(status. last rev. under further full? construction October 2019)
Ah, all those terrible myths and simplistic concepts around Grow Lights.
In statu nascendi !!
Let's start with a simple fact: Grow lights should help and enhance the growth of plants, indoors or outdoors if needed, nothing more nothing less. This means that the lights need to provide for a sufficient light yield output and of the correct wavelengths that suit plants. But first things first so ....
First let's have a short talk about natural light and the human perception of it. (examples and info from source: https://maxmax.com/aVisionFAQ.htm)
Full natural light is composed of different wavelengths which we perceive as "colors". Those bundled color make us see white light, but a rainbow clearly shows the composing wavelengths. The wavelengths are defined in "nanometer" (nm) and one nanometer is a billionth of a meter. Sometimes it is also defined in "degree Kelvin" which corresponds to its "heat".
Most common lamps have their wavelength or "color" expressed in "°kelvin" aka a "warm white" is 2700-3000 °K which is mostly also taken up in the commercial type number ie a lamp number 827 = 2700 °K. Cool white = #840 = 4000°K etc.
Humans are able to see from about 400nm (violet) to 750nm (red). Below 400nm is ultraviolet light. Above 750nm is infrared.
The human eye is not equally sensitive to all colors. We see color best during the day when we are using the cone sensors in our retinas. Below is a graph showing how the eye sensors are more sensitive at some wavelengths than others. The eye is most sensitive in the green - yellow portion at around 550nm at night, which is why the most efficient street lights are the "orange" sodium lights.
All this to explain that "human vision" is completely different from what plants "see" as useful light to grow and thrive. See some graphs below ...
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Back to artificial lights now.
It is very unfortunate but it is impossible to give usable info about "synthetic light" with out going technical, so dig your heels in!
So, "Grow lights" also need two major components: wavelength (color) and intensity ie how much light they can produce.
Lamps have a light intensity, look at it, you can just see it. Unfortunately the human eye is about the worst detecting system for evaluating light as it works like a camera lens, dimming light if too strong or otherwise and then the perception in the brain is doing the rest. Bummer.
So, the industry has set a scientific "measurement" system to classify lamp light intensities so people know what sort of lamp they buy. At the same time the lamp "color" is also mentioned as not all lamps, by design, radiate the same color of light, commonly referred to as "wavelength". That is where the warm white, cool white, daylight etc comes from. Unfortunately these "settings" are focused on "human" perception.
Basically this is where the entity "Lumen" is coming from, which, let us further complicate the things, is not the unit of light intensity (which is "candela") but the result of the unit candela radiating in a spherical environment at a set distance! LOL!
To set out a common standard the general concept is to set a surface size that is illuminated by x Lumen. Typically this calls for "Lumen per square meter" and this unit is named "Lux".
OK, this is only ballast to tell you that all this mumbo-jumbo is really based on serious science!
Here is a graphical representation of the concept "Lumen" and "PAR" (see below) compared to the full, visible light spectrum.
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Now let's talk about the useful light for plants and the concept of "PAR" that derives from it. (PAR being Photosynthetic Active Radiation)
In the graphs below one can see which parts of the light color spectrum are most important for plants, from there it is clear that the red and blue wavelengths bear the most importance, the yellow green part is less important and is mostly reflected so plants look "green".
From there it is also clear that a Lux meter is not very efficient as its main measuring is right in the middle of the least efficient light for plants.
So a PAR meter is more efficient in an evaluation of what is present as useful light levels. For the completeness of the chapter here is what PAR is standing for:
PAR is essentially a measurement of light emission within the photosynthetic range of 400-700nm. This represents the area of light that plants use for photosynthesis, or to grow. PAR is measured by the amount of micro moles of light per square meter per second. Although PAR is a key measurement, it only tells one part of the story. There are many areas within the PAR scale which plants absorb only in small quantities, such as in the green range (560 nm). Thus, a reading of a light’s spectrum must be used in conjunction with the PAR reading to determine from what bandwidths those micromoles are being generated, and in what amounts. A spectrum reading is obtained by using a piece of equipment called a spectrometer. These two readings together will give a complete picture of a light’s effectiveness; PAR showing strength, and spectrum showing that this strength is in the proper wavelength proportion for what the plant can actually utilize.
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