This article is intended to help people understand what lighting technology is best for them. I understand that this may be too lengthy or detailed for some people, however this was the route i chose as i felt it could be helpful for both the beginner learners and the more advanced who wish to take their knowledge even further.

I highly recommend visiting my article on Light Metric Systems to have a better understanding of the metrics used to quantify light and how they relate to grow light technologies.

Grow Light Characteristics:

Lighting Efficiency:
Light is emitted by energy provided by atoms with a higher energy level. With electrical lighting, this energy is typically in the form of electricity provided by a source. Such as Nuclear Fission, Combustion, Wind, Solar, Hydro, and Chemical Conversion. How we use that energy to make usable light is done in different ways. Such as Incandescent, Florescent, Light Emitting Diode and Gas Discharge.

How well an electrical entity can turn electrical energy into usable energy is called electrical efficiency. This is calculated by the total power input divided by the useful power output. Any energy that is not converted into usable energy, is converted into another form of energy. Such as heat, where heat is not desired. Or visible light, where light is not desired. Do not assume that heat is automatically the byproduct of inefficient electrical conversion. As heat can be a form of energy that is desired. Such as with convection heating devices. With heating systems its inefficient conversion byproduct is energy that did not convert into usable heat energy, such as visible light. With lighting devices where energy is not converted into visible or usable light, heat is typically the byproduct of inefficient light conversion.

Horizontal Light Uniformity:
Uniformity is very important to the overall efficiency of a grows ability to process photons. Uniform light allows energy to be divided equally over the grow and share the photosynthetic load. This also allows a plant to develop without bias and grow symmetrically. The uniformity of light is determined by how light is emitted from a source and if or how light guiding equipment is designed. Such as reflection hoods or optical lenses. Fixture height also changes the characteristics of light uniformity, as light travels further, photons diverge more over space. As such uniformity is improved with increased distance or height.

Vertical Light Uniformity:
Penetration is important for the overall quality of a grow. As photons travelling deeper into a grow, provides better vertical growth uniformity. Factors determining good penetration are light distribution characteristics and total light energy. Light sources with more collimating properties allow photons to travel further and ignore the rules of the inverse square law, which only apply to point sources. Light sources with more light output, provide more overall energy and allow more light to bleed through open gaps. Some lights also provide very diffusive characteristics (gas discharge), which can penetrate foliage easier than collimated light (LED).

Light Coverage:
Light footprint is important so light is not wasted by photons that completely miss the intended canopy. Wide angle directivity is a common trait for grow lights as the focus on design is heavily impacted and encouraged by the commercial grow industry. This is because large commercial grow facilities configure their grow light systems to allow optimal light uniformity. As wide angle light systems provide superior uniformity through divergence. However for personal indoor grow systems, we typically do not have as much control over space. It is ideal to prevent light from leaving the intended area and focus or collimate light as much as possible to capitalize on available resources.

Lifespan:
As light fixtures age, their electrical efficiency degrades, this is inevitable and is important to understand when it is best to replace a lighting fixture or bulb. Although most technologies can last longer than their recommended lifespan, it is usually more beneficial to replace the fixture or bulb as the degraded efficiency reduces the effectiveness of the light source and replacing it with a new product can recover costs from increased production and yield.

Spectrum:
Although it is often thought that spectrum has a large effect on the effectiveness and efficiency with plant growth. Its actually less important than you might think. Research shows that when using broad spectrum light sources, light quantity is more important than light quality. This follows previous and current research and understanding in photobiology, and the efforts from those such as bruce bugbee, making it very clear the impacts that light has on plant anatomy and growth. As long as sufficient levels of blue (10-15%) are provided, light quantity is the most important factor. 

Grow Light Technologies:

Incandescent-Halogen:
Incadescent lights have a filament made of tungsten and surrounded by nonreactive (inert gas) gas such as argon. When electricity flows through the filament, the lamp heats to about 4500 degrees. Its this intense heat that makes the filament glow with light. The inert gas provides a mean to prevent the filament from burning up which would otherwise happen if exposed to oxygen. In a halogen lamp, filament evaporation is slowed by a chemical process that redeposits metal vapor onto the filament, extending its life. In all other aspects, the halogen lamp is the same as a incandescent lamp.

Incandescent lights are the least efficient technology with around 0.3umol per watt and should only be used for practical purposes. It is only mentioned here as a reference and a honorable mention to the first electrical light invented.

Fluorescent:
The central element in a flourescent lamp is a sealed glass tube. The tube contains a small bit of mercury and an inert gas, typically argon, kept under very low pressure. The tube also contains a phosphor powder, coated along the inside of the glass. The tube has two electrodes, one at each end, which are wired to an electrical circuit. When current flows through this circuit there is a considerable votage across the electrodes, electrons will migrate through the gas from one end of the tube to the other. This electrical current causes some of the mercury to vaporize into gas. Electrons then collide with some of this mercury and cause electrons to shift to a higher energy level and then release photons when they return to their original energy level. Flourescent lights naturally produce UV light that we cant see or use, the phosphor powder on the inside of the glass is whats used to change the UV energy into visible or more usable light.

Fluorescent lights are a more practical choice, not the most efficient but they are still considerably more efficient than incandescent at around 0.8umols per watt. Fluorescents are a good choice for the grower on a budget or who is just willing to experiment and practice their grow skills. They are also good for grows where space is limited, as the low flux levels of the lights allow them to be placed at very close proximities, minimizing the space required for a grow. They are also very good for use with young plants such as clones, seedlings or early vegetating plants. Their spectrum is of sufficient quality for all stages of growth but as said they are very low flux level lights with moderate efficiency. Fluorescent lights typically have a lifespan of around 10,000 hrs or 2 years of averaged 18 hr and 12hr light cycle.

Metal Halide:
Metal halide has a arc tube made of quartz, with two electrodes, one on each end. Inside the arc tube is argon gas, mercury and metal halide salts. When current flows through the quartz tube, the arc is initially through the argon gas. As the lamp heats up, the mercury starts to vaporize and further increases the lamps temperature. Its at this point the bulb acts and looks like a mercury vapor lamp. When the lamp gets hot enough, the matal halide salts start to also vaporize. The halide salts are made from various chemicals that each give off their own colors, they mix with the blue colors of the mercury and produce a wide spectrum with multiple peaks given off from the various chemicals.

Metal halide (MH) has traditionally been used in the vegetation stage as their spectrum and efficiency provide good qualities to support healthy, strong growth in plants. They are more efficient than fluorescent at around 1.2umol per watt but less efficient than some of the other light technologies such as sodium discharge and LED's. MH has the highest UV levels of all the grow lights available. Sometimes the bulbs come with a warning about their dangerous levels of UV. Metal halides lifespan is around 5000 hrs or 9 months at 18hr light cycles.

Ceramic Metal Halide:
Ceramic Metal Halide (CMH) is almost identical to Metal Halide, except that the quartz tube is replaced with a ceramic tube. The ceramic tube allows higher arc tube temperatures, which some manufacturers claim results in better efficacy, color rendering and efficiency.

CMH is not new but has just been improved on since its introduction over 30 years ago. Research shows that the photon flux efficiency (total photon output devided by input power) is comparable to sodium discharge technology with around 1.5umol per watt. But definitely not superior. Because its efficiency is identical to sodium discharge, this suggests that its heat output would also be identical with equivalent wattages. Despite what is claimed by retailers, CMH does not produce any significant amount of UV light.

CMH does however offer some clear practical advantages. Their spectrum is very flat with a near neutral white balance. This allows a better overview of a plants condition and a more enjoyable experience. This would be of good choice to new growers who demand good yields but would like the simplicity of being able to see their plants clearly. You also gain the advantage of being able to use the same light through the whole grow, this could be of advantage compared to sodium discharge as they are ideally used with metal halide which is less efficient. The only downside is their short bulb life, with around 5000 hrs or 12 months at 18hr and 12hr averaged light cycles.

Light Emitting Diode:
A light emitting diode is a semiconductor such as gallium arsenide (GaAs) and Gallium Phosphide (GaP) that forms a P-N junction. As current flows in a forward bias, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light is determined by the energy band gap of the semiconductor. Blue is the most used color by LEDS today, although different colors such as red are possible. Other colors are typically made from blue LEDS with the use of phosphor coatings. Allowing the conversion of blue into any desired color.

Light emitting diodes (LED) have come a long way over the last 20 years. Of the last decade LED's have become more efficient than all other technologies. However to capatilize on these efficiency advantages, becomes impractical for economical reasons. The reason for this is that, to obtain high efficiency levels, LED chips must be run at low power. This is because a LED's efficiency is directly related to its operating current. As a result single LED modules are very weak when trying to achieve high efficiency levels. To provide a fixture with sufficient flux levels and efficiency, many chips in parallel are required. Doing so becomes costly and impractical to the consumer. As a result, many light manufacturers sacrifice the efficiency of the product for a more attractive price. Many products on the market currently, are of poor quality compared to cheaper alternatives such as gas discharge lamps. Quality LED fixtures are easy to tell apart from low quality products, simply by looking at the price. More expensive fixtures are typically of higher quality and produce far better efficiency values compared to cheaper fixtures. The quality is quite simply related to the material costs. The more LED diodes used and hence the higher the cost, the more efficient it can be made. Most of the cheaper LED fixtures on the market are less efficient than gas discharge technology. With typically around 1-1.5umol per watt. The more expensive but higher quality fixtures can obtain levels as high as 2-3umol per watt.

Some benefits of LED are of course its attainable efficiency levels, but also their ability to use optics to control light. This provides the practicality for a fixture to control the angle of light and improve efficiency through less energy that would otherwise be gone from radiated or reflective losses. As semiconductors improve over time, this technology will become cheaper and more efficient. Eventually it will superseed other light technologies and become the dominant form of all light products. LED fixtures lifespan varies depending on the quality of design and the amount of heat that the fixture has to endure during operation. The more efficient a fixture is made and the more heat that can be extracted from the fixture, the longer it will last. Cheap fixtures or inferior designs such as chip on board (COB) or clustered designs, will degrade more quickly. Also fixtures that have poor cooling designs and fail to adequately remove heat, will also degrade faster. But typically LED fixtures will last anywhere between 30,000-50,000 hrs. Or between 6-10 years at a averaged 18hr and 12hr light cycle.

High Pressure Sodium:
High pressure sodium has an arc tube made of polycrystalline alumina and two electrodes, one on each side. Inside the tube is xenon gas, mercury and sodium. The arc tube is made with polycrystalline alumina because it can withstand the heat while pressurizing the chemicals Hence the name, High Pressure Sodium. When current flows through the sodium bulb, the xenon gas allows electrons to heat the bulb and vaporize the mercury. The mercury heats the bulb even hotter which vaporizes the sodium and completes the chemical reaction. The colors from the mercury (blue) combine with the monochromatic color of the sodium (orange), to create a light source that is more broad spectrum than the former chemicals on their own.

High pressure sodium (HPS) has been the king for nearly 60 years, with an efficiency of about 1.5umol per watt. Its been a tried and true technology that even today still competes well with other light technologies. Although LED can be made to be more efficient, this comes at a cost. HPS at this time is much more cost effective as the initial cost is low as well as its great efficiency levels. Double ended gas discharge bulbs, provide good electrical efficiency of around 1.7umol per watt and high output capacitiy. These are commonly used in commercial grows for these reasons alone. Its only downside is that HPS has very little light in the blue spectrum. Which is important for plant development during the vegetation stage. As such Metal Halide is commonly used during the vegetation stage, while HPS is used for the flowering stage. Sodium discharge has a bulb life of around 10,000 hrs or 24 months at 12hr light cycle.

As always please comment your thoughts or any questions on the subject.