PPFD vs PAR vs Lux: What to Check for Grow Lights

Three terms, three jobs

A grow light listing can show PPFD, PAR and Lux in the same line, and the three look alike at a glance. Each term answers a different question about the light. PAR names the range of wavelengths that plants use. PPFD measures how much of that range lands on a surface. Lux measures how bright the light appears to a person. Knowing which question each term answers is the first step to reading a listing that carries any of them.

The order to read them matters. PAR comes first, because it says whether the light covers the band plants can use. PPFD comes second, because it says how much of that usable light arrives at the plant. Lux comes last, and for plant light it is the least useful of the three. A listing may state one, two, three, or none of them; the catalogue reads what a listing actually states.

PAR: the range plants use

PAR stands for photosynthetically active radiation. It is the name for the band of light wavelengths that plants use in photosynthesis, commonly described as the 400 to 700 nanometer range. PAR is not an amount; it is a slice of the spectrum. A light can be described as delivering PAR even when the listing gives no figure at all.

When a listing does state a PAR value, that value is tied to conditions, so the number on its own is incomplete. The maker's page and the maker's manual state where the light was placed when the value was taken. The article on full spectrum versus targeted wavelengths explains how sellers describe spectrum in listing words.

PPFD: how much lands on a surface

PPFD stands for photosynthetic photon flux density. It counts the photons in the PAR range that arrive at a surface, such as the leaves of a plant, per unit of area and per unit of time. It is a spot measurement: a figure taken at one place at one moment. A PPFD taken near the center of a light's footprint differs from one taken near the edge.

PPFD is the closest single term to how much usable light reaches the plant. When two listings both state PPFD, the figures can be compared only when both makers describe the same conditions of measurement. That description is on the maker's page. A separate article on this site explains how PPFD over a day becomes a daily light figure, DLI.

Lux: a measurement for human eyes

Lux measures illuminance, the amount of visible light that falls on a surface as the human eye perceives it. The eye is most sensitive to the middle of the visible spectrum, so lux weights yellow and green light heavily. Plants use a wider band of wavelengths, including blue and red, that lux does not weight in the same way.

A lux figure tells you how bright a light looks in a room, not how well it feeds a plant. This is why grow light sellers often state PAR or PPFD instead of lux, and why a listing that gives only lux is giving less information about plant light. When lux is the only light measurement stated, the watts and coverage on the same listing are the details to compare.

Where the figures live on a listing

On this site, the catalogue reads the details a listing states: light type, watts, coverage, dimmable and timer words. PAR and PPFD figures rarely appear in the title, and when they do, they are a seller's statement under that seller's conditions. The full specification lives in the maker's manual and on the maker's page for the model.

The UMN Extension page on lighting for indoor plants describes the light levels that different houseplants and seedlings need in plain terms. The DesignLights Consortium describes how horticultural lighting products are verified against stated performance. Both are useful reading alongside a listing.

What to do when a listing only states watts

Many listings state watts and coverage and stop there. That is enough to sort a light by job. A windowsill of herbs suits a clip-on or a bulb, a seed starting shelf suits strips, a 4x4 tent suits a board or a set of bars. The wattage and the stated coverage are what the catalogue compares.

When PAR and PPFD are absent, there is no honest way to invent them. The right move is to compare the watts and coverage the listing does state, and to read the maker's page if the buyer wants a figure the listing lacks. The buying guides on this site sort lights by these stated details.

Read the maker's page next to the listing

A listing is one seller's summary; the maker's page for the model is where the full specification sits. Makers such as GE Lighting and Vivosun publish product pages from their main sites; the page for the exact model is the place to read stated figures. When a seller states a PPFD figure, check the maker's page for the same model and compare the two.

When the maker publishes a PAR map, that map is a picture of PPFD across a footprint. Reading a PAR map is covered in a separate article. The same rule applies to every measurement term on a listing: watts, coverage, dimmable and timer words come from the listing, and figures like PPFD come from the maker.

What to pick for your job

Questions

Is lux useful for choosing a grow light?

Lux describes how bright a surface looks to the human eye. Plants use wavelengths that lux does not weight the same way. When a listing states PAR or PPFD, those are the figures to read. When it states only lux, the watts and coverage on the same listing are the details to compare.

What does PAR mean on a listing?

PAR names the range of wavelengths plants use in photosynthesis. It is a term, not a measurement. A listing may say a light delivers PAR without stating a figure. The maker's page states the value and the conditions it was measured under.

Where is PPFD printed?

A seller may print PPFD in the bullet list or in the image set of a listing. The maker's manual and the maker's page for the model state the figure and the distance it was measured at. Read both next to the listing.

Can I compare two lights by PPFD?

Only when both makers state how the figure was measured. The conditions, such as the distance and the area sampled, need to match. That explanation is on each maker's page.

Is PPFD the same as DLI?

No. PPFD is a spot measurement of light arriving at a surface. DLI adds time over a day to that figure. The article on DLI explains the difference.

What if a listing gives no PAR or PPFD?

Most listings state watts and coverage instead. Those are the honest terms to compare. The site's guides sort lights by the details a listing states.