PPFD vs Lux: Which Should You Measure for Grow Lights?

A lux meter and a quantum sensor can sit under the same fixture and produce numbers that disagree by a wide margin, without either instrument being faulty. The reason is that the two units count different things: lux describes how bright the light looks to a human eye, while PPFD counts the photosynthetically active photons landing on the canopy. This article sets out what each unit measures, why a single conversion factor cannot be trusted across lamp types, and what to measure instead.

Two units that answer different questions

Lux is a photometric unit. It is built from the sensitivity of human vision, so a wavelength the eye responds to strongly contributes more to a lux figure than a wavelength it barely registers — even when both carry the same number of photons.

PPFD is a quantum unit, expressed in µmol/m²/s. It counts the photons in the 400–700 nm waveband that arrive on one square metre of canopy each second, giving every photon in that waveband equal weight regardless of how bright it looks to us.

Virginia Tech Extension sets the two side by side in a table of light units and then states the practical consequence directly: it is not recommended to use units of candela, lumen, foot-candles, lux, watts or joules when measuring light levels for plants.

Peer-reviewed lamp testing uses the same equal-weight definition. Shelford and Both, reporting technical performance characteristics for 18 lamp types, define PAR as measured across the 400–700 nm waveband while giving equal weight to each of the photons across that waveband, and note that an equivalent measurement is PPFD, in units of µmol/m²/s.

The two systems therefore rank light sources differently, and only one of them ranks them for a crop.

What each unit counts, and whether it converts

The table below separates the units that describe light for plants from the units that describe it for people or for the electricity meter.

Sources: unit definitions and the recommendation against candela, lumen, foot-candles, lux, watts and joules from Stallknecht, Virginia Tech Extension (SPES-720, 2025); the equal-weight PAR and PPFD definition from Shelford and Both, AgriEngineering (2021).

Why one conversion factor cannot cover every lamp

If lux could be converted to PPFD with a single number, comparing instruments would be trivial. It cannot, because the conversion depends on the spectrum of whatever is being measured — and different horticultural lamp types have different spectra.

Virginia Tech Extension publishes example conversion factors between foot-candles and PPFD, and they differ by light source. The published worked example uses the same numbers in one direction: full sun has a PPFD close to 2,000 µmol/m²/s, and multiplying that by 5.01 gives 10,020 foot-candles.

The spread across lamp types is what breaks a single factor. The sunlight factor is 5.01 and the high-pressure sodium factor is 7.62, a ratio of about 1.5. Apply the sunlight factor to a high-pressure sodium installation, or the reverse, and the estimated PPFD moves by roughly 50% before any measurement error is counted.

Lamp-to-lamp variation is the second reason. Shelford and Both compared 18 lamp types with sunlight and reported a substantial range of performance characteristics, concluding that a careful assessment is needed before selecting a light source for a horticultural application. One photometric multiplier cannot represent that range.

Source: Stallknecht, Virginia Tech Extension (SPES-720, 2025), Table 2, which states that the conversion factor depends on the light source, and links to a longer published list of factors. The example in that publication multiplies a PPFD value by the factor to obtain foot-candles, so the reverse conversion divides by the same number.

Where a lux reading is still useful

Saying that lux is the wrong unit for plant light is not the same as saying a lux meter has no use. It has several, provided no absolute PPFD value is claimed from it.

• Trend and uniformity checks. Comparing one point in a canopy with another, or the same point before and after a change, is meaningful with a cheap meter as long as the fixture and its spectrum stay the same between readings.
• Relative comparisons at a fixed dimming level. LED output is linearly related to PPFD, so if nothing changes except the dimming level, the ratio between two lux readings and the ratio between the corresponding PPFD values agree.
• Spotting a fixture that is not running. On a multi-fixture installation a lux meter quickly shows a dark corner or a lamp that has failed.
• Reading a controller that reports in photometric units. Michigan State University Extension's guide to light sensors covers light units and how to convert the values a greenhouse environmental control computer reports into more meaningful figures — the situation where a photometric reading arrives whether you wanted it or not.
• Not for absolute values. Because the weighting differs between the two systems, a lux figure cannot be quoted as a PPFD figure with a fixed multiplier.

How to get a PPFD number instead

The range of measurement options is short, and the cost differences between them are large.

1. Borrow a quantum sensor first. A standalone quantum sensor measures PPFD directly in µmol/m²/s. Virginia Tech Extension puts the price of standalone quantum sensors from under $100 to over $1,000, and notes that when paired with a computer to record readings over time they give accurate information about the light environment through the day and across the year.
2. Move to a spectroradiometer when the spectrum varies. Spectroradiometers measure intensity and colour together; Virginia Tech notes they are less useful for sunlight, whose spectrum is consistent, than for LEDs, which come in varying combinations of colours, and that their cost runs from $1,000 to over $2,500.
3. Treat a phone app as a screening tool. Virginia Tech describes apps that use a phone's internal photodiode to measure PPFD as typically the most cost-effective option, and suggests a $20 app can provide useful information for homeowners. It does not replace a logged sensor where production decisions are being made.
4. Turn intensity into a daily total. Multiply the PPFD at the canopy by the hours the lights run and by 0.0036 to obtain the daily light integral in mol/m²/day; 200 µmol/m²/s over 16 hours works out to about 11.5 mol/m²/day.
5. Check the total against the crop before adjusting anything. Virginia Tech lists 5 to 10 mol/m²/day for seedlings and cuttings, 12 to 17 for lettuce, and 20 to 30 for tomato, cucumber and zucchini.

What a PPFD figure still does not tell you

A PPFD number is a point measurement, usually taken under the centre of a fixture at a stated distance. Uniformity is a separate question: Virginia Tech Extension notes that as a light source is moved closer to the canopy the PPFD rises, but the overall uniformity of distribution decreases, so fewer plants are effectively illuminated. A reading taken at the centre can therefore sit well above what the edge of the canopy receives.

Spectral quality is separate again. The same 18-lamp comparison that motivates measuring in photons also found a substantial range of performance characteristics across lamp types. A PPFD figure is the right starting point for comparison, not a complete description of a light source.

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