Measurement Values Explained

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

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

PAR

Photosynthetically Active Radiation (PAR) describes the wavelengths (i.e. colors) of light that plants use for photosynthesis. PAR comprises photons with wavelengths between 400 and 700 nanometers (nm).

ePAR

Extended Photosynthetically Active Radiation (ePAR) expands the PAR range by including far-red photons (i.e. light) up to 750 nm. When the ePAR switch is turned on, the PPFD and DLI readings on the main screen automatically change to their extended version (ePPFD and eDLI).

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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, the app is able to extrapolate ePAR values.

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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

The following types of measurements are available in the app. You can swap between them 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 typically measured in micromoles of photons (i.e. light particles) per square meter per second (μmol/m²/s). This is what PAR meters or quantum meters usually 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 (e.g. Sun or LED FS + Red).

DLI

The Daily Light Integral (DLI) measures the total amount of photosynthetically usable light a plant receives over an entire day, assuming the light intensity remains constant for the configured photoperiod. Set the photoperiod (i.e. amount of hours your light is on) to get the correct DLI. The DLI is typically measured in moles of photons per square meter per day (mol/m²/d).

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Note that the DLI reading won’t make sense when measuring sunlight as the sun’s intensity changes during the day.

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 proportion of far-red light (e.g. Sun or LED FS + Red).

Illuminance

Illuminance is measured in lux or foot-candle (fc) and describes how much light, as perceived by humans, reaches a surface. The total light output of a lamp (i.e. its radiant flux) is measured in lumen (lm). Lux (lm/m²) is how many lumen reach a square meter whilst foot-candle (lm/ft²) describes lumen per square foot.

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Note that lux and fc values will change depending on how close you get to a lamp, whilst the lumen value only changes when changing the power (e.g. by dimming) of the lamp.

Illuminance should not be used when measuring light for plants. Learn 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 2700K describes a warm, yellowish light. A higher CCT such as 6500K 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 (2000-3000K) at sunrise and sunset, to cooler white light (5500-6500K) at midday. This natural cycle occurs due to changes in the sun's position and the amount of atmospheric gasses its light travels through.

Other Grow Lighting Metrics

You might have heard of other plant lighting-related metrics such as PPE, PPF, YPF, YPFD, or the McCree curve. Head over to our blog and have a look at the full grow light metrics glossary:

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