Learn About Light

Learn About Light

What is the best yardstick for measuring lighting for plants?

Here are the very simple rules to follow:

DO NOT use watts to compare lights!
Use Lumens or LUX if you are buying a light for human use
Use PPF or PPFD if you are buying lights for plantsl
Compare the spectrum as well as brightness
   Choosing a Grow Light seems simple.
   Just get a nice, bright light. But, how bright is bright?
   And, will my plants like it? 
   Most grow lights are measured by wattage.  This is an OK yardstick if all the grow lights are the same type, and if all the lights had the same light output. But that is not the right way for comparing COB LED lights to HPS / Metal Halide lights!
    Many manufacturers are comparing lights using the wrong yardstick. Wattage is a measure of electricity, not light, so we cannot compare lights by power consumption unless we are specifically interested in measuring electrical usage and lighting efficacy..
   What is bright to my eyes may be very dim to my plant. As you know by looking at a rainbow, light visible to the human eye can be separated in colors:  Red, Orange, Yellow, Green, Blue, Indigo, and Violet.
   We measure these colors in wavelength (nanometers). We can also measure the amount of light by a specific wavelength.
   Sunlight is a mixture all wavelengths that humans see as white light.
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   Humans can see colors from deep violet to deep red.

But the human eye favors wavelengths around the color green (555 nanometers in wavelength), so the light meters we use to determine the luminosity of sunlight for humans is biased toward the color green. The colors deep red and deep blue are not weighted as much as green. A specific measuring system was developed to measure light mesured in Lumens and Lux.

   A Lumen is a unit of measure of brightness of a light at a single point as seen by a human.

   LUX is the unit of measurement of Lumens over an area of one square meter.

   The plants that we care about live in sunlight. Plants cannot “see”. They use the energy of sunlight to power photosynthesis which is the process that plants use to convert carbon dioxide and water to make food for the plant.

   Plants are able to use all wavelengths from 400 nanometers to 700 nanometers, but some wavelengths are more effective than others in photosynthesis. This range is call PAR or Photosynthetic Active Range.

   Early academic studies on the use of artificial light for indoor plant culture determined that the blue and red portions of the spectrum were most important for healthy indoor plants. This lead to the use of Blue and Red (Burple) LEDs only in horticultural light fixtures.

   Subsequent studies, particularly the NASA Study on Sole-Source Lighting for Controlled-Environment Agriculture showed that a full spectrum light source across the PAR is required for vigorous, healthy plants.

   Chlorophyll is the chemical in a leaf that is primarily used in photosynthesis, and it best captures red and blue light.  A leaf reflects green and it also allows some green to penetrate through the cell structure in the leaf.  Other chemicals in the photosynthesis process use red, yellow, and orange as well as a little green light.

   Since plants use green the least, a measurement system like Lumens (which is weighted toward green) does not provide a good yardstick.  We need a yardstick which can measure light in the same way that a plant leaf uses light if we want the best light for the plant.

   Photosynthetic Active Radiation (PAR) measured in micro moles is this new yardstick.  It is about the same wavelength range as seen by the human eye. But, PAR light meters are calibrated to a plant’s requirements, not to the human eye.

   PAR light meters all light between 400 and 700 nanometers.

   The output of a PAR meter is called Photosynthetic Photon Flux (PPF). The unit of measure is micro moles per second.  (Basically, count the number of photons per second at one spot).

   Since plants need light over the entire plant, not just at one spot, we need to be able to measure light over an area. Photosynthetic Photon Flux Density (PPFD) is the number of photons hitting a square meter per second. It is measured in micro moles per square meter per second.

   Unfortunately, typical LED grow lights and commonly available HIDS / HPS grow lights do not provide a full spectrum light source.  These lighting sources are insufficient for full plant growth without sunlight.

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   LED grow lights are cooler and more electrically efficient than HPS / HIDS units. But, older style LED grow lights used individual LEDs in both Blue and Red, and cannot deliver full spectrum light.

   While these units delivered some of the spectrum, they lacked the green, yellow, and orange shades necessary for light penetration below the top-most leaves on a plant with dense foliage.

   Full spectrum light is now available from the new Chip On Board (COB) LED grow lights with reduced heat impact and increased electrical efficiency.

   This is the full spectrum output of the Broomstraw Farm Grow light using the Cree CXB3590 Chip On Board  LED:

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As depicted, the CXB3590 COB LED minimizes harmful UV-A and UV-B radiation while providing an excellent approximation of sunlight!

How is light measured?

   Light may be measured as brightness at the source of the light (Lumens or PPF) or by how much light is available for a specific area at a specific distance from the light (Lux or PPFD).

   Don't try to judge light brightness by eye!  The human eye is wonderful for vision, but it is not a calibrated measurement tool.  Also, you never want to look directly into a bright light.

   Instead, rely on scientific measurements to compare lights.  You will notice that many manufacturers do not include the scientific measurements. Be wary of products that do not have measured light output.

Here is the full details on Lumens, LUX, PAR, PPF, and PPFD from Wikipedia:

"The lumen (symbol: lm) is the SI derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source. Luminous flux differs from power (radiant flux) in that radiant flux includes all electromagnetic waves emitted, while luminous flux is weighted according to a model (a "luminosity function") of the human eye's sensitivity to various wavelengths. Lumens are related to lux in that one lux is one lumen per square meter. "

"The lux (symbol: lx) is the SI derived unit of illuminance and luminous emittance, measuring luminous flux per unit area.[1]It is equal to one lumen per square metre. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. It is analogous to the radiometric unit watt per square metre, but with the power at each wavelength weighted according to the luminosity function, a standardized model of human visual brightness perception. In English, "lux" is used as both the singular and plural form." 

"Photosynthetically active radiation, often abbreviated PAR, designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more or less with the range of light visible to the human eye. "

"Photosynthesis is a quantum process and the chemical reactions of photosynthesis are more dependent on the number of photons than the energy contained in the photons. Therefore, plant biologists often quantify PAR using the number of photons in the 400-700 nm range received by a surface for a specified amount of time, or the Photosynthetic Photon Flux Density (PPFD).[5] "

   The problem is that many, if not most, suppliers do not list these critical measurements.  Inside, they compare their LED lights to older HPS / HIDS light equivalent as measured in electrical watts.  While this may be of value to a grower who is replacing older lights, it does not provide enough information to clearly compare grow lights on a measurable basis. 

   While comparing electrical consumption as measured in electrical Watts (W) or Kilowatts (KW) is great for measuring lighting efficiency, these measures cannot be used to compare the visible light output of a unit.

 Here are the very simple rules to follow:

  • Do not use Watts to compare lights
  • Use Lumens or LUX if you are buying a light for human use
  • Use PPF or PPFD if you are buying a light for plants
  • Do your research and compare lights based on science.


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