Scientific Horticulture Metrics


Hint: If you like to refresh the differences between PAR, PPF and PPFD – check the menu above about horticulture lighting metrics.


PAR (Photosynthetically Active Radiation) is the segment of the electromagnetic spectrum (400 to 700 nm) that is presumably used by plants for photosynthesis.

PPF (Photosynthetic Photon Flux) is a metric that describes how much PAR a lamp emits in total. PPF does not measure PAR at a specific location (e.g. your crop canopy), but describes the amount of light in the PAR range, coming out of the light-source, every second.

PPFD (Photosynthetic Photon Flux Density) measures the amount of photons within the PAR range at a defined distance (e.g. the amount of light delivered to your plant bed) every second. If you have a PAR meter, it is reporting PPFD (μmol/m²/s) measurements. Modern spectrometer can calculate PPFD and the similar W/m² and show you also the spectral allocation, enabling you to better understand if the lamp fits your recipes, plants, research direction.

To efficiently evaluating a horticulture lighting system, we need to break with some of the conventional metrics and values we are used to. Try to follow these approaches:


Lamp Power: Don’t use electrical watts to compare grow lights

LED Power: Don’t focus on the watts of the LEDs used in the lamps (1W, 3W, 5W, etc.)

Light: Don’t use lumens to compare grow lights, plant needs PAR

Promises: Don’t believe a magical one-fits-all growth spectrum

Measurements: Don’t only look at a single PPFD measurement directly under the lamp


It is important to start to use values and metric that reflect the needs of the plants, the former values came from ambient illumination, taking the human eye as reference. These values won’t fit plants. Over the last decades we became experienced to use Watt and Lumens to compare lamps. With the introduction of LED technology this is misleading and need to be revised.  



Lamp Power: Don’t Use Electrical Watts To Compare Grow Lights

Based on long learned habits, Watt or Watt/m² are common values to compare light sources. Due to the change of technology this can be a very misleading when attempting to compare LED horticulture lights.

In a classic scenario the input power was the valid reference in comparing lamps and the output was defined mainly by the lamp technology. Nowadays the output can vary up to 200% from LED based lamps with the same W input. As the PAR output is your relevant value you want to eliminate all the losses in electronics and LED design and straight measure the desired light. Having the PPF and the power consumption available you can understand what the long-term costs are and make a sophisticated decision. High quality components cost more but delivery better short and long term performance.

LED Power: Don’t focus on the watts of the LEDs used in the lamps (1W, 3W, 5W, etc.)

What LEDs do you use? Are they the latest from manufacturer xy? Do they have 1,3 or 5W? Fact is that high power LED are less efficient then smaller LED so you can get better efficiency by not using 5W LED. Over the years many different types of LED came up for very different applications. We are now in the position to select the right LED in terms of wavelength, power input and optical output to create a SUNlike spectrum that is flexible.

The single information of one of the many different LED is not helpful in comparing the growth lights, in the end the optical output in PAR plus maybe additional spectral bands outside PAR is. Make sure you focus on what is important in your daily business. Don’t get trailed away by fancy marketing.

Light: Don’t use lumens to compare grow lights, plant needs PAR

A lumen is a unit to describe of how bright (white) light appears to the human eye. The human eye is very insensitive to blue and red, directly inverse to plants. Therefore, since human vision is not correlated to photosynthetic growth, checking lumens is a bad fit for plants.

Promises: Don’t believe a magical one fits all growth spectrum

Growth lamps have developed a lot as well as the knowledge about plants requirements for light. It started off with blue and red lights, according to frequently referenced Chlorophyll A and B absorption spectrum chart. As it turned out plants are less picky and do appreciate a full spectrum, as they received over the last million years. As the research progresses more, more effects of light in the growth cycle of a plant are being discovered.

We are for sure not finished with researching. So we want to make sure that you have all the options with a sun like light which you can tune as needed. The latest research is evaluating the effects of UV and IR, being outside the PAR range yet still can change a plants color, taste, smell besides other aspects. So don’t forget about this option.

Measurements: Don’t only look at a single PPFD measurement directly under the lamp

To properly equip your phytotron or greenhouse you usually want to maintain homogeneous conditions over your growing area. To do that with light, it is important to understand how the lamp is distributing the light. While a fall off to the sides is normal there are various technics available to focus the light much more directly under the lamp. You need the distribution over the canopy to plan the fixtures and not a single great value (looks nice, I know) as sometimes provided.

We can create PAR maps according to your installation scenario and can also attempt to compare them to the existing lighting scheme.

Make sure you understand the light distribution and the best way is a PPFD map with the same measurements points and height for all fixtures to be tested.


You are looking for light to grow and develop your plants. In our opinion, you want to buy a lighting system that delivers the required amount of light to your plants with the best price-performance-ratio, while consuming the fewest electrical watts over the life time possible. Collet the following pieces on information: PPF, input watts, and PPFD maps for your intended coverage area. With this information, you can calculate: PPF/$, μmol/J, light distribution patterns, and uniformity levels and can be insured that you have control over your purchase decision.