Indoor growers would have fantastic yields from grow lights if they knew which lights perform better for their crop. LED grow lights allow fine tuning of light spectrum specific to the needs of plants to develop through different stages. But, if the right lights are not chosen yields will be too low. First time buyers often find it confusing how to determine light output from grow lights. Unlike traditional bulbs which are measured in watts, grow lights performance is typically determined using Lumens / Lux or PAR (Photosynthetic Active Radiation). Growers must be familiar with these terms to evaluate their horticulture lighting system.


Lumen is defined as a measure of luminous flux or brightness. It determines the total quantity of light, as perceived by the human eye, emitted by a light over time. The lumen measurement includes the wavelengths of light perceived by the human eye which is between 400nm to 700nm and excludes infrared and UV lights. Lux is similar to lumens but defined as lumens per square meter. Lux takes into account the space over which the light intensity is spread and because of this it is a better measurement of brightness. In indoor growing, an optimal amount of exposure to lumens is vital for healthy plants. But at the same time, it is important to know that the plants do not use all of the lumens produced by the light. For instance, the yellow light of visible spectrum is used less by plants for photosynthesis. Lights with higher lumen rating may appear brighter to the human eye, but they may not necessarily be effective in plant growth. 

PAR (Photosynthetic Active Radiation) or PPF

PAR describe the wavelengths of light between 400nm and 700nm which are used by plants for photosynthesis. Since the main purpose of using supplemental light in the greenhouse is to augment photosynthesis, light sources are often rated on PAR. It indicates the light energy available for plants to use in photosynthesis which means how many photons in the spectral range fall on the plant each second. Plants do not utilize all of the light emitted by the source but only takes a part of radiation for photosynthesis. Specifically, plants respond well to blue and red lights present in the visible spectra. While blue is helpful in vegetative growth and root development, red helps in stem growth, flowering and fruit production. Hence, checking the PAR outputs of light sources is important because it indicates the useful light spectra that influence the yield of indoor garden.

Which is a better indicator for plants?

From a plant’s perspective, they require light that is useful for photosynthesis but not the one that is brighter for humans. The human eye has peak sensitivity for yellow and green wavelengths but these colors are not as effective in plant growth. Traditional bulbs may have high light intensity, i.e lumens, but may not have high PAR or PPF values. This is one of the reasons why traditional bulbs are poor choices for indoor plant cultivation. While choosing better grow light for plants, higher value is given to wavelengths that promote plant growth and development. That means, in grow light selection, higher preference is given to PAR rating than lumen/lux value.