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Lux is a photometric quantity weighted to the human eye. PAR is a quantum quantity counting photons in the 400 to 700 nanometre band that drive photosynthesis. Irradiance is a radiometric quantity reporting power per unit area across a defined spectral range. They are not comparable because they answer different questions. A lux meter cannot tell you whether a plant will grow. A PAR meter cannot tell you whether a room is bright enough for reading. A solar irradiance meter reports total power arriving at a plane, which is what a photovoltaic system needs. The instrument must match the question. The listings for solar irradiance meters show the third category clearly; the first two require different sensors and different calibration standards.
What each instrument is actually counting
Light is a spectrum, and every light measurement is a weighted average of that spectrum. The weight is the whole story. A lux meter applies a weighting curve that approximates the photopic response of the human eye, peaking near 555 nanometres and falling to near zero in the ultraviolet and the deep red. The unit is lumens per square metre. It is a photometric quantity, built for lighting design and workplace illumination.
A PAR meter counts photons between 400 and 700 nanometres. The unit is micromoles of photons per square metre per second, written µmol m⁻² s⁻¹. It applies no eye weighting; every photon in the band counts equally. That is why a red light and a blue light can have very different lux readings but similar PAR readings. The sensor is a quantum sensor, and its calibration is traceable to a spectroradiometer.
An irradiance meter reports power per unit area, usually in watts per square metre. It measures the total energy flux across a defined spectral range, often the full solar spectrum from roughly 300 to 1100 nanometres for a silicon photodiode sensor. The reading is radiometric. It is what a photovoltaic panel or a solar thermal collector responds to. The explanation of solar irradiance covers the spectral assumptions that sit behind the number.
These three quantities are not convertible with one factor because the conversion depends on the spectrum of the source. A fluorescent tube, a white LED and the sun have different spectral power distributions. The same irradiance in watts per square metre can produce different lux and different PAR depending on the spectrum. Any single conversion factor is valid only for one specific light source.
Why a lux reading cannot be turned into a PAR reading
Suppose a grower measures 20,000 lux under a white LED and wants to know the PAR. The temptation is to multiply by a number found online. That number is a conversion factor derived from one particular lamp spectrum, and it does not hold across lamp types. The reason is that lux weights the spectrum by the eye’s sensitivity, while PAR weights it flat across 400 to 700 nanometres. Two lamps with the same lux output can deliver different PAR if their spectra differ, particularly in the red and blue regions where the eye is less sensitive but photosynthesis is strong.
The same trap applies in reverse. A PAR meter reading of 500 µmol m⁻² s⁻¹ does not tell you whether the space is comfortable to work in. It tells you how many photosynthetically active photons arrive per second. The human eye may find that space dim or bright depending on the spectrum, and the PAR meter is blind to that distinction by design.
For solar work, the question is usually irradiance. A photovoltaic panel’s output is driven by the total power in the solar spectrum, not by the eye’s response and not by the photosynthesis band alone. A solar irradiance meter is built for that task. The relationship between irradiance and panel output is linear within the panel’s rated conditions, which is why an irradiance reading is the correct reference for a performance check.
The sensors inside the three meter types
The sensor determines what the instrument can honestly report. A lux meter uses a filtered photodiode with a colour correction filter designed to match the photopic curve. The filter is never perfect, and the mismatch is published as f1 prime, a percentage error against the standard curve. A good lux meter has a low f1 prime, but the reading is still a photometric approximation.
A PAR meter uses a quantum sensor, often a photodiode with an interference filter that passes 400 to 700 nanometres and a diffuser to collect light from the hemisphere. The calibration is in micromoles per square metre per second, and the sensor’s spectral response is designed to be flat across the band. The quality metric is the spectral mismatch error, which the listing may or may not publish.
A solar irradiance meter uses a photovoltaic detector, often mono-crystalline silicon, with a diffuser and a cosine correction filter. The cosine correction matters because the sensor must respond correctly to light arriving at an angle. Without it, a low sun angle produces an artificially low reading. The guide to choosing a solar irradiance meter explains why the sensor type and the cosine correction are the first specifications to check.
Some instruments combine functions. The Fluke IRR2-BT is a solar irradiance meter with a mono-crystalline sensor rated to 1400 W/m² and a built-in temperature sensor, plus an option for an external probe. The Triplett SPLT400 adds tilt and compass readings to irradiance and temperature, which helps with array placement. Neither instrument reports lux or PAR, and neither is intended to. They are built for the solar question, and their sensors are chosen for that spectral range.
What the numbers look like in practice
The table below shows the unit, the spectral weighting and the typical application for each measurement type. The figures in the example column are illustrative of the quantity, not readings from any instrument listed here.
| Measurement | Unit | Spectral weighting | Typical application | Example figure |
|---|---|---|---|---|
| Lux | lumens per square metre (lx) | Photopic curve, peaking near 555 nm | Workplace lighting, safety, photography | An office desk might read 500 lx |
| PAR | micromoles per square metre per second (µmol m⁻² s⁻¹) | Flat across 400 to 700 nm | Horticulture, plant science, aquarium lighting | Full summer sun might read around 2000 µmol m⁻² s⁻¹ |
| Irradiance | watts per square metre (W/m²) | Broadband, often 300 to 1100 nm for silicon sensors | Solar panel testing, meteorological reference | Full sun at sea level might read around 1000 W/m² |
The example figures are not interchangeable. The full sun values are quoted for the same clear sky, yet they sit in three different unit systems with three different weightings. Converting between them requires the solar spectrum and a set of integrals, not a single multiplier. That is the core of the incomparability.
Where a listing publishes a range, that range tells you what the instrument is built to see. The TENMARS solar power meter spans 0 to 1999 W/m² and also reports BTU/(ft²·h), a unit still used in some solar thermal work. Its accuracy is stated as plus or minus 10 W/m² or plus or minus 5 percent, which is a typical specification for a silicon photodiode irradiance meter. The listing does not publish a spectral response curve, so the user cannot check how far its sensitivity departs from the solar spectrum. That is a common gap in this class.
When the instrument you have is the wrong one
The most common mistake is using a lux meter to judge plant lighting because it is the instrument already on the shelf. The reading will be high under a white LED and low under a red and blue grow light, even when the PAR is the same. The grower then adjusts the light based on a number that does not track photosynthesis. The correct instrument is a PAR meter, and no conversion factor repairs the error.
The second mistake is using a PAR meter to check a solar installation. A PAR meter reports only the 400 to 700 nanometre band, which is a fraction of the solar spectrum that a photovoltaic panel converts. A panel responds to a broader range, including near infrared. A PAR reading of 400 µmol m⁻² s⁻¹ on a cloudy day does not tell you the irradiance in W/m², and the panel output will not track the PAR value in any simple way.
The third mistake is treating an irradiance meter as a general light meter. It is not. It reports total power in its spectral band, which is the right quantity for a solar panel and the wrong quantity for a lighting design. The Fluke 393FC clamp meter with irradiance function is an example of a tool that combines electrical measurement with an irradiance reading for solar work; it is not a substitute for a lux meter or a PAR meter, and its listing does not claim to be.
Where a product listing mixes categories, read the sensor description. A silicon photodiode with a cosine correction filter is an irradiance sensor. A filtered photodiode with a photopic correction is a lux sensor. A quantum sensor with a 400 to 700 nanometre bandpass is a PAR sensor. The label on the box matters less than the sensor and the unit.
Practical selection rules for each measurement
For workplace or domestic lighting, use a lux meter. It answers the question the standards ask. For plant growth, use a PAR meter and report the result in micromoles per square metre per second. For solar panel performance, use a solar irradiance meter and report watts per square metre. If you need more than one quantity, you need more than one instrument, or an instrument with separate calibrated sensors for each. The listings in this category are irradiance instruments; they do not publish lux or PAR calibration, and they should not be used for those purposes.
Temperature is a separate variable that affects both photovoltaic output and plant growth. Some irradiance meters include a temperature sensor, as the Fluke IRR2-BT does with its built-in sensor and optional external probe, and the Triplett SPLT400 reports temperature alongside irradiance. That combination is useful for solar work because panel output falls as cell temperature rises, so an irradiance reading without a temperature reading gives an incomplete picture. The meters with temperature probes page covers that combination in more detail.
For wireless or remote logging, the wireless solar irradiance meters page explains the trade-offs in sensor placement and data transmission. None of these instruments cross the boundary into photometry or quantum measurement. They stay in the radiometric domain, and that is the honest scope of the category.
The short answer to the comparison question
Lux, PAR and irradiance are not comparable because they are weighted differently, reported in different units and built for different decisions. A lux meter serves lighting design, a PAR meter serves plant science and a solar irradiance meter serves photovoltaic work. The instruments named here are irradiance meters; their listings do not publish lux or PAR calibration, and they should not be used as substitutes. Match the instrument to the question, and treat any single conversion factor between the three as valid only for one specific light spectrum.
Frequently Asked Questions
Can I convert lux to PAR with a fixed multiplier?
No. The multiplier depends on the spectrum of the light source. A factor derived under one lamp type will be wrong under another. To convert honestly you need the spectral power distribution and the two weighting curves, which is a calculation, not a single number.
Is a solar irradiance meter the same as a lux meter?
No. A solar irradiance meter reports watts per square metre across a broad spectral band, often 300 to 1100 nanometres for a silicon sensor. A lux meter reports lumens per square metre weighted to the human eye. They use different sensors and different calibrations, and their readings are not interchangeable.
Why does my PAR meter read zero under a white LED that looks bright?
It should not read zero if the LED emits in the 400 to 700 nanometre band. If it does, check that the sensor is not saturated or that the meter is set to the correct range. A PAR meter is blind to light outside that band, so an infrared or ultraviolet source will read zero by design.
Do the irradiance meters listed here measure PAR or lux?
Their listings do not publish lux or PAR calibration. They are radiometric instruments reporting watts per square metre, sometimes with BTU/(ft²·h) as an alternative unit. They should not be used for photometric or quantum measurements.
What does cosine correction mean on an irradiance sensor?
It means the sensor's response is corrected for the angle at which light arrives. Without cosine correction, a low sun angle produces a reading lower than the true irradiance on a horizontal plane. The correction is a diffuser and filter design that makes the sensor respond according to the cosine of the incidence angle.
If I only need one meter, which should I buy?
Buy the meter that matches the question you ask most often. For solar panel work, a solar irradiance meter with temperature measurement is the practical choice. For plant lighting, a PAR meter. For workplace lighting, a lux meter. No single instrument covers all three honestly, because the sensors and calibrations differ.