Grow Light Dictionary: Shining A Light on PAR, PPFD, μmols & More

grow light dictionary, light spectrum blog header

You are ready to buy an indoor grow light, but you’ve started to notice it’s like speaking a different language. PAR, PPF, and μmol/s….What do all these technical terms mean? 

On your journey to indoor cultivation, you’ll need to grasp the basics of lighting science to implement the latest in lighting technology. 

Below you’ll find an easy-to-understand breakdown of all the technical terms, acronyms, and measurements in the grow light industry.

The Different Types of Grow Lights

High Intensity Discharge (HID) 

High-Intensity Discharge (HID) grow lights are a legacy technology still widely used in the industry today. The lights are glass tubes containing various chemical vapors, producing intense light when an electrical arc flows through them.

There are several kinds of HID lights, but the two most notable are metal halide (MH) and high-pressure sodium (HPS) for indoor growing.

HID lights typically consume a lot of energy and produce a lot of heat. Compared with newer technologies, namely LED grow lights, they also have a substantially shorter lifespan. Still, they remain one of the most affordable and established technologies for growing indoors. 

Metal Halide (MH)

Indoor cultivators use MH grow lights specifically for vegging. Metal halide fixture contains mercury and metal halide vapors, which produces an intensely bright white light that leans towards the blue side of the light spectrum. Blue waves are ideal for creating vegetative, bushy growth in plants.

High-Pressure Sodium (HPS)

Growers often transition plants into flower by moving them into rooms with HPS light fixtures. HPS grow lights contain sodium gas and when an electrical arc travels through it, the tube produces a much warmer tone of light, falling into red and infrared wavelengths. This is ideal for producing flowers.

Light Emitting Diodes (LED)

Light Emitting Diodes, or LEDs, are the most recent evolution of grow light technology. Hundreds of small semiconductors, called diodes, are tied together and charged with an electrical current. 

LED grow lights offer improved spectrum, substantial energy savings, and a much better ecological footprint than their traditional counterparts. In addition, their intensity and color spectrum can be adjusted to the crop and stage of growth.

Light emitting diodes
Light emitting diodes, LEDS, are the next evolution of grow light technology.

All About the Light Spectrum

Electromagnetic (Light) Spectrum 

This simply refers to the various frequencies of electromagnetic energy. The term electromagnetic technically covers a broad range of energy waves, from x-rays to radio waves to light waves. 

Growers need only to grasp the frequencies (measured in nanometers or nm) perceptible by humans and plants. This spectrum is between 300 to 800 nanometers.

Visible Spectrum

Humans cannot see the entire electromagnetic spectrum. The visible spectrum is what the human eye can naturally see, between 740 nm to 380 nm. Ultraviolet and infrared light falls outside the visible light spectrum, but it’s still important for indoor cultivation.

Plants and animals often can see and process wavelengths far outside the spectrum visible to the human eye.

Full Spectrum

Many grow light brands market their fixtures using the term full spectrum. Full-spectrum light spans the entire electromagnetic spectrum, both the visible and invisible wavelengths. The most familiar source of a full-spectrum light is straight-up sunlight.

In the grow light industry, the term full spectrum is often used as a bit of a marketing tactic. If you see the full spectrum on the box, it may mean a variety of things, including greater productivity, better visibility, better spectrum for plants (or humans), among many other possibilities.

According to Rensselaer Polytechnic Institute, “Light sources promoted as full-spectrum can cost over ten times as much as nearly identical products that do not bear the full-spectrum claim.” Because of this problem, it’s best to select grow lights using other parameters rather than a basic claim of full-spectrum lighting.

Lighting Science, Acronyms and More

Photosynthetically Active Radiation (PAR)

If the visible spectrum describes what humans can see, PAR describes what plants can ‘see.’ PAR describes the light spectrum that plants use for photosynthesis. 

A PAR reading (using a PAR meter) will tell you the quantity of light expressed by your grow light, but it doesn’t necessarily tell you quality. So it’s a critical piece of data for choosing a grow light, but not the only piece.

Photosynthetic Photon Flux (PPF) and Photosynthetic Photon Flux Density (PPFD)

These two definitions are best explored together. First, let’s start with PPF. This is a value assigned to how many photons within the PAR range are emitted every single second. If it falls outside the photosynthetically usable range, it doesn’t count. We measure PPF using micromoles per second, or μmol.

But, just because your grow light emits an impressive amount of light (high PPF) doesn’t mean it’s useful by the time it falls on your canopy. 

That’s where PPFD comes in. PPFD measures how many micromoles are hitting every square meter per second. We measure this by μmol/s/m2.

In sum, you’ll want to look at PARPPF, and PPFD to compare different grow lights.


The intensity of traditional grow lights is often measured by watts (w). Simply put, watts measure the units of power generated or consumed. You’ll typically see both MH and HPS lights sold in 400w, 600w, or 1000w capacities.

This indicates the level of power drawn, and for HID technologies, it is an accurate measurement of the intensity discharged. The higher the energy used by HID lights, the more light produced.

Do not rely on watts when shopping for LED lights. This is an inaccurate translation between two totally different technologies. LED grow lights naturally draw much less energy and are often less intensity. 

But because LED grow lights tend to produce better PAR values, they can be less ‘intense’ than traditional lighting and remain just as effective for plant growth.


Although somewhat important to your gro light purchase, voltage has nothing to do with the light emitted, PAR values, or PPFD. 

Voltage is essentially the electrical ‘pressure’ pumping through the electricity network in your area of the world. Different countries have different standard voltages. For example, in the US, you’ll need devices wired for 110v and 220v, while in Europe, you’ll need a device suitable for 230v. 

HydroGuide Grow Lights will all work in North America, but double-check the voltage if you choose to shop elsewhere. 

Learn the Lingo, Understand Your Grow lights

When you shop for grow lights, you don’t have to feel overwhelmed with all the technical mumbo-jumbo. Take a moment to grasp the basic concepts of light science and grow light technology, and it’ll pay off in your grow room.

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