When comparing horticultural light bulbs we should know if we are wanting color spectrums in the PAR range balanced for a vegetative or a flowering growth. Then we should focus on the growth light’s quantity of photons (PPFD) emitted and reaching the plant from the bulb at a certain distance and finally how many of those emitted photon reaches the plant per day (DLI) in the applied photo-period.

Confused? Then read on to understand these concepts perfectly.

Watts in Horticulture

Most of us are accustomed to referring to Watts in regard to the strength of a light bulb, when wattage means nothing in terms of light output . Wattage measures the consumption of electricity. But as we grew up with the incandescent light bulb, we had a relative understanding by knowing the wattage. A 20 watt incandescent gives half the light as a 40 watt, for example. However now, we are exposed to many different bulb types. Besides, plants respond to light differently than our eyes. We are into indoor horticulture and have come to want to compare other classes of lighting fixtures such as florescent, Hid, HPS, MH and LED. So comparing wattage doesn’t work for us anymore. It wont even work comparing LED to a LED. So lets get educated.

Lumens in Horticulture

Lumens have nothing to do with horticultural growth. A company using this term for horticultural topics… should not be. Regrettably however, the bulb rating many growers are familiar with is “lumen”. A lumen is the total light produced within the range of the human eye right next to the source, i.e. the light bulb. Plants have a completely different sensitivity to light colors than humans. Lumens tells us nothing about the distribution of that light energy over the spectrum. Neither does it tell us how much is useful for plants. Lumens are only interesting if we are talking about home, work, store front or parking light illumination, not horticulture.

Lux in Horticulture

Similarly, lux tells us very little about a lamp’s plant-growing power.  Unlike lumen, which tells us how much human eye-light is GIVEN off at the bulb, lux tells us how much human eye light is RECEIVED at at any particular location. A bulb giving off 1000 lumins is fine but how much light do you receive from it one meter away or two meters or even one kilometer? What a lux will tell you is how much light is received at any one place.  Again, because lux meters are meant for measuring the amount of light usable by humans, they don’t tell us anything about how plants will respond.

So in summary the difference between the units Lumen and Lux is that Lux takes into account the area over which the luminous flux is spread at a fixed distance. Lumens is the amount of light coming from a light source irrespective of the distance to any object.

nm in Horticultural Lighting

A nm is an abbreviation for nanometers. In lighting it is a distance measurement. A nanometer is a billionth of a meter. It tells us the width of the wave length signifying what color it is in the global light spectrum. Plants require the spectrum in between 400-700 nm. This range is often called growth light, however the term “PAR” is a bit more descriptive and correct.

PAR in Horticulture

Plants have a completely different sensitivity to light colors than humans. The only part of the global radiation spectrum which can be used by a plant for its photosynthesis is between 400-700 nm, this is called the PAR-light (Photosynthetically Available Radiation). The amount of photons in the PAR region are called growth light, which are measured in micromol (μmol). A plant in the vegetative growth stage uses mostly the spectrum in the 450nm range while the flowering stage uses more 650nm.

Relative Quantum Efficiency (RQE)

If you have Googled around for LED horticultural lighting, I am sure you would have seen all the fuse about “growth light Blue” (450nm) and “flowering light red” (650nm). In very general terms this is correct. But it’s not that simple. The PAR range offers much of the light that plants need, but for optimal growth result, UV light (280 – 400 nm) and/or far-red light (700-800 nm) might be important. For example far-red is critical for the flowering of many plants. A lot of research has been conducted regarding the optimal light spectrum of different plants. During this time a new word has been coined, the Relative Quantum Efficiency curve. It considers the photosynthetic rate of the plant (by measuring CO2 uptake), the energy of light at different wavelengths and the plants absorption of light i.e. what stays in the leaf and is not reflected away or transmitted through it. However, producing a plant which is sell-able is not only about photosynthesis. Also other aspects like shape, flowering, color of leaves, color of flowers, taste, smell, root development, etc. are important to have a high quality plant.

The relative quantum efficiency curve (RQE, white line in graph) is one of the best approximations of an “optimal photosynthesis” spectrum. The graph also shows the spectrum of an HPS lamp (yellow line) and the spectrum of a typical narrow bandwidth red and blue spectrum LED light (red line). Nor the HPS or the narrow red-blue led do a good job in matching the RQE

The white line in graph is the RQE. It is the “optimal photosynthesis” spectrum. The graph also shows the spectrum of an HPS lamp (yellow line) and the spectrum of a typical narrow bandwidth red and blue spectrum LED light (red line).

Graph courtesy Valoya- Plants, Production and Products A Horticultural Lighting Guide

Photosynthetic Photon Flux Density (PPFD)

Photosynthetic Photon Flux Density(PPFD) goes by various names, Quantum Light or simply Photon Flux. In all cases, it is a unit of measure to express the light quantity relating specifically to photosynthesis in units called micro-moles. The PPFD is expressed in micro-moles of photons per meter squared, per second (μmo/ms/sec). You could say, the PPDF is to PAR as Lux is to lumens. The prior is horticultural specific, while the latter, refers to the capacities of the human eye. A mole is a quantity, an amount, a very large amount. It is a number equal to the number of atoms in 0.012 kg of carbon-12. A micromole is one millionth of a mole.

The table below provides a reference to light levels for a sampling of plants and sun conditions

  • House Plants 30 to 200 PPFD
  • Leafy Crop Plants (i.e. Lettuce & Basil) 200 to 600 PPFD
  • Tomatoes & Other Fruit Crops 400 to 1000 PPFD
  • Full Sunlight in Summer @ Noon 2000 PPFD
  • Full Sunlight in Winter @ Noon 1200 PPFD

Meters that measure the PPFD are called “quantum meters“. A quantum is the amount of energy carried by a single photon. A quantum meter gives you the entire wavelength measurements of the total number of photons/second as well as the YIELD PHOTON FLUX (YPF) of the particular lamp being measured. You can find all the right equipment for horticultural light by downloading this PDF file from Spectrum Technologies Inc. You will be amazed. I was.

Yield Photon Flux (YPF)

Photosynthesis is driven by photon flux rather than the general overall energy flux. But remember, not all absorbed photons yield equal amounts of photosynthesis. So, two measurements of photosynthetically active radiation have emerged. These are photosynthetic photon flux (PPF), which values all photons from 400 to 700 nm equally, and yield photon flux (YPF), which weights photons in the range from 360 to 760 nm, only taking into account the plant photosynthetic response.

Daily Light Integral (DLI)

An efficient lamp for plant growth must convert as much electrical energy as possible into PAR energy. The term ‘daily light integral’ (DLI) or Light Quantity, refers to the number of light particles, or photons, received during one day in the PAR region of 400-700 nm. The DLI, measured in mol/m2/day, can have a dramatic difference on root and shoot growth, root development of cuttings, and finish plant quality. These qualities include stem thickness, plant branching and flower number. The average DLI received outdoors ranges roughly from 5 to 60 mol/m2/day and depends on location and time of year. For potted crops 10-12 mol/m2/day is normal. Plant quality generally increases as the average DLI increases. The DLI is important to monitor in your green house so as to know you are applying the right amount of light to your plants.


Photoperiodic is the interval in a 24-hour period during which a plant is exposed to light. For example the photoperiodic of a some well known flowering plants is 12 hrs. while  the photoperiodic of a normal plant in vegetative stage is 16 hr.

Using Plant Led Light Terminology

Just for fun I have made up a sentence using all the words relating to plant light:

” The OST LED vegetative growth LED fixture’s average growth light is 200 micromol (μmol) in the PAR range of 456nm with a 200 PPFD. At a distance of 18″ from the plant and a 16hr. photoperiod you have a DLI of 10mol/m2/day.”