How to Use This Page
The thirty terms are grouped by where they sit in a purchase, and the section before the last one covers the measurement and certification vocabulary that decides whether a published number can be trusted at all. Start with the five that do most of the work, then take the groups in the order your own questions run: what the numbers mean, what the light is made of, what you are physically buying, whether the number can be trusted, and what happens in the room.
The 30 Terms at a Glance
The table below is the fastest way in: scan all thirty, then jump straight to the entry you need. Each row gives the full name, the unit it is stated in, and the one thing the term settles in a purchase.
| Term | Full name | Unit | What it tells you |
|---|---|---|---|
| PAR | Photosynthetically Active Radiation | band (400–700 nm) | Which photons count as plant light at all |
| Photon & Micromole (µmol) | the counting unit of plant light | 1 µmol = 6.022 × 10¹⁷ photons | The unit every photon number is counted in |
| PPF | Photosynthetic Photon Flux | µmol/s | Photons the fixture emits each second |
| PPFD | Photosynthetic Photon Flux Density | µmol/m²/s | Photons landing on one square metre each second |
| DLI | Daily Light Integral | mol/m²/day | Photons banked across the full photoperiod |
| PPE | Photosynthetic Photon Efficacy | µmol/J | Photons per joule: the running-cost figure |
| Lumen & Lux | human-eye light metrics | lm · lx | Brightness as people see it, not as plants use it |
| Watt | electrical input | W | Circuit load and running cost, not light output |
| Spectrum / SPD | Spectral Power Distribution | nm (380–800 plot) | The full wavelength mix a fixture emits |
| Full Spectrum | a label, not a standard | none | Nothing verifiable on its own; judge from the SPD graph |
| CCT (Kelvin) | Correlated Colour Temperature | K | White-light tone for people, not a plant metric |
| CRI | Colour Rendering Index | scale to 100 | Colour fidelity for the crew's inspection work |
| Blue-to-Red Ratio | spectral balance | ratio | Vegetative structure versus flower mass in the mix |
| Far-Red | just past the PAR boundary | 700–750 nm | Extends canopy photosynthesis when mixed; invisible to PPE |
| UV | ultraviolet | below 400 nm | Switchable channels doing work no PPE figure credits |
| Diode (LED Chip) | the emitter | package rating | The efficiency ceiling; a package rating is not a lamp rating |
| Driver | AC to DC conversion | none | The usual failure point; brand and placement belong in the quote |
| Dimming | output control signal | % · 0–10 V | How far below nameplate the fixture will run |
| LED vs HPS | technology comparison | µmol/J | The settled efficacy gap between the two technologies |
| Form Factor | bar · board · point source | none | Shape drives light distribution as much as the diodes do |
| Beam Angle | the emission cone | degrees | Why fixtures with identical PPF light rooms differently |
| Mounting Height | distance to canopy | distance | The biggest buyer-controlled lever on delivered PPFD |
| Footprint & Coverage | area held at spec | ft² · m² | Canopy area divided by footprint gives the fixture count |
| Uniformity | evenness across the canopy | min ÷ avg | Whether the corners get light, not just the average |
| Photoperiod | daily light schedule | hours | Sets how many photons a fixture banks per day |
| Canopy Penetration | depth of usable light | none | Why a strong top reading can hide an underlit lower canopy |
| Under-Canopy Light | bars below the canopy plane | W | Lights the lower bud sites top light cannot reach |
| Light Burn | too much intensity or heat | none | Bleached upper growth; fix distance and dimming first |
| Light Leak | light in the dark period | none | Small dark-hour leaks that stress a flowering room |
| Flux Maintenance (L90 / Q90) | output over life | hours | What the fixture still delivers late in its life |
The Five Terms That Do Most of the Work
| Term | Unit | One line | Where it bites |
|---|---|---|---|
| PAR | 400–700 nm | The waveband plants can use for photosynthesis | Defines what is being counted at all |
| PPF | µmol/s | Photons the fixture emits each second | Compares fixtures on output |
| PPFD | µmol/m²/s | Photons landing on one square metre each second | Decides intensity at the canopy |
| DLI | mol/m²/day | Photons banked across a full photoperiod | Decides the daily dose the crop receives |
| PPE | µmol/J | Photons per joule of electricity | Decides running cost |
Everything else in this glossary either refines one of these five or describes the hardware that produces them. The pairs most often used interchangeably, and wrongly so, get their own table near the end.
Light Measurement: The Chain from Wall to Canopy
PAR Photosynthetically Active Radiation · 400–700 nm
The waveband of light plants can drive photosynthesis with. It is a range of wavelengths, not an intensity: a fixture cannot be "high PAR", it can only emit more or fewer photons inside this band. The DLC kept this boundary in 2021 rather than extending it to 750 nm, which is why far-red photons sit outside every PPE figure. The 400–750 nm extension circulates under the name ePAR; it is a research proposal, not part of the standard.
In a purchase conversation: PAR sets what is being counted. Every other number on the sheet is a measurement inside or outside this band.
Photon & Micromole (µmol) the particle of light · 6.022 × 10¹⁷ photons per µmol
Light delivers energy in photons, and horticultural metrics count them the way a warehouse counts boxes: each photon counts once, whatever its colour. Human-eye metrics such as lumens weight colours differently, which is why the two systems disagree about the same lamp. The counting unit for photons is the micromole: 6.022 × 10¹⁷ of them, roughly six hundred quadrillion. PPF, PPFD and DLI are all photon counts in micromoles, differing only in where they are measured and over what timescale.
In a purchase conversation: ask whether photons were counted or weighted for eyes, and treat any figure that gives µmol without a per second, per square metre or per day as an incomplete number.
PPF Photosynthetic Photon Flux · µmol/s
The number of PAR photons a fixture emits each second, measured on the assembled fixture in an integrating sphere. It is a property of the hardware and carries no directional information: where the photons land is a separate report.
In a purchase conversation: no PPF figure means the claimed efficacy cannot be checked. See how to verify a claimed µmol/J.
PPFD Photosynthetic Photon Flux Density · µmol/m²/s
Photons landing on one square metre of canopy each second. Set by PPF plus mounting height, optics and spacing. Commercial flower rooms commonly run 900–1200 µmol/m²/s at the canopy under ambient CO₂, and up to about 1500 with CO₂ enrichment.
In a purchase conversation: the number a layout is actually bought on. Ask for a map run at your dimensions: PPFD & DLI targets by stage.
DLI Daily Light Integral · mol/m²/day
The photons a square metre banks over a full photoperiod: PPFD × hours × 0.0036. At 12 hours, 30–45 mol/m²/day comes from roughly 700–1050 µmol/m²/s; the higher flower-room band of 40–65 requires CO₂ enrichment to be worth running.
In a purchase conversation: crop targets are written in DLI, so the fixture and the light schedule are sized together, not separately.
PPE (µmol/J) Photosynthetic Photon Efficacy · µmol/J
PPF divided by wall watts: the running-cost number of horticultural lighting. Mainstream commercial fixtures sit at 2.8–3.1 µmol/J, and on current LED technology the published fixture-level ceiling is 3.4 µmol/J for white-plus-red spectra.
In a purchase conversation: read it as a power-bill number, never a growth number. The full treatment: what µmol/J really tells you.
Lumen & Lux lm · lx · weighted for human eyes
Photometric units weighted by the sensitivity of the human eye, which peaks in yellow-green and discounts the deep red and blue plants use most. A lux meter therefore systematically misreads a horticultural fixture, and no conversion factor exists between lm/W and µmol/J because the ratio depends on the spectrum.
In a purchase conversation: a lumens-first spec sheet is a document about human vision; treat it as a signal about what is not being shown.
Watt W · electrical input
The power a fixture draws from the wall. It tells you circuit load, heat and running cost, and it is the denominator of PPE, but it says nothing about how many photons leave the fixture: two builds at the same wattage can differ widely in PPF.
In a purchase conversation: ask which wattage a claimed efficacy used, the wall draw or a diode rating. The gap between the two is the oldest trick on the market.
Spectrum and Colour: What the Photons Are Made Of
Spectrum / SPD Spectral Power Distribution · nm
The plot of how many photons a fixture emits at each wavelength, usually drawn from 380 to 800 nm, so it shows the UV edge and the far-red tail that PPE does not count. It is the most complete single description of a light source: every other term on this page is either derived from it or deliberately ignores part of it.
In a purchase conversation: ask for the SPD, and ask which channel state it was measured in. Our controller page shows how a fixture's SPD is displayed per model.
Full Spectrum a label, not a standard
Generally means a fixture covering the PAR band broadly, usually a white base plus red diodes and sometimes far-red or UV channels, rather than narrow red and blue peaks. The phrase is not standardised: two fixtures can carry the same label with very different SPDs. Some vendors rebrand the same idea as PBAR (280–800 nm), which is likewise a description, not a standard.
In a purchase conversation: judge "full spectrum" from the spectrum graph alone. The label carries no information on its own.
CCT (Kelvin) Correlated Colour Temperature · K
Describes the tone of white light as the human eye reads it: 2700–3000 K reads warm, 4000 K neutral, 5000–6500 K cool. Plants do not perceive Kelvin. Two fixtures at the same CCT can differ widely in red content, and therefore in photon output.
In a purchase conversation: useful for work-light comfort and inspection under canopy; not a plant metric and not a substitute for the SPD.
CRI Colour Rendering Index · scale to 100
How faithfully a light renders colour to human eyes. It says nothing about photosynthesis. Its commercial use is practical: higher-CRI white makes pest scouting, deficiency spotting and ripeness judgement easier for the people walking the rows.
In a purchase conversation: a people metric, not a plant metric. Worth having for the crew, priced at whatever efficacy it costs.
Blue-to-Red Ratio spectral balance · dimensionless
The share of blue versus red photons in the emitted spectrum. Blue builds compact vegetative structure; red drives flower mass and measures higher in µmol/J because a red photon carries less energy, so the same joule buys more of them.
In a purchase conversation: before crediting hardware for a higher PPE, check whether the difference is simply the ratio.
Far-Red 700–750 nm · outside the PAR definition
Sits just past the PAR boundary, so its photons cost watts but appear in no PPE figure. Mixed at up to roughly 30 percent of the photon flux, a planning range that shifts with crop and intensity, it raises canopy photosynthesis as much as an equal number of PAR photons; on its own it does almost nothing.
In a purchase conversation: a lower PPE on a far-red-equipped fixture is a design trade, not a defect. Ask which channel state was measured: why the number stops being the answer.
UV ultraviolet · below 400 nm
Sits below the PAR band. UV-A appears in some commercial programs through switchable channels; UV-B is biologically active at low doses, easy to overdo, and a personnel-safety question before it is a plant question: any UV-B device belongs behind shielding, interlocks and a schedule, and published biomass claims stay unsettled rather than promised. Because UV photons are outside the count, a fixture running its UV channels measures lower PPE while doing work the metric does not credit.
In a purchase conversation: if the fixture carries UV, ask whether the quoted PPE was taken with the channels on or off.
Fixture and Optics: What You Are Actually Buying
Diode (LED Chip) the emitter · a package rating, not a lamp rating
The semiconductor that emits the light, and the ceiling on what a design can reach. Diode datasheets are package measurements taken at ideal conditions; a finished fixture reads lower once the driver, the junction heat and the optics take their share. A package rated 3.2 µmol/J can land in a fixture measuring 2.8.
In a purchase conversation: a diode rating is not a lamp rating. Ask which of the two a claim describes: the four losses between them.
Driver AC to DC conversion · the usual failure point
The component that converts mains power to the current the LEDs need, and the first thing to fail in a cheap build. Integrated drivers save fixture cost; remote drivers move heat and weight out of the canopy zone and make service a two-minute job instead of a scaffold job.
In a purchase conversation: driver brand and placement belong in the quotation, not the footnotes: external-driver series.
Dimming 0–10 V or PWM · % of rated output
Running a fixture below nameplate output. The signal type decides compatibility: most commercial fixtures accept 0–10 V, some drivers take PWM, and a room that mixes them needs a controller that speaks both. The classic failure is a PWM dimmer wired to a 0–10 V driver, which is why a fixture can flicker on one controller and behave on another. Efficacy at a dim level is not automatically the nameplate figure.
In a purchase conversation: check signal compatibility before buying the controller, and ask for efficacy at your setpoints: multi-zone control.
LED vs HPS technology comparison · µmol/J gap
The comparison the market has settled. The best double-ended HPS sits near 1.7 µmol/J; the DLC qualification floor for LED is 2.5 and mainstream commercial LED sits at 2.8–3.1. The residual question is heat: HPS radiant heat is a cost in a cooled room and can be a resource in one that needs warming.
In a purchase conversation: quote LED against LED. Against HPS, the fixture comparison ends before it starts: retrofit economics.
Form Factor bar · board · point source
The physical shape of the light-emitting surface. Bars form an extended source that spreads evenly across a frame; boards pack diodes densely; point sources concentrate. Shape drives distribution and mounting behaviour as much as the diodes do.
In a purchase conversation: pick the form for the room before comparing efficacy figures across forms: top-light range.
Beam Angle the emission cone · degrees
How tightly the optics focus photons as they leave the fixture. Narrow beams carry intensity further and concentrate it; wide beams trade peak intensity for even coverage. It is one reason two fixtures with identical PPF produce different footprints.
In a purchase conversation: beam angle explains most "same lamp, different room" surprises before installation quality even enters.
Mounting Height distance to canopy · sets delivered PPFD
The biggest lever a buyer controls over delivered intensity. Bar fixtures are extended sources, so the inverse-square intuition people carry from bulbs does not apply cleanly: lowering a bar fixture brightens the centre more than the edges.
In a purchase conversation: a PPFD map is only valid at the height it was measured. See dim it or raise it.
Footprint & Coverage the area a fixture holds at spec
The canopy area a fixture can hold at target PPFD with acceptable uniformity. Quoted footprints assume a mounting height and a dim level; strip either condition away and the number is decoration.
In a purchase conversation: divide canopy area by footprint for a fixture count, then verify with a map: worked room example.
Uniformity minimum ÷ average PPFD · dimensionless
How evenly photons land across the canopy, stated as the dimmest grid reading divided by the average. Around 0.70 is workable, 0.85 and above is excellent, and below 0.70 the corners run short no matter what the average claims.
In a purchase conversation: a modest fixture with good uniformity beats an intense patchy one at the same average. The 9-point grid shows the test: reading a PPFD map.
Measurement and Certification: How the Number Is Checked
Every published figure is the output of a method, and each step in that chain settles something the step before it could not. Two fixtures can carry the same headline number and mean different things, because the method behind it differs. These are the terms that turn up on test reports and rebate paperwork.
| Where the number comes from | What it settles | What it cannot settle |
|---|---|---|
| Quantum sensor (PAR meter) | The PPFD at one spot, measured in your own room. | The whole fixture's output, or accuracy against a calibrated lab. |
| Spectroradiometer | The full spectral curve at high resolution. | Photon output across every angle, which needs integration. |
| Integrating sphere | The whole fixture's photon output, and the efficacy that follows from it. | How the light behaves in a room, at your own height and spacing. |
| How it is tested and certified | What it settles | What it cannot settle |
|---|---|---|
| LM-79 | How a fixture measures under a standardised test. | Long-term decline: it is a single-point report. |
| LM-80 and TM-21 | How the emitters hold output over time. | Instant performance, or how the driver behaves as it ages. |
| L90 and Q90 | That maintenance was reported on photon output, the basis the DLC asks for, rather than on lumens. | That the figure applies to your thermal and drive conditions. |
| UL 8800, ETL, CSA | Electrical safety for the damp, high-hour conditions of a grow room. | Energy performance. Safety and rebate listing are separate, and one can be present without the other. |
| DLC QPL | That a specific model clears the current efficacy and reporting threshold, and usually the rebate gate with it. | Safety. It is versioned and model-specific, so confirm the exact model number rather than the brand. |
Room Reality: Where Numbers Meet Plants
Photoperiod hours of light per day · 18/6, 12/12
The daily light schedule. DLI is PPFD multiplied by photoperiod, so the same fixture banks a different daily dose at 18 hours than at 12. Photosensitive crops also need the dark hours to stay dark, which is where the light-leak entry below starts to matter.
In a purchase conversation: state the schedule before sizing fixtures; a veg room and a flower room are different loads on the same hardware.
Canopy Penetration how deep usable light reaches
How far usable intensity extends below the top of the canopy. Upper leaves absorb red and blue readily; green penetrates deeper and reaches lower foliage. Penetration explains how a strong top-canopy reading and an underlit lower canopy coexist in the same room.
In a purchase conversation: a top-light PPFD map says nothing about the lower canopy; that layer needs its own plan.
Under-Canopy Light bars mounted below the canopy plane
Fixtures placed under the canopy to light the lower bud sites the top light cannot reach. A separate discipline from top lighting, with its own spectrum, wattage per row and watering cautions, and documented yield results in commercial rooms.
In a purchase conversation: judge it by its own numbers, not by top-light logic: the under-canopy research guide.
Light Burn intensity or heat damage · also called photobleaching
Bleached or crisped upper growth, cupped leaf edges and stalled tops caused by excessive intensity or radiant heat at the leaf. Research literature calls the same symptom photobleaching, or light stress. It mimics nutrient problems, which is why it is diagnosed against the PPFD map and the mounting distance before anyone touches the feed schedule.
In a purchase conversation: the fix is distance, dimming or distribution, in that order: when PPFD runs too high.
Light Leak light during the dark period
Any light reaching plants during the dark hours of a photosensitive schedule. Small leaks, including fixture indicator LEDs and door edges, can interrupt the dark period a flowering crop depends on and stress the room. Reported response thresholds for sensitive crops sit near 0.002 µmol/m²/s, below what a standard PAR meter resolves, so treat the figure as a planning number and verify in the dark. They are invisible in daylight, which is why they go unfixed.
In a purchase conversation: audit the room with lights off before blaming the genetics, and ask how a fixture's indicators behave in the dark.
Flux Maintenance (L90 / Q90) output over life · hours
How output decays as fixtures age. L90 is the hours before output falls to 90% of its initial value, L70 the same test at 70%, and Q90 the share of units still above 90% at rated life. The DLC requires a photon flux maintenance claim of Q90 at 36,000 hours or better on the 400–700 nm band.
In a purchase conversation: a fixture with no maintenance claim is priced on its first year: how efficacy claims age.
The Terms That Get Confused
Most spec-sheet arguments are two people using one word for two things. These five pairs cover most of them:
| The pair | The difference in one line |
|---|---|
| PAR vs PPF | PAR is the waveband; PPF is the photon count measured inside it. |
| PPF vs PPFD | PPF leaves the fixture (µmol/s); PPFD lands on the canopy (µmol/m²/s). |
| PPE vs PPFD | PPE prices the photons (µmol/J); PPFD counts what arrives (µmol/m²/s). |
| Lumens vs µmol/s | Lumens weight light for human eyes; µmol/s counts photons plants use. |
| DLI vs PPFD | PPFD is an instant rate; DLI is that rate held across the photoperiod. |
The Numbers That Do Not Choose a Fixture
These metrics describe light as human eyes, or as old incandescent bulbs, saw it. None of them decides how a plant grows, and a supplier who leads with them is describing something other than photosynthesis. One entry still earns its place for a different reason, and is marked as such.
| Metric | What it actually measures | Why it does not choose a fixture |
|---|---|---|
| Lumens | Light output weighted to the human eye. | Plants do not see the way eyes do, so high lumens can mean low PAR. |
| Lux | The same eye-weighted light falling on a surface. | Room brightness, not the photon count at the canopy. |
| Candela | Intensity in one direction, eye-weighted. | A spotlight metric from the HID era, with no place in canopy planning. |
| CRI | How naturally a light reveals colour to people. | Not a plant metric. It does earn its place for inspection and scouting under canopy. |
| CCT (Kelvin) | The apparent warm or cool tone of white light. | It folds the whole spectrum into one number and hides where the red and blue photons sit. |
| Nominal wattage | A size class compared against an old HID bulb, often printed as 1000W equivalent. | Not the wall draw, so it cannot be divided into photon output to give a real efficacy. |
What Comes Next
These thirty cover the conversations a lighting purchase actually runs on, and they are Part 1 of three. Part 2 is live: Grow Room Terms, the thirty environment and engineering words, VPD, CO₂ enrichment, BTU per hour, sensible heat ratio and the photomorphogenesis terms that describe how light shapes a plant rather than feeding it. Part 3 is live: Legacy Grow Light Terms, the HID-era vocabulary from candela and ballast to footcandle and 1000W equivalent, translated for a buyer reading an old sheet or planning a retrofit. The deep dives already published in this series cover PPFD and DLI targets by stage and grow light efficacy.
FAQ
What do the abbreviations on a grow light spec sheet mean?
Five carry most sheets: PPF is the photons a fixture emits per second, PPFD is the photons landing on one square metre each second, DLI is the photons accumulated over a photoperiod, PPE in µmol/J is photons per joule of electricity, and CCT is the tone of the white light. If a sheet leads with lumens or watts, it is describing light for human eyes; ask for the photon numbers.
Is PAR the same as PPFD?
No. PAR is the 400–700 nm waveband of light plants can use for photosynthesis; PPFD is how many photons from that band land on one square metre of canopy each second. PAR is a range of wavelengths, PPFD is an intensity measured inside that range, and the two are not interchangeable.
What does µmol mean on a grow light?
A micromole: 6.022 x 10^17 photons, roughly six hundred quadrillion. It is the counting unit for plant light. PPF is stated in µmol per second, PPFD in µmol per square metre per second, and DLI in mol per square metre per day. A figure that gives µmol without a per-what is not a complete number.
What is the difference between lumens and PPFD?
Lumens weight light by the sensitivity of the human eye, which peaks in yellow-green and discounts the deep red and blue plants use heavily. PPFD counts photons per square metre per second with no eye weighting. A lux meter systematically misreads a horticultural fixture, and there is no conversion factor between lm/W and µmol/J because the ratio depends on the spectrum.
What does full spectrum mean?
There is no standard definition. It generally means a fixture covering the PAR band broadly, usually a white base plus red diodes and sometimes far-red or UV channels, rather than narrow red and blue peaks. Two fixtures can carry the same label with very different spectra, so judge it from the spectral power distribution graph, not the wording.
What is CCT and does Kelvin matter for plants?
CCT (correlated colour temperature) describes the tone of white light for human eyes: 2700–3000 K reads warm, 4000 K neutral, 5000–6500 K cool. Plants do not perceive Kelvin. Two fixtures at the same CCT can differ widely in red content and photon output, so treat CCT as a work-comfort and inspection figure, not a plant metric.
What do 18/6 and 12/12 mean?
They are photoperiods: 18 hours of light with 6 of dark is a common vegetative schedule, and 12/12 triggers flowering in photosensitive crops. Because DLI is PPFD multiplied by hours, the same PPFD banks half again as many photons at 18 hours as at 12, which is why fixtures and schedules are sized together.
Why is wattage not a measure of light output?
Watts are electrical input: circuit load, heat and running cost. Photons out depend on efficacy, so two fixtures drawing the same wattage can differ widely in PPF. That is why the industry quotes PPF and PPE, and why a claimed efficacy should be checked against wall-draw watts rather than a diode rating.
What does 1000W equivalent mean on a grow light?
It is a size class compared against an old 1000 W HID bulb, not the power the fixture draws. The figure your efficacy maths and your electricity bill depend on is the input wattage at the wall, measured under the test conditions stated in the report. A wattage a manufacturer will not qualify is a marketing figure, and it cannot be divided into photon output to produce a real efficacy.