Measurement Values Explained

Learn about PAR, PPFD, DLI, Illuminance, and CCT

Photone allows you to measure different properties of light. All measurements (except CCT) are light intensity measurements. This means they indicate how much light reaches a surface at any given time.

PAR

Photosynthetically Active Radiation (PAR) is the part of the light spectrum that plants use for photosynthesis. It covers light wavelengths between 400 and 700 nanometers (nm) and is measured as PPFD.

ePAR

Extended PAR (ePAR) expands the PAR range from 700 to 750 nm by including far-red light. When the ePAR setting is enabled, the the PPFD and DLI values on the main screen automatically switch to their extended equivalents: ePPFD and eDLI.

ℹ️

How it works: Your device’s camera is only able to capture photons within 400 to 700 nm. But since the relative amount of far-red photons in the selected light source is known, Photone is able to extrapolate ePAR values.

⚠️

Unfortunately, ePAR readings are not available when using the built-in ambient light sensor (ALS). ALS sensors are designed for illuminance measurements and thus lack sensitivity in the red range.

Measurement Values Available in Photone

You can swap between the following measurements by swiping left or right on the main screen:

PPFD (PAR Meter)

Photosynthetic Photon Flux Density (PPFD) describes how much photosynthetically usable light your plant receives at any given moment. PPFD is measured in micromoles of photons per square meter per second (μmol/m²/s). PPFD is the most relevant measurement in plant lighting and what all PAR meters or quantum meters display.

ePPFD

Extended Photosynthetic Photon Flux Density (ePPFD) is similar to PPFD but accounts for the ePAR range instead of the PAR range. The unit remains the same (μmol/m²/s), but measurement values will be slightly higher if you’ve selected a light source with a high proportion of far-red light such as Sun or LED FS + Red.

DLI

The projected Daily Light Integral (DLI) estimates the total amount of PAR your plants would receive over an entire day if the current light intensity remained constant throughout the configured photoperiod (the number of hours your lamp is on). Adjust the photoperiod using the + and − buttons.

🌞

When measuring sunlight, light intensity changes continuously throughout the day, so the projected value is not meaningful. To determine the actual DLI under sunlight, use the Logging tool to record measurements from sunrise to sunset.

eDLI

The extended daily light integral (eDLI) is similar to the DLI but accounts for the ePAR range instead of the PAR range. The unit remains the same (mol/m²/d), but measurement values will be slightly higher if you’ve selected a light source with a high high proportion of far-red light such as Sun or LED FS + Red.

Illuminance

Illuminance describes the amount of visible light reaching a surface as perceived by the human eye. It is measured in lux (lumen per m²) or foot-candles (lumen per ft²). Lumen describe a light sources total visible light output.

Because illuminance is based on human vision rather than plant response to light, it is not suitable for measuring light for plants. Learn more about why on the Photone blog:

CCT

Correlated color temperature (CCT) refers to the color appearance of a light source and is measured in Kelvin (K). In other words, the CCT describes how "warm" or "cool" a light appears. For example, a lower CCT like 2700 K describes a warm, yellowish light. A higher CCT such as 6500 K describes a cool, bluish light.

The CCT is an important factor in plant lighting as it can influence a plant’s properties during various growth stages such as flowering and fruit production, leaf size and thickness, and the overall plant shape and structure.

The sun's CCT varies throughout the day, ranging from warm reddish-orange light around 3000 K at sunrise and sunset, to cooler white light around 6500 K at midday. This natural cycle occurs due to changes in the sun's position and the amount of atmospheric gasses its light travels through.

All Plant Lighting Metrics Explained

Ever felt like plant light metrics are just a pile of random letters? PAR, PPFD, DLI — and if that weren't enough, there's PPF, PPE, YPF, YPFD, McCree, and even more. They all sound confusing until someone explains them properly — so we broke every one down in plain English. Pop over to our complete grow light metrics glossary and grow with confidence:

Back