What Is PAR and How Does It Differ From Lux?
PAR and lux both describe light, which is why the two are so often compared — and why the comparison is misleading. They are not two scales for the same thing. PAR defines a waveband; lux measures a brightness, weighted to the human eye. This guide sets out what each one actually quantifies, why no fixed conversion between them exists, and which units to specify a grow light in instead.
What PAR is
PAR stands for photosynthetically active radiation. It is not a unit and not an intensity: it is the spectral range that photosynthesis uses, defined as radiation between roughly 400 and 700 nanometres — the blue and red region of the spectrum where a plant's ability to produce sugars peaks. University of Missouri Extension defines it exactly that way, then makes the point that is so often missed: PAR does not refer to intensity, only to the wavelengths emitted, and the intensity of light in the PAR range is measured as photosynthetic photon flux.[2]
That photon-flux measurement is the useful one, because photosynthesis is a photon-counting process. As the Stark-Einstein law states, one photon excites one electron regardless of its colour, so counting photons in the 400–700 nm band is the appropriate way to predict photosynthesis. Michigan State University's summary of the measurement systems puts the same conclusion in one line: counting photons “considers all colors of light equally and is the most appropriate way to measure light intensity for photosynthesis and plant growth.”[1]
In practice PAR is quantified by three derived quantities:
- PPF — photosynthetic photon flux (µmol/s): how many photosynthetic photons a source emits each second. One micromole of light contains 62 quadrillion photons.[2]
- PPFD — photosynthetic photon flux density (µmol/m²/s): how many of those photons land on a given area of canopy at a set distance. It rises as the light source is brought closer to the plants.[2]
- DLI — daily light integral (mol/m²/day): the total light delivered over a day, combining PPFD with the photoperiod.[2]
What lux is
Lux is a photometric unit. One lux is one lumen per square metre, and a lumen is a measure of visible light weighted by the sensitivity of the human eye — so lux describes how bright a surface appears to a person, not how many photons a plant receives.[1]
The weighting is the whole problem. Human vision responds strongly to green and yellow and poorly to deep blue and deep red, so a photometric scale discounts precisely the two colours that are most useful for plant growth. Michigan State University Extension states it directly: these units of measurement are based on the perceived brightness of the human eye, the eye perceives green and yellow much better than blue or red, and the measurement system is therefore “completely biased toward people and is not appropriate for plants.”[1]
Two consequences follow, both of which matter in a grow room. First, an intensity of deep blue and deep red light that looks dim to us can be bright light for a plant.[1] Second, because most lighting is bought for people, lamp efficiency is almost always quoted as luminous efficacy in lumens per watt, which is a figure optimised for human spaces rather than for photosynthesis.[1]
PAR and lux side by side
| Dimension | PAR family (PPF / PPFD / DLI) | Lux family (lumen, lux, foot-candle) |
|---|---|---|
| What it defines | A waveband (400–700 nm) and the photon flux within it | The apparent brightness of light to a human observer |
| Base quantity | Photons per second | Luminous flux |
| Weighting | Equal for every photon in the band, regardless of colour | Weighted by the eye's spectral sensitivity, peaking in green–yellow |
| Typical units | µmol/s (PPF); µmol/m²/s (PPFD); mol/m²/day (DLI) | lm (lumen); lx (lux, = lm/m²); fc (foot-candle; 1 fc = 10.8 lx) |
| Appropriate for plants? | Yes — the recommended system for photosynthesis and plant growth[1] | No — “highly misleading and inappropriate for plant applications”[1] |
Why the ratio changes with the light source
Because the two systems weight the spectrum differently, the number of lux that corresponds to a given PPFD depends on the spectrum of the source in question — which is why any calculator offering one universal lux-to-PPFD factor is producing an estimate, not a measurement.
The published comparison makes the effect visible. In the Michigan State University table, a blue LED has the highest photon efficiency of the four sources listed, at 1.8 µmol/J, yet the lowest luminous efficiency at 47 lm/W — while a cool-white fluorescent has the lowest photon efficiency at 1.2 µmol/J and the highest luminous efficiency at 90 lm/W. A high-pressure sodium lamp sits at 1.6 µmol/J and 130 lm/W, and a red LED at 1.7 µmol/J and just 17 lm/W.[1] The two efficiency scales rank the same four sources in completely different orders.
| Light source | Photon efficiency (µmol/J) — for plants | Luminous efficiency (lm/W) — for people |
|---|---|---|
| Cool-white fluorescent | 1.2 | 90 |
| High-pressure sodium | 1.6 | 130 |
| Blue LED (peak 455 nm) | 1.8 | 47 |
| Red LED (peak 655 nm) | 1.7 | 17 |
The practical reading is that switching from a white source to a red-dominant spectrum can raise the light delivered to the canopy while the lux reading falls, because the eye values that light less. Judging a grow light by lux therefore penalises exactly the spectra that are most photon-efficient for plants.
The units to use instead
For a grow-light specification, extension guidance points to the PAR-derived quantities rather than photometric ones: University of Missouri Extension advises growers shopping for fixtures to focus on the PAR range and to avoid lights specified in lumens, lux, candelas and Kelvin colour temperature.[2] Michigan State University goes further and recommends that the horticultural industry discontinue the use of photometric units such as lumens, lux and foot-candles altogether for plant applications, because they are a misleading indicator of plant growth and flowering.[1]
Three practical notes follow from that.
- Measure, don't convert. PPFD is measured directly with a quantum sensor in µmol/m²/s. Where a manufacturer publishes PPFD at stated heights, those figures can be used; otherwise the value should be measured at canopy level.[2]
- Know the band's limits. The 400–700 nm definition is a convention, and there is a case for extending it to include far-red light beyond 700 nm, which is termed extended PAR or ePAR and can increase plant growth.[3] Two fixtures can carry the same PAR figure and still differ in their far-red content.
- Intensity is not the whole specification. A fixture can deliver the right PPFD on average and still spoil a canopy through uneven distribution, so uniformity has to be managed as a dimension in its own right.[3]
Frequently asked questions
Can I convert lux to PPFD?
Only approximately, and only if the spectrum of the specific light source is known. Lux is weighted by the eye's sensitivity and PPFD counts photons between 400 and 700 nm, so the relationship changes with the source: the same four lamps in the Michigan State University comparison rank in opposite orders on the two scales.[1] Any single conversion factor is an estimate rather than a measurement.
Is PAR the same as PPFD?
No. PAR is the waveband (roughly 400–700 nm); PPFD is a measurement of how much light within that band lands on a given area per second, in µmol/m²/s. Extension guidance makes the distinction explicitly: PAR refers to wavelength or colour, not to intensity, and the intensity of light in the PAR range is measured as photon flux.[2]
Why does my grow light look dim but perform well?
Because human vision is a poor judge of plant-useful light. Deep blue and deep red light that appears dim to us can be bright light for plants, and the Michigan State University table shows a blue LED with the highest photon efficiency of the four sources listed but the lowest luminous efficiency of the three that are not red.[1] Brightness as you perceive it is not a guide to photon delivery.
Should I choose a fixture by lumens per watt?
Not for plants. Lumens per watt is luminous efficacy, which describes how efficiently a source produces light for human vision. For horticultural use the equivalent figure is photosynthetic photon efficacy in µmol/J, and Michigan State University recommends discontinuing photometric units for plant applications because they mislead.[1]
What light level should I aim for?
Aim for a PPFD band appropriate to the crop and stage, then check it against the daily total. For example, University of Missouri Extension gives lettuce an optimum PPFD of 250–350 µmol/m²/s and notes that light above that level wastes energy, while excessive light can cause photoinhibition.[2] Because PPFD depends on distance, the target has to be checked at canopy height, not at the fixture.
About the publisher
Zhongshan City Ruixian Electronics Factory (XineLam) designs and manufactures LED lighting products and LED grow light modules. The company has 17 years of experience in the LED lighting industry and holds 300+ patents in China and internationally.
This page is a technical explainer assembled from the published research and extension guidance listed below. The units and comparisons described are general to horticultural lighting; nothing on this page is a performance claim for a specific fixture.
References
- Runkle, E. and Bugbee, B. Problems with Foot-candles, Lux and Lumens. Michigan State University Extension. https://www.canr.msu.edu/uploads/resources/pdfs/footcandles-lux-lumens2.pdf (accessed 10 October 2026).
- Cabrera-Garcia, J. and Ernst, M. Controlled Environment Agriculture: Understanding Grow Lights (Publication No. G6987). University of Missouri Extension, February 2025. https://extension.missouri.edu/publications/g6987 (accessed 10 October 2026).
- Runkle, E. Extended photosynthetically active radiation can increase plant growth. Michigan State University Extension, 12 August 2024. https://www.canr.msu.edu/resources/epar (accessed 10 October 2026).