Lighting Figures – 6 Metrics You Should Know

This page gives an overview of the most important measurement units related to light for plants. When researching grow lights, one is confronted with countless metrics and abbreviations – Watt, PAR, PPFD, PPF, etc. are just some of the figures used to compare lighting systems for horticulture. We have already discussed in the blog why lumens are not used as a benchmark for plant lamps.

Understandably, customers and interested parties find it difficult to compare different products and put them in the right context. Therefore this list should show which metrics are relevant for plant lighting and what they are used for.

Important Grow Light Metrics


Photosynthetically Active Radiation – all wavelenghts between 400 – 700 nm are weighted equally


Daily Light Integral (of PAR)


Photosynthetic Photon Flux


Efficiency – Photosynthetic Photon Flux per Watt of electrical energy


Photosynthetic Photon Flux Density on a cultivation area


Weighted photon flux (Yield Photon Flux) according to McCree’s action spectrum of photosynthesis

Light plants can use: Photosynthetically Active Radiation (PAR)

It is generally known that the main part of photosynthesis is excited by light that lies in the spectral range between 400 and 700 nm, i.e. precisely the visible part of the light spectrum. This is exactly what photosynthetically active radiation says: All photons falling within this range are evaluated equally. Light outside this range is meaningless for the metric PAR.

In contrast, the measuring unit lumen represents the sensitivity of the human eye. The graph clearly shows why lumen values hardly serve as a standard for plant lighting.

PAR im Vergleich zu Lumen
Grafik DLI Richtwerte verschiedener Pflanzenarten

Quantity of light for plants: Daily Light Integral– DLI

The necessary intensity is derived from the total light requirement of the plant throughout the day. An index for the different lighting requirements is the daylight integral, DLI (Daily Light Integral). It describes how many photons hit a certain surface in 24 hours.

For different plant species it is again known how much light they need or can sensibly use in the course of a day.


Calculate Daily Light Integral (DLI):

PPFD in µmol/m²s [Number of photons per second per square meter] * 3600 [number of seconds in an hour] * x  [number of hours of light per day] / 1.000.000 = DLI in mol per day

Quantity of PAR light grow lights produce – PPF

This information should not be missing in a grow light’s spec sheet: How many photons are emitted in the range of photosynthetically active radiation? That is the photosynthetic photon flux (PPF). It is the most important metric for comparing different light sources, because it is determined in a standardized test setup (integrating sphere). Therefore one can objectively judge how good the lamp is at generating light in the PAR space between 400 and 700 nm.

Efficiency  – PPF/W

In terms of photon flux, it is possible to objectively assess how efficient and economical a lighting system for plants is. To do this, the PPF is divided by the electrical power consumed. PPF/W is expressed in µmol/J.

Efficiency of various lighting technologies

Floureszent lamps: ca. 1,2 µmol/J
Latest HPS 600W: ca. 1,8 µmol/J
Latest HPS 1000W DE: ca. 2,1 µmol/J
Latest LED: über 3 µmol/J

What really hits the canopy – PPFD

These theoretical values are very good for finding out what a light source emits in photons and how efficiently it works. Meanwhile, what is actually received by the plants cannot be determined with theoretical values, but depends on the individual usage scenario.

The so-called photosynthetic photon flux density (PPFD) indicates how many photons of the PAR area hit a square meter. In doing so, one should consider the needs of the plants (see DLI) and strive for a distribution that is as even as possible.

Issues with PPFD as a benchmark

PPFD is actually more relevant in practical use than pure output, i.e. PPF. In fact, it is easy to measure how much PAR reaches the plants. However, the PPFD can only be determined in relation to the specific environment. In this context, the distance to the plants (or to the sensor), light guiding ( any reflectors or optics), the reflection of the walls and the bottom surface, etc. play an enormous role. Even the temperature in the room has a certain influence on the measured values. Since these factors are not standardized, the measurement is unique in each test scenario. Comparability across manufacturers is therefore by no means guaranteed. To avoid misconceptions, we endeavor to make our own PPFD tests as transperent as possible.

Darstellung der PPFD von 10 FLUXengines auf 1,44 m²

How plants deal with light quality – spectrum and YPF


As diverse as the flora and fauna on our planet is, so different are the needs of plants. Therefore, the required light quality differs between different species and stages of development. There is a consensus, however, that healthy plant development is only possible under a broadband spectrum that covers the PAR range. In the growth phase an increased proportion of blue is advantageous (e.g. from 4000K with white light), in the flowering phase an increased amount of red (e.g. up to 3000K with white light). In addition, there are various wavelengths which influence photomorphogenesis (development of the growth pattern) and the production of secondary plant compounds.


It is disputed which wavelengths contribute most effectively to photosynthesis. In 1973 McCree determined the action spectrum of photosynthesis on the basis of 22 species. The measure YPF (Yield Photon Flux) weights the light of different wavelengths by this action spectrum, also called McCree curve. However, since the measurements have fallen under criticism, this metric should be used with caution. McCree had examined individual leaves in isolation, not the plant as a whole. In addition, the measurements were made under relatively low lighting conditions.

Grafik Photosyntheserate, PAR und McCree Wirkspektrum