamp to the floor (i.e. the grid). The middle values in the chart always show the biggest PPF values, seeing as the light is most intense there and travels the shortest distance to the floor/grid. As you get further away from the center point of the grid, the light intensity and therefore also the PPF decreases. Especially cheap lights can have really bad values at the outer edges of the chart. The PPF values also decrease as the light is raised from 18 inches to 24 inches.
Seeing as the PPF values are highest in the center, it is often a better idea to buy multiple small grow lights than one big grow light if you have the budget. That way, the PAR gets more evenly distributed to the plant canopy. Some companies that hype up their marketing with high PPF values only refer to the center values and do not take an average of the entire grow grid. This is most often the case with cheaper Chinese grow lights, but false marketing is a problem in the entire industry.
*All Readings Taken Using LI-COR LI-250A Light Meter. [source]
Photon: light particles
Photosynthetic photon flux (PPF): total number of photons within PAR range produced by a light fixture every second. Measurement units: [umol/s]
Photosynthetic photon flux density (PPFD): number of photons in the PAR range between 400-700 nm emited each second over a square meter area. In simpler terms, the amount of PAR that arrives on a plant every second. [source, source2] PPFD is a spot measurement of a specific location on the plant canopy. This value is often misused in marketing.
In the words of Fluence (by Osram), “Lighting companies that only publish the PPFD at the center point of a coverage area grossly overestimate the true light intensity of a fixture”. The PPFD right below the grow light tends to be strongest. The PPFD goes down the further away from the center of the grow light you get. See PAR chart for more info. Also, manufacturers can easily manipulate PPFD data. The manufacturer needs to specify the vertical and horizontal measurement distance from the grow light, the number of measurements included to get an average value, and min/max ratio. Ideally, an independent third party should make the PAR chart. Measurement units: [umol/m2/s] [source]
Photon efficacy (PE): how efficient does a grow light transform electrical energy into PAR photons. Watts (same as Joules/second) describes the electrical input needed. Measurement units: PE = PPF/P = [umol/s]/[J/s] = [umol/s]*[s/J] = [umol/J].
Higher values are better. [source] Some grow lights have a PE value of over two!
The PPFD and PE values are important values to consider when purchasing a grow light. All grow lights provide Watt values. However, if the PE is bad, the light is not good at converting electricity into useful PAR.
Let’s look at an example. The popular Vivarspectra 300W grow light. [source] Already from the title, it is clear they are lying. Going a bit further down in the description, and you can see the power draw is roughly 130W. That’s more than two times less than shown!
This is correct, so hats off to them for mentioning this.
Let’s calculate the PE next. Taking an average of the PAR chart in the right image above, I get an average of 115 umol/J. Dividing this by the 130 W power draw, we get 115/130= 0,8 umol/J. This is pretty good value for money, considering this light can often be found for under $100.
LED light spectrum: The two most important colors that plants use are blue and red. Blue light is very important during the vegetative growth stage (e.g. a chili pepper plant growing from a starter plant until it starts the bloom). Red light is then used during the blooming/flowering cycle (e.g. when a chili plant starts flowering and grows chili peppers).
White light contains a lot of green and yellow light, which is still efficient for plants to use. Plant leaves are green because they do not absorb as much green light as red and blue. This green light is reflected back to us. Green light that is not reflected penetrates the upper canopy. This leads to higher growth rates in the lower canopy with white light, increasing the entire plant growth rate and yield.
White vs blurple LEDs: White LEDs produce light in the entire color spectrum. Blurple, on the other hand, focus solely on the blue and red spectrums, which gives off a blue/purple light, i.e. blurple. Researchers have found that white light most often leads to higher yields and quicker growth rates than blue and red lights. White light can penetrate the canopy better and provide more light to the lower leaves, increasing the entire plant growth rate.
Diode vs luminaire efficiency: Some companies may claim a really good efficiency value by basing it on the diode efficiency. The LED efficiency (i.e. a single LED) is higher than the luminaire (i.e. a complete electric light unit) efficiency. This is because there are multiple efficiency losses in the entire luminaire. The LED driver efficiency, LED drive current, LED junction temperature, optical losses in the fixture, etc all reduce the total efficiency of the PAR that the plants receive. [source]