The typical rating most growers are familiar with is the “lumen.” The definition of the lumen is the total light produced within the range of the human visual response. It tells us nothing about the distribution of that light energy over the spectrum, and most importantly, it doesn’t tell us how much is useful for plants.
The problem with lumens is especially pronounced when measuring light at the far ends of the human visual response curve. Consider three lamps—red, green and blue—each emitting the same number of watts of optical energy. The red and blue lamps would have much lower lumen ratings compared to the green lamp, simply because the human visual response is very low at red and blue, and highest at green. That’s why a high lumen rating does not necessarily make a lamp better suited to growing plants. Lumens, kelvin temp, CRI, CCT, CIE Chromaticity, all these terms are standard lighting measurements that tell us how the light looks to the HUMAN EYE. These terms do not represent the quality of a light spectrum nor do they take into consideration the McCree Curve. The McCree Curve represents the average photosynthetic response of plants to light energy. The McCree Curve, also known as the Plant Sensitivity Curve, begins at 360nm and extends to 760nm. This curve can be placed over a special distribution chart to see how well a light source can affect plant growth.
Similarly, light meters that measure in “lux” tell us very little about a lamp’s plant-growing power. The light sensors in lux meters have their own spectral response curves which may over- or under-measure light at various colors. This is why lux meters usually have different settings for “sunlight,” “fluorescent” and “incandescent” lamps. 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.
Plant biologists define light in the 400nm to 700nm spectral region as “photosynthetically available radiation,” or PAR. The unit for measuring PAR, micro-mols per second (μmol/s), indicates how many photons in this spectral range fall on the plant each second. Inexpensive PAR meters use sensors that respond over 400-700nm spectrum, and have their own sensitivity curves that require different calibration for sunlight, fluorescent and HID lighting.
To properly measure the amount of energy present for photosynthesis we must use a spectroradiometer. This instrument measures energy in watts at each specific wavelength over a range of wavelengths. A spectroradiometer can provide a direct comparison of each lamp’s ability to produce light that plants can use for photosynthesis. Spectroradiometers are expensive instruments, not usually found outside laboratories. (A more common instrument called a spectrometer can show relative light output over a spectral range, but does not measure energy in watts.)
Manufacturers should publish spectroradiometric data showing the energy per wavelength produced by their lamps. This data will allow growers to accurately compare different lighting technologies—whether HPS vs. LED or different LED horticultural lights—and know how much usable light their plants will receive from each system. PAR is essentially a measure of Energy per Area (this energy is in the 400nm - 700nm range). With that said the two types of PAR readings you will see are W/m2 and µmol/m2. Working with mols because they are universal and talk about a quantity of photons reaching a particular area, while working with Watts requires you to measure the energy of the wavelengths not the actual photons. As plants absorb photons, using mols makes more sense it's just another way. PAR was derived from the McCree Curve. It is a TOTAL count of light energy (in photons) between 400nm and 700nm. The PAR measurement is fast becoming a popular metric of the growing power of a light source. However, PAR measurement has two fundamental flaws. First off, the wavelengths between 380nm to 400nm and 700nm to 780nm are excluded. Secondly, all photons are weighed equally regardless of wavelength. The McCree Curve clearly shows plants respond to energy outside the 400nm to 700nm range. Furthermore, plants respond differently to energy within the PAR range. PAR does not distinguish which photons of light are present, rather it only counts the total of photons present in those nanometers.
That being said, PAR Watts (W/m2 = J/m2s) can be directly related to PPF (µmol/m2s) by equating Joules of energy to Moles of photons and the energy given off by a single photon at a particular wavelength.
To relate PAR to PPF you factor in the amount of time in which the sample was taken (or exposed to PAR). In an ideal situation you're testing for 1s, so your PPF equal your PAR, but this may not always be the case. Be careful when reading about a lights PAR output (if a company takes a cumulative PAR reading over 10 seconds and gets 10,000 PAR they could theoretically report a PAR of 10,000 and provide no more information which would be a very misleading number. In this case your PAR was a cumulative measure of photons per area rather than accounting for the time during which the reading was taken (making a PPF reading of 1000 PAR/s). PPF is a measure of the quantity of photons moving through an area during a particular period of time, PAR is simply a measure of the photons present in a particular area, PPF is the better unit for determining light output. The Sun gives off a PAR reading of 2000 µmol/m2 (or in this situation one could say that the sun gives off a PPF reading of 2000 µmol/m2s). Be careful when lighting your grow space. If you go too far beyond 2000 µmol/m2 you will run into other issues with your plant.
It has been asked how to relate PAR/PPF to Lux/Lumen. This is not a very valid conversion. Lumen is a measure of visible light perceived by the human eye, not an actual measurement of the total visible light available, while PAR is a measurement of the total photons available. Lux to Lumen is simply taking the luminous flux (lumens; Lm) and accounting for the area it is lighting (Lm/m).
Talking about HPS or MH lights (or even CFLs) it is generally advisable to deal with the lumens/lux measurements which have become a standard. PAR of 1000 from 610 - 680 nm, the PAR rating on most standard lighting, is irrelevant until the full spectrum and wavelengths can be met. With an LED however you are only producing specific wavelengths of light. Your standard Red/Blue LED grow light is producing Red light somewhere from 630 to 660 (this is pretty standard) and your Blue light somewhere from 440 to 480 (this is pretty standard). Don't be fooled by hype on LEDs exhibiting "Multiple wavelengths!" a 3 band light system of Red/Blue/Orange produces stupendous yields. There is just simply not enough research done yet on broadband LED lights to make it worth spending the extra money when the increase in yields are not statistically significant. When buying your standard LED light these days they have a very high ratio of Red : Blue, so it is advised to pick up an additional Blue LED supplemental lighting, critical for vegetative growth and still indeed valuable during any flowering cycle.
In general use Lux reading over Lumen reading, and use PPF reading over PAR reading (although most PAR meters will give you PAR Watts, which you then simply need to convert into µmol units).
Plants cannot photosynthesize without the proper amount and type of light. A plant’s leaves act as solar collectors absorbing light energy. Photosynthesis occurs when light waves generate electrons. The electrons trigger the Chlorophyll in the plant’s leaves when combined with water to start a chemical reaction. Carbon dioxide atoms convert into starch, which produces energy for the plant, and Oxygen. Diffuse light increases plant energy production because the light bends around corners to reach the lower leaves, not just the upper canopy. The typical rating most growers are familiar with is the “lumen.” The definition of the lumen is the total light produced within the range of the human visual response. It tells us nothing about the distribution of that light energy over the spectrum, and most importantly, it doesn’t tell us how much is useful for plants.
With a majority of cases, lumens or foot candles measure light intensity. Light bulbs, grow lights, and natural light meters use these measures almost exclusively. These measures describe the wave lengths visible to the human eye. There is not a direct correlation between Lumens and the light used by plants because the human eye cannot see all light. PAR (Photosynthetic Available Radiation) light values measure the light used by plants for photosynthesis. Direct or diffuse light do not have different PAR values. This means that our eyes perceive differences in lumens between direct and diffuse light. Diffuse light appears dimmer to us even though total light transmission is not decreased.
White light comprises all colors of the rainbow. Each color in the spectrum represents a specific wave length. Of the visible spectrum, violet wave lengths are the shortest while red are the longest. Ultraviolet rays are too short for the human eye to see while infrared rays are too long. The most visible rays to humans are those in the middle of the color chart, yellow and green. In contrast, light on the opposite ends of the spectrum, blue light and red light, are generally the most productive for plants. The light that generally produces the most efficient photosynthesis extends beyond the visible spectrum on both sides of the color chart.
Illumination for plants, also known as "irradiance", is sometimes measured in PAR watts per square meter (W/m2). Another means of measuring light quantity for plant growth involves discrete units of quantum flux in the PAR region called "photons". Photon flux is commonly measured in units of micromoles per square meter per second (µmoles/m2/s), where 1 mole of photons = 6.022 x 1023 photons.
This is an objective measure since it directly indicates how much light energy is available for plants to use in photosynthesis. However, lamp manufacturers typically rate their lamps in lumens, a measure of light in the spectrum visible to humans. Moreover, most lighting engineers measure lighting levels in lumens per square meter (lux) or per square foot (foot-candles). Since the spectral sensitivities of plants and humans are quite different, there is no direct method of converting the units without evaluating the full range of spectral characteristics for a given light source.
Under controlled conditions, Initial Reference PAR Value refers to the lamp light output after 100 hours of powered operation. Due to variations in systems and service conditions (in particular the burning cycle and the operating system), actual lamp performance can vary from the Initial Reference
Bulb. Color Temp. Initial Lumens. PAR MMOLES/SEC
HPS PSL 400W Bulb 2.1K 56,500 725
HPS PSL 600W Bulb 2.1K 90,000 1100-1150
HPS PSL 750W Bulb 2.1K 104,000 1350-1415
Here is where supplemental light can help your indoor garden. Those familiar with greenhouses understand diffuse light and par ratings in such situations. Trying to replicate nature inside is the base fundamental of growing indoors but we want to take that steps farther as well by increasing the PAR value our plants are receiving on each canopy tier. Light manipulation can increase your PAR without adding bulbs. In a fully enclosed indoor garden you may want to add supplemental bulbs in addition to your main bulbs to up the PAR value for your plants, thusly allowing the lower canopy to absorb the full spectrum. Keeping your par range in accordance with your room size is key when playing with PAR as you truly do not need as high of a rating as you think you may.
Staying on top of bulb and light maintenance will keep your PAR and lumens rating within the values your plants need for proper size and shape.
