What is light?
Light is an electromagnetic wave. This wave has a wavelength that can be measured in meters, nanometers to be exact. Mathematically speaking that is 0.000000001. This is abbreviated by using nm. White light is a mixture of all wavelengths of light. The hue or color that a light gives off is due to the amount of light from a curtain wavelength. Below 400nm is called ultra-violet or UV. Above 700nm is infrared or IR lighting. The wavelength of each color is as follows: (note that as colors get closer to each other the hues blend together)
UV C: 100-280nm
UV B: 280-315nm
UV A: 315-400nm
IR A: 700-1400nm (near infrared)
IR B: 1400-3000nm (mid infrared)
IR C: 3000nm-1mm (far infrared)
The human eye sees light from ~400nm to ~700nm. There are a large number of measurements used in lighting and optics. I am going to cover the common ones in this thread.
This is directly from Wikipedia, as they explain it very well.
“ The lumen (symbol: lm) is the SI derived unit of luminous flux, a measure of the total “amount” of visible light emitted by a source. Luminous flux differs from power (radiant flux) in that luminous flux measurements reflect the varying sensitivity of the human eye to different wavelengths of light, while radiant flux measurements indicate the total power of all light emitted, independent of the eye’s ability to perceive it. A lux is one lumen per square meter. “
Lumens are skewed toward what the human eye sees better. Therefore, lights that contain a high amount of greens and yellows have a higher value given to them than those that contain less or no green and yellow. This is an okay measurement to use when comparing two bulbs/panels of the exact same lighting type or wavelengths, but very poor when comparing light sources of different types. For example: When comparing two HPS bulbs (with roughly the same spectral output) the one with the higher lumens will be a brighter bulb. However, if you are attempting to compare two CFL bulbs one that is cool white and one that is warm white, the warm white will have roughly 5 more lumens per watt.
Photosynthetically active radiation
Photosynthetically active radiation or PAR is a measurement of all light between 400 and 700nm. This is wavelengths of light that control MOST photosynthesis. Unlike lumens, PAR does not skew the results of the photon count toward any given color and is expressed in µmol photons/ m2/second. It simply records the number of photons of the wavelength(s) being measured. You may also see the term PPFD or photosynthetic photon flux density expressed as W/m2. This is the measurement that is most valuable to us as plant growers.
“Radiant flux or radiant power is the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light). The power may be the total emitted from a source, or the total landing on a particular surface.” Again, Wikipedia has a great definition. This is mostly used in technical publications and development, but is the most accurate measurement that can be given. It would be great to use this measurement for growing but you are not likely to see it on any packaging.
Chlorophyll and light absorption.
Chlorophyll is the main part of photosynthesis. This is why leaves are green when illuminated by white light. Chlorophyll is a green pigment which is found in all plants. However, there is more than one type of chlorophyll. There are at least six known types of chlorophyll and possibly more (the last chlorophyll to be discovered was in 2010). We all know that chlorophyll is what makes plants grow and they absorb the sun/light to make that happen. Many people are not aware of all of the other photoreceptors (light catchers) in plants. There are between 200 and 300 photoreceptors in every plant. Some botanists believe that there are still hundreds if not thousands to be discovered. These antenna or accessory pigments play a much larger role than most people think. The reactions that occur within a plant are very complex. Light hits the leaf, excites a photoreceptor pigment that then performs a chemical reaction that is passed on to the next. As PS1 loses and electron it gains one from PS2, this is called the Hill reaction. This process occurs in chlorophyll A, move through lutein, zeaxanthin, beta-carotene, lycopene, and then to chlorophyll A. This is just for the production of sugars that the plant uses to grow. When we talk about cannabis or other higher plants there are a number of other reactions that occur for the building of cannabinoids and other compounds used within the plant. Below are some of the wavelengths for plant photoreceptors. (Note that most photoreceptors have multiple peak absorption points.)
Chlorophyll A: 430, 662
Chlorophyll B: 453, 642
Alpha Carotene: 442, 444, 478
Beta Carotene: 425, 480
Zeaxanthin: 423, 451, 483
Lutein: 420, 447, 477
If you compare that list with the above list of wavelengths, you will see that none of these photoreceptors are in the green or yellow region. This is another example of how the lumen measurement does not equate to the growth of plants.
This leads me to the Emerson Effect or Emerson Enhancement Effect. This has been used in almost every plant photoreceptor study since he published his first paper on it in 1957. When illuminating plants with 700nm wavelength red light, he noticed that photosynthesis was slowed greatly. The same occurred when illuminated with 650nm red light only. However, the combination of the two generated a symbiotic response and increased the growth rate to double that of the single sources combined (a fourfold over each light alone). His later work indicated that there was not one but two photosynthesis traps. We now call these PS1 and PS2, where each system transfers energy and electrons to each other increasing productivity.
Types of Lighting
HID lighting is high intensity discharge. This refers to Metal Halide (MH) and High Pressure Sodium (HPS) most often. HPS is the second most efficient light source and has been used in the growing lighting and greenhouse industries for years. They work very well, put out a lot of light, and even more heat. I’ll go over coverage areas later, but HPS and MH lights can cover a lot more area from a single point source than any other lighting type. That is something to keep in mind if you are running a commercial grow operation or have a very large flowering room.
CFL and Tube Fluorescents
Compact and Tube Fluorescent lights are used extensively for growing. They are cheap to purchase, widely available, produce little heat, and have a wide range of color temperatures. When using CFLs, you can place them very close to your plants (within 2 to 3 inches) due to their low heat output. This helps the light penetrate deep into the plant and increase photosynthesis. Be careful about using a large portion of CFLs in a small confined space. While they put off a small amount of heat, placing large amounts in a confined space without proper ventilation can cause the heat to rise rapidly. Fluorescent lighting is not as efficient as HID or LED lighting, so as your grow area increases, the energy inefficiency causes a greater problem.
Light Emitting Diode (LED)
LED lighting is the newest form of lighting and has gotten a really bad rap for the most part. I’ll be honest; I’m an LED person and have been testing them for years. This probably makes me a little bias when it comes to this subject. LEDs have come a long way over the past few years. Everything we thought we knew about lighting and what produced more light and what grew plants better is out the window when speaking of LEDs. First, the measurement of lumens that we have all went by for decades only applies to a very small portion of LEDs, those that produce white light. Because lumen is a scale based on brightness to the human eye, color LEDs have a very low lumen count. Radiant Flux is the measurement used by designers when attempting to make panels and get a certain percentage of each type of light. The photoreceptors in plants use different wavelengths to perform different functions. This is the main reason that LEDs got a bad rap over the past 5 years or so. When panels first started coming out they only had 2 colors 630nm red and 480nm blue. While this will grow plants, it is missing a lot of the key elements that the plant needs to perform to its full potential. As companies (and individuals) started testing these LEDs to grow, they noticed this. Soon you seen 4, 5, 7, 11 and even 15 band LED grow panels. The results started to improve but the bottoms of the plants where not producing very well if at all. Then secondary optical lenses came to improve penetration. Most companies have learned that hitting wavelengths for accessory pigments and adding these lenses finally made LEDs with it for growing. I want to point out that it was the MMJ/Cannabis growers that performed the large amounts of these test and drastically improved the industry. There are some great people on this forum and many others that have contributed to the advancement of LEDs for growing and the production industry noticed. They started producing better diodes in the color sector as well because of these people and they deserve credit for their work. LEDs are the most energy efficient form of lighting, produce the least amount of excess heat, and have the highest lm/w rating of any lighting type. The area where LEDs fall short is in coverage area and penetration.
The following is just for reference on energy efficiency in lighting types and is not to be construed as suggesting a lighting type for your grow room. A 100% efficient light would produce 680 lumens per watt. The following is the current efficiency of available lighting.
Incandescent: 1.9-2.6% (10-17lm/w)
Halogen: ~3.5% (35-60 lm/w)
CFL and Tube Fluorescent: 9-11% (CFL 50-70 Tube 30-100 lm/w)
HPS: 27-32% (100-150 lm/w)
MH: 25-30% (75-100 lm/w)
LED: 17-46% (1mw -167+ lm/w)