Glossary Of Grow Light Terms

glossary of grow light wordsYou want an LED grow light with a high PPFD.

That’s nice.

Now what the heck is PPFD? And why do I want it?

I get a lot of questions about various terms that are standard jargon in our industry. And I know I’m partially to blame for that.

I try to avoid grow light specific terms in my articles, but I often catch myself using them anyway. Since I can’t seem to avoid them and since everyone else in the industry suffers from the same problem, I figured the next best solution would be to create a glossary of all the terms we use when talking about grow lights.


Definitions of Words Used in the Indoor Lighting Industry

These are sorted alphabetically.


Air-Cooled Reflector

An air cooled reflector is a grow light reflector that is sealed by a glass panel or tube. It has ventilation ducts on either end, allowing the hot air from the bulb to be ventilated from the reflector directly out of the grow room through ducting.


Ampere (Amp)

The ampere is a unit of electric current. It is equal to a flow of one coulomb per second. It is usually abbreviated as ‘amp’ or just using the letter ‘A’ as in 5A. As growers, we often need to calculate the amps required to run a grow light, which you can do with the following formula: A = P / V, where P=power in watts and V=volts.


Arc Tube

The tube inside an HID (high intensity discharge) bulb that contains a noble gas and two electrodes. When electricity is sent through the bulb, the noble gas allows it to travel as an arc between the electrodes. This arc is what produces light.



For our purposes here, an array is a grid of LED emitters affixed on a single chip, such as a COB LED (see below).



A ballast is a device which provides the high voltage needed to ignite a fluorescent or high intensity discharge lamp. Once the bulb has been ignited, the ballast switches to providing a steady flow of electricity to ensure the lamp runs continuously without flickering.


Base Type

The base is the part of a bulb that connects to the socket on the reflector. Different bulbs have different base types, like E27, mogul, etc. The base type of the bulb must match the socket on the reflector or lamp.


Candela (cd)

Candela measures the luminous intensity of a light source in a given direction. It is equal to one lumen (see below) per steradian (an angle).


Candlepower (CP)

A unit of light intensity that is equal to the light emitted by a candle of a specific size and type. It is also equal to 0.981 candelas. The candlepower is not generally used anymore as a unit of measurement.


Ceramic Metal Halide (CMH) Lamp

CMH lamps are a fairly new variant of metal halide lamps. They use a very hot ceramic tube to ionize various gases and salts. The exact makeup of the gas and salt mixture determines the spectral output of the lamp. CMH bulbs have a very high CRI (up to 96), meaning that objects look almost completely natural under CMH light. On top of that, they also contain far more red light than a standard MH bulb. For this reason, ceramic metal halide bulbs are becoming very popular among indoor growers: they can fulfill the roles of both the MH and the HPS bulbs.



COB stands for Chip-on-Board and is a newer technology for packaging LEDs. Multiple LED chips are mounted directly on a substrate so that they form one single module. The individual LEDs are tiny and function as a sole light source. They appear like a lighting panel, not like a bunch of little lights.


Color Temperature

The unit of measurement to indicate the color spectrum of light emitted by a lamp. It is expressed in Kelvin, with a higher number indicating a color spectrum closer to the blue end (cool light) and a lower number indicating a spectrum closer to the red end (warm light).


Compact Fluorescent (CFL) Bulb

CFL bulbs are those spiral fluorescent bulbs we all know. They have a built-in ballast and a standard E27 base, meaning they can screw into any standard household light socket. They emit a white light, but come in various color temperatures. Warm white bulbs have temperatures around 2700 to 3500K and cool white bulbs have temperatures around 5000 to 6500K. The warmer bulbs are better for flowering and the cooler ones for vegging. You can also get bulbs with a color temperature in between. Due to their low power usage and heat output, they make excellent grow bulbs for very small grows or great supplemental bulbs to fill in shaded areas for a large grow.


CRI (Color Rendering Index)

An international system to rate a light’s ability to render object colors. It is measured on a scale of 1 to 100, with a higher number indicating that colors are rendered more true to life. Thus, a CRI of 100 means that objects under that light will appear exactly as they are, while a light source with a much lower CRI will discolor everything.


Electromagnetic Interference (EMI)

EMI is the electrical interference, or noise, that is generated by devices with electrical components. EMI levels from high-frequency devices are regulated by the Federal Communications Commission (FCC).


Electronic Ballast

Electronic ballasts use electronic components to provide the voltage needed to start a lamp and the current regulation needed to keep it running. Their counterpart are magnetic ballasts.


Fluorescent Lamp

Fluorescent bulbs use a phosphor coating to transform ultraviolet energy into visible light. They provide more output than incandescent bulbs while using less power. They also emit less heat, meaning they can hang very close to the plants. But they are less efficient that HID or LED lighting, especially when a lot of light is needed.



Frequency refers to the number of cycles of electromagnetic radiation per second. It is measured in Hertz (Hz) and Kilohertz (kHz). In the United States, the electrical grid runs at 60 Hertz.


Halogen Lamp

Halogen lamp refers to a tungsten-halogen lamp, which is a high-pressure incandescent lamp that contains halogen gases, like iodine or bromine. Halogen gases make it possible for the filament in the lamp to run at a higher temperature and thus more efficiently. The drawback are higher temperatures and a limited spectrum, which makes them unsuited as grow lights.



The Hertz is a unit of measurement for the frequency of alteration of electric current. It used to be called Cycles Per Second.


HID (High Intensity Discharge) Lamp

A type of lamp that contains an arc tube filled with various gases and metal salts under high pressure. The lamp produces light when an arc is ignited between two electrodes in the gas mixture. Common types of HID bulbs are mercury, metal-halide and high-pressure sodium.


Hot Spot

The hot spot is the area immediately beneath a grow light where the intensity is highest. Hot spots can cause uneven growth and even damage to the plants.


HPS (High Pressure Sodium) Lamp

High pressure sodium lamps are a type of HID lamp that contains a sealed ceramic arc tube filled with sodium, mercury and xenon gases. Those gases are ignited to produce a light with a yellow, red and orange color spectrum. This spectrum is well suited for stimulating flowering and fruit production, which is why many indoor growers use HPS bulbs during the blooming stage of growth.


Incandescent Lamp

This is the standard light bulb we all have at home. It produces light by heating a thin filament wire made from tungsten until it glows white. These bulbs are not very effective as grow lights, because they do not have an ideal spectrum, they generate too much heat and need too much power to operate.


Initial Lumens

This measures the amount light a lamp produces after 100 hours of operation.


Input Voltage

The voltage provided by a power supply to a ballast or driver.


Input Watts

This is the total power consumed to produce light, i.e. the power used by the bulb plus the power used by the ballast.


Kelvin Temperature (K)

The temperature unit Kelvin is used to indicate the color spectrum of light from a lamp. See the entry for ‘color temperature’ above for more.


LED (Light Emitting Diode)

An LED is a solid state device that creates light by passing electricity through a chemical compound to excite it and light it up. No filament needs to be heated. LEDs as grow lights are highly efficient in that they produce more output per watt of power than other light sources and generate less heat during operation.


Light Bleaching

Light bleaching occurs when the radiation from the light is too intense for the leaves, causing the chloroplasts to turn white, so that they can no longer process any light. This damage is permanent and it is why you need to always ensure sufficient distance between your grow lights and your plants.



The lumen is a unit of measurement for light output. One lumen is equal to the amount of light from a single candle on one square foot of surface located exactly one foot from the candle. The lumen is used to indicate the power of a standard light bulb or an HID bulb. It does not tell us much about the power of an LED light (see PPF and PPFD below).


Lumen Depreciation

Lumen Depreciation refers to the decrease in lumen output of a light source over time.



One lux is equal to one lumen per square meter.


Metal Halide (MH) Lamp

Metal Halide lamps are another type of HID lamp. Their arcs are filled with a mixture of metal halides. This mixture produces a white light similar to natural daylight. It leans toward the blue end of the spectrum, which makes it ideally suited for vegetative growth. As a result, MH bulbs are generally favored during the early phases of plant growth and are replaced by more reddish HPS bulbs when it is time for plants to flower.


PAR (Photosynthetic Active Radiation)

PAR refers to light in the color spectrum that is used by plants for photosynthesis. This spectrum is in the range of 350 to 650 nm. PAR is often given by LED manufacturers as a measurement of their light’s output, but PAR is not actually a measurement. What they are really giving you is PPF (see below). They are mistakenly referring to that measurement as PAR.



The chemical process in which plants take light energy, water and carbon dioxide and convert it to carbohydrates on which they feed.


PPF (Photosynthetic Photon Flux)

PPF measures the amount of Photosynthetic Active Radiation (PAR) emitted by a light source every second and tells us how much usable (by plants) light is emitted by the source in question. It is given in units called micro moles (umol) per second. PPF does not take into account where the light goes, which makes PPFD a more useful measurement for our purposes.


PPFD (Photosynthetic Photon Flux Density)

PPFD measures the amount of light usable by plants that actually arrives at those plants (or just on a given surface).  It is measured with a PAR meter and given in micromoles per square meter per second. PPFD is the most useful measurement for indicating how much usable light a source provides to your plants.


Power Factor

The power factor indicates the ratio of real power (in watts) to apparent power (volts multiplied by amps). It measures how efficiently a ballast uses its power.



Reflectivity is a measure of the reflective quality of a surface. It tells us how much of the light that hits that surface is reflected back without being absorbed, diffused or otherwise lost. Here is a good article on different reflective materials for a grow room.


T5, T8 and T12 Fluorescent Bulbs

These are the fluorescent tubes you see in many office buildings. The ‘T’ stands for tubular and the number indicates the diameter of the tube in eights of an inch. Thus, a T5 tube has a diameter of 5/8 of an inch and a T8 is 8/8, or 1 inch, in diameter. For indoor growers, T5 bulbs are the most popular, due to their high efficiency and the fact that they are available in various color temperatures.


UV (Ultraviolet) Light

UV light is radiant energy that has a wavelength between 100 and 400 nm. It is classified into three different types: UVA with a wavelength from 315 to 400 nm, UVB  with a wavelength from 280 to 315 nm and UVC with a wavelength from 100 to 280 nm.


Volt (V)

The volt is a unit of electric potential. A single volt is the difference in potential between two points of an electric conductor, when a one amp current flowing between those points dissipates one watt of power.  You can calculate the voltage with the following formula: V = P / A where ‘P’ is the power in watts and ‘A’ is amperes.


Watt (w)

The watt is a unit of power that indicates the rate of doing work. For our purposes, one watt is the power produced by a current of 1 amp across a potential difference of one volt. In other words, W = A x V where ‘A’ is amperes and ‘V’ is volts.

November 8, 2017

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