TYPES OF LIGHTS
IntroductionThere are a number of different approaches used for growing plants under lights. To make an informed decision as to what type of lighting should be employed, the fundamentals of light, colour and lighting systems should be understood. In this article we will examine the how light is qualitatively appraised with respect to color and intensity. Different lighting systems will be examined, and most available types of lights will be discussed. Examples of some "real world" lighting systems will be given and analyzed with respect to effectiveness, initial cost, operating expense and longevity.
LIGHT AND COLOUR
What is lightVisible light is that part of the electro-magnetic spectrum that lies between the wavelengths of ultraviolet and infrared. That's probably more that you need to know for the purposes of home growing.
White light is all colors
When we see a rainbow, we are seeing white light split up into it's component colours, hence the expression "all the colours of the rainbow". Plants, in general, absorb red and blue light and reflect green light. Our eyes are most sensitive to the color green.
Sunlight is different in different places in the worldSunlight contains, more or less, equal portions of all colours of sunlight. Northern sunlight, that is, sunlight in areas north of the fortieth parallel, has more blue than equatorial sunlight because of absorption of all other colours, or wavelengths of light, by the atmosphere.
This is the same effect that causes underwater photos taken below three feet to be so blue. Just as the atmosphere absorbs non-blue light so does water, except water absorbs non-blue light at a much greater rate. Almost all non-blue light below three feet of water is absorbed.
How is light measured?Light quality is expressed and measured in many ways. Light colour can be measured in degrees Kelvin (K) and the colour rendering index of a light source can be measured and expressed as CRI. (0 degrees Celsius = 273 degrees Kelvin; 0 degrees Kelvin = absolute zero)
Colour temperature - degrees KelvinWhite light can have different "warmths". A bit more red/yellow makes white light appear "warmer". A bit more blue and light appears "cool". This can be quantitatively assessed by the assigning of a colour temperature, given in degrees Kelvin. Think of colour temperature as the colour of a block of iron as it is heated to various high temperatures. A warm, reddish light is around 3500 degrees Kelvin, and above 6000 degrees Kelvin the light takes on a bluish tone. Sunlight is somewhere around 5000 degrees Kelvin.
Colour rendering index (CRI)The colour rendering index identifies the degree of colour shift objects undergo when illuminated by a particular light source compared to a standard source. In simpler terms, the CRI expresses the degree to which a light source renders the true colour impression. The CRI is an index and ranges from 0 to 100. A light source having a CRI of 100 means objects illuminated by it look like they're supposed to; that is their natural color is not distorted. A light source having a very low CRI would tend to make objects appear to be a different shade or even colour that they really are. An example of light with a high CRI is, obviously, sunlight. Some fluorescent tubes such as Daylight, Chroma 65 or Vita-Lite have a very high CRI. Some light sources such as Gro-Lux or sodium vapour lamps have very low CRI's.
The color rendering is important when examining flowers under different light. Because cool white (blue) fluorescent lights lack red, red flowers look dull, almost grayish. Sodium or mercury lights are even worse for distorting color.
A light meter may be used to measure the amount of light, measured in foot-candles, emitted by a light source, measured at some distance from the source. Growing information for a plant will give an indication of the amount of light the plant requires, usually stated in foot candles at the surface of the leaf.
Light meters for use in photography are designed to be sensitive to the same wavelengths as the human eye. That is not what a plant sees! To measure correctly what a plant sees, you must use a meter that provides a measure of photosynthetically active radiation (PAR), the wavelengths of light most important for plant health. These wavelengths, between 400 and 700 nanometers, are critical for the photosynthesis and chlorophyll production that drive plant growth. (A nanometer is one billionth of a meter.)
Good light, free, but hard to controlThis is of course what plants are used to and it can hardly be argued that this is anything less than the most natural. However coaxing enough sunlight onto your plants throughout the whole year from the top rather than from the side as through a window can be problematical.
Sunlight is the certainly the cheapest way to illuminate your plants, although it is unreliable and very difficult to regulate. This is subject to geographical variation, of course. If you live in California and have a skylight over a plant stand, you might be getting enough light. If however you live in an area that does not get a lot of sunlight or your plants are stacked in rows in a basement, you will obviously need supplemental lighting.
Cheap, low quality lightIncandescent lights are the ubiquitous screw-in bulbs you most likely have lighting your home. An incandescent bulb consists of a glass bulb with a tungsten filament in a near vacuum; just a small amount of argon or krypton is present. When current flows through the filament, it heats up and glows giving off both heat and light.
Halogen bulbsA variation of the incandescent bulb is the halogen bulb. This is an improvement to incandescent bulbs invented by GE in 1958 for the wing tip navigation lights of the Boeing 707. In a regular incandescent bulb, the tungsten filament evaporates, and over time the inside of the bulb is coated with a fine coat of tungsten from condensed tungsten vapour. This coating will severely limit the light output of the bulb. In a halogen bulb, a small amount of one of the halogens (Iodine or Bromine are used) is present and combines with the evaporated tungsten. This Tungsten Iodide or Tungsten Bromide molecule has an affinity for the tungsten filament and returns there and splits. The tungsten from this molecule returns to the filament while the halogen returns to the atmosphere inside the bulb. This process does not work unless the bulb jacket is at least 200 degrees Celsius. This is why halogen lamps are so hot and must be taken into consideration. Halogen lamps are 25-30% brighter than regular incandescent bulbs. The halogen cycle, as it is called, takes place in a very small capsule, as it is easier to maintain the high temperature required for the halogen cycle to operate in a smaller space. This capsule is placed inside another glass capsule which serves as the bulb's outer casing and although it is still plenty hot, it is not as hot as 200 degrees Celsius.
Output spectrum is biased towards the redThe output spectrum of incandescent light, halogen or regular, is biased heavily toward the red. Non-halogen bulbs have a colour temperature of 2700K, while halogen bulbs have a colour temperature of 3000K - they are a slightly more whitish light. Both have a CRI of 100. A diagram of the spectrum looks rather like a triangle, starting with almost no output in the green and rising at an almost linear rate to the far red and infra-red. Although incandescent bulbs are very inefficient, they are a very good source of near and far red light which is certainly very important. They are sometimes used as supplements in systems which are deficient in the red end of the spectrum.
EfficiencyThe great disadvantage to incandescent lights is their inefficiency - you don't get a lot of light compared with how much energy you put apply. One saving grace in this respect is that the efficiency increases proportionally to the wattage, for example a single 100 watt bulb is much brighter than two 50 watt bulbs. The energy that does not get converted to light is wasted by being given off as heat. All but the smallest wattage bulbs can generate an awful lot of heat, and this must be taken into consideration. Another point to consider is, because the heat is so great, a splash of water on a hot bulb can shatter it.
Halogen bulbs are more efficient than "regular" incandescent bulbs by virtue of remaining brighter longer; they still give off 95% of their initial light output at the end of their lives, which are about twice as long as regular incandescent bulbs. They are also more expensive.
The great advantage of non-halogen bulbs is of course their extreme low cost for initial purchase, and of course their great availability; you can buy them anywhere. Halogen bulbs are on the average 5 to 10 times as expensive as their non-halogen counterparts and can usually be found at larger hardware stores. Since their primary market is yuppie track lighting they are usually found as spot or flood lights. Of potential interest to grower is the low voltage bulbs used in some track lighting systems. Operating as 12V, these bulbs are quite small and would be good to use a supplemental light augmenting a fluorescent setup. They are also the cheapest of halogen bulbs. While I have seen them at $30 each in fancy designer light stores, I have also seen them in Price Club at 3 for $12. Sylvania makes a series of bulbs called Capsylite that come in "regular" bulb shapes plus the large parabolic reflectors sometimes used to illuminate the outside of houses. Osram makes a large array of different shapes and sizes, most of which look like the vacuum tubes. They are probably the most useful to growers because of their smaller size and wide range of wattages; from low power bulbs all the way up to 150 watts. They are however not cheap and can be quite a challenge to find somewhere that stocks them.
LongevityIncandescent bulbs have a lifespan of about 1000 hours. Halogen bulbs have a life of about 2000 hours. One interesting personal note here; although regular incandescent lights are rated at 1000 hours, we've all had some bulbs that seem to burn on forever. The Guinness book of world records lists the longest lasting light bulb as being an incandescent bulb in a firehouse in, I believe Boston that is some 70+ years old; it is never turned off, which is a key point. This is why your parents always gave you hell for flicking the lights on and off really quickly, the wear on the filament from having current suddenly shot through it is quite great. If you'll notice, most bulbs fail when turned on, not in the middle of operation, or when they are turned off. The halogen bulbs I have throughout my home seem to be on a timer; when 2000 hours is up *poof*, they expire. I curse them out, do a rough calculation and come to the conclusion that their 2000 hours just expired.
FLUORESCENT LIGHTINGThis is a bunch of data on the commonly available fluorescent tubes from GE, Sylvania and Philips.
Cheaper To Run, More Expensive To InstallFluorescent lights are very common in our day to day lives. They are cheap to operate as they emit about four times as much light per unit of electricity as incandescent lights do. On the other hand they are more complicated to install because they require a ballast to operate. You may be familiar with the regular "cool white" and "warm white" tubes sold in hardware stores but what you may not know is that fluorescent tubes come in hundreds of shapes, sizes and spectral output.
How They WorkFluorescent lights work by placing an anode and a cathode at opposite ends of a glass tube. Inside the tube is a partial vacuum and a small amount of mercury vapour. When energized, the mercury vapour is ionized and emits ultraviolet radiation. The inside of the tube is coated with a phosphor - a powder that "fluoresces" (gives off light) when stimulated by ultraviolet radiation, thus producing visible light. The chemical composition of the phosphor determines the spectrum or colour of the emitted light. (Fluoresce has nothing to do with flour.)
Replace Tubes Every Six MonthsAlthough fluorescent lights are very energy efficient, there is a particularly nasty phenomenon known as "cathode decay" that causes, over time, less energy to be transferred through the mercury vapour. The net effect is that the tube will emit less and less light as it gets older. To all appearances, the tube will put out the same amount of light until it suddenly stops dead one day, (which can take years), but for all practical purposes, because the drop off in light output is an exponential decay, the tube should optimally be replaced every six months or at the very least once a year. Writing the installation date on the tube itself with a permanent magic marker can be a big help here.
Types Of Fluorescent TubesThere are many different types of fluorescent tubes. They differ in the physical size, composition of the phosphor and the wattage. When fluorescent tube is mentioned, the standard T12 four foot tubes usually comes to mind. This tube has a diameter of 1.5 inches and is available in 18", 24" 36", 48", 72" and 96" lengths. T12 tubes are available in HO (High Output) or VHO (Very High Output) which draw more and much more current respectively, but produce more light than regular T12 tubes. T12 tubes are also available in U-shaped, that is a four foot tube is bent back on itself so it forms a large U, and is about 24" long. The T8 or "slimline" fluorescent has a 1" diameter tube and is available in 24", 36" and 48" lengths. Circular tubes are available with several different radii, and in several different types. In the last few years, compact fluorescent tubes have become very popular mostly as replacements for incandescent bulbs. These tubes come in all sizes, from a 3" 5 watt bulb to much larger bulbs that replace 40W four foot tubes, yet are just one third of the size.
The phosphor chemistry is what makes the difference between a cool white and a daylight tube and every tube is available with a dizzying array of choices in this area. As the composition of the phosphor changes so does the spectrum of the visible light being emitted by the tube.
For illumination for plant growth only a small percentage of the dozens of available tubes are appropriate. They fall into the following broad categories: industrial, full spectrum, daylight, plant growth, actinic, tri-phosphor, special purpose and HO/VHO.
Use Four Foot TubesAlthough fluorescent tubes come in many sizes, volume of scale dictates that there is really only one size - the T12 four foot length. Some ninety percent of all fluorescent tubes made are this size, and because of this volume this is the cheapest size, although this needs to be qualified. If you are buying tubes through normal retail channels, the markup is generally high enough that they can play with prices and a 24 inch tube costs less than a 48 inch tube but more than an 18 inch tube. If however you are buying tubes through other channels, such as lighting distributors, you may find that the four foot tube is cheaper than any other size. T12 tubes that are smaller or larger will cost you more. Additionally, the four foot size has the longest lifespan and also the highest ratio of lumens (light output) per watt. Thus, where space allows, use four foot tubes. If there is not enough space for these, individual compact fluorescents may be called for.
ManufacturersIn North America the "Big Three" in fluorescent tube manufacturing are General Electric (GE), Sylvania and Philips. They all make, almost without exception, the same tubes, under different trade names although there are some notable exceptions. Smaller and off-shore manufacturers include Duro-test in the US and Osram who make some tubes in North America and some in Europe.
Industrial TubesThese tubes include the ubiquitous "cool white" and "warm white" usually used in home and industrial lighting applications. These tubes are tuned to produce the brightest possible illumination for the least amount of electricity. Since the human eye is most sensitive to green, these tubes peak in the green portion of the visible spectrum. In fact they rise and fall quite sharply either side of the green peak. Warm white is shifted a bit toward the red end of the spectrum thus accounting for the "warmer" appearance.
If all you want to do is illuminate your plants these tubes are fine. These tubes are cheap, and they don't look terrible. Recent evidence suggests that although plants require mostly red and blue light, ANY light, in high concentration must be applied for the plants to open their stomata thus permitting respiration. This goes a long way toward explaining why some people are able to grow beautiful plants with just cool white and warm white tubes. Enough light, of any type will grow plants. These tubes are far from optimal however and they really are almost completely devoid of the necessary red and blue portion of the spectrum. If you can grow decent plants under these lights, you will do even better under more appropriate lights. These tubes are available anywhere fluorescent tubes are sold and are the cheapest tubes available.
DaylightDaylight tubes are the next big improvement in more natural light (that is a more closer approximation of sunlight) as a result of an improved phosphor formulation. Although daylight tubes output a spectrum that although does not fully emulate sunlight, it is significantly better than earlier cool white and warm white tubes. These tubes are occasionally available at hardware and department stores. They are not uncommon and any lighting supplier should have them or be able to order them. They cost a bit more than cool white, but are not expensive. Figure about $3 to $4.
Plant Growth LightsEpitomized by the Sylvania Gro-Lux® tube, plant growth lights are, unlike all other fluorescent tubes, meant solely for promoting plant growth; you won't find these illuminating somebody's home or office - with one exception. Where I work, a receptionist thought it would be nice to have pink lighting in the lobby and ordered and had installed some plant growth tubes. You do get used to it, but they are most disconcerting when initially encountered.
GE's version of this tube is called "Gro-N-Sho". Gro-Lux type tubes have an output spectrum with two large spikes, one in the blue, and one in the red portion of the spectrum. There is almost no light emitted in any other portion of the spectrum and as such, they cast an eerie purplish glow and do not appear very bright. The spikes in the red and blue occur quite abruptly and are quite steep. This spectrum was chosen as it matched the absorption of visible light by chlorophyll in a test tube. In the 50's a study was conducted on various lighting types and phosphor formulations on plant growth, the results of which were published in the book "Lighting for Optimal Plant Growth" (Kent State Press) The phosphor formulation of Gro-Lux type tubes was improved upon. Instead of two steep abrupt spikes in the red and blue, there are two slow rising large "bumps"; the peaks in the red and blue were not as high, nor did they rise as sharply. Instead of concentrating all the energy in these two narrow energy bands, the output was tuned to produce wider bands still centered around red and blue. It became commercially available from Sylvania as Gro-Lux Wide Spectrum; GE named theirs Gro-N-Sho Wide Spectrum. These are more pinkish than purple and are indeed what is in the lobby of the building where I work.
Incidentally, you could never get away with regular Gro-Lux (as opposed to Gro-Lux wide spectrum) tubes in a lobby; they look dark, don't illuminate well and are a very deep purple. The Wide spectrum plant lights are brighter and don't look like a 60's psychedelic poster shop when used to illuminate a room like a regular Gro-Lux would.
Philips makes a plant light they named "Agro-Lite", which is a minor variant of the wide spectrum Gro-Lux. They commissioned a study at a major American university comparing their Agro-Lite to wide spectrum plant lights. The Philips tube resulted in 2 - 10% greater growth in a variety of terrestrial food crops when compared to other wide spectrum plant lights.
Since these tubes are quite commonly used for houseplants they are reasonably common in hardware stores or nurseries, although what typically happens is a store will only sell one vendor's fluorescent tubes. Even worse, they don't recognize the difference between plant lights and wide spectrum plant lights with the result being you will usually find plant lights or wide spectrum plant lights from one manufacturer in a store. Wide spectrum tubes are reasonably inexpensive, although regular Gro-Lux type tubes tend to be a bit more expensive still - the chemical that makes up the phosphor which produces red is the expensive part. In a pet shop these can be between $10 and $20. From a lighting supplier a Gro-Lux tube is about $9 while a wide spectrum tube is about $7.
Full SpectrumFull spectrum tubes imitate natural sunlight as closely as possible by emitting light in every spectral range. All the different colours of visible light and a very small amount of ultraviolet is emitted. The Duro-Test Company produces "Vita-Lite" tubes. GE produces "Chroma 50", Philips produces "Colortone 50", Sylvania produces "Designer 5000K". All these tubes have an output spectrum that is similar to sunlight - about as close as modern chemistry can bring us. These tubes try to imitate equatorial sunlight at noon, which has a colour temperature of around 5000K.
Noon-day sunlight from northern climes has a larger amount of blue in the spectrum, having a colour temperature of 7500 Kelvin. Since the red pigment in plants is limited by blue light, these are sometimes useful. Duro-Test sells a "Vita Lite 75", GE sells a "Chroma 75" and Philips sells a "Colortone 75".
There is quite a disparity in availability and price of these tubes. The Vita Lites have very good distribution. They can be found in most aquarium stores (and many pet stores as they are also used for illuminating lizards who need the Vitamin D from the ultraviolet light). The downside of this is like anything you buy in a pet store that you can buy in a hardware store, the price can be quite high when buying them from a pet store: $15 - 20+. The same Vita-Lite tube from a lighting supplier is about $7, and the Chroma 75 I have obtained for less than $5. They are nearly identical.
Tri-PhosphorPhilips makes the most popular range of T12 tri-phosphor tubes, the "Ultralume" series. Recognizing that the primary light colours are red, green and blue, Philips made a tube that fluoresces very sharply only in these three narrow wavelengths. The light emitted appears white, and very bright. They are used primarily in clothing stores because they completely lack emitted ultra-violet, which bleaches clothes. Ultralumes come in colour temperatures of 3000, 3500, 4000, 4500, and 5000 which is accomplished by varying the amounts of red, green and blue phosphors. Since red is the most difficult colour light to obtain from fluorescent tubes and the Ultralume 35 has the most red, this is probably the most interesting tube from our perspective. Ultralumes are in the $7 range and can be found at better pet/aquarium stores. Philips tubes seem to be difficult to find in some areas, notably the West coast although I have occasionally seen Ultralumes on sale in department stores there. Again, a lighting supplier can usually get any of these tubes.
ActinicThese tubes emit light only from the blue end of the spectrum and are used in marine setups to supply the blue that is missing from normal aquarium lighting but is required by marine algae, anemones and corals. They are usually only available from specialty aquarium stores and are not cheap. They have little or no application for growing plants.
Reflector and ApertureOf the large manufacturers of fluorescent tubes, only Sylvania makes reflector and aperture tubes. Many of the new aquarium specific tubes have reflectors, but have little data to back up their assertion that the reflector is worth the extra cost. Sylvania however, has a data sheet on their reflector and aperture tubes.
Quoting from the "Sylvania Engineering Bulletin O-338"
"Aperture and reflector fluorescent lamps differ from standard fluorescent lamps in that they allow a certain amount of control over the direction in which the light is being sent. As sketched in Figure 1, a reflective coating is placed between the outer glass and the phosphor coating. This reflective coating provides the direction control by reflecting most of the incident light and directing it through the uncoated surface or clear window of the aperture lamp.
The total light output of reflector lamps is actually less than that of standard lamps. These lamps are intended for applications which can best utilize their special light distribution. The light is often too bright for direct illumination, but when used with reflectors it can be a very effective means of controlling the light."
Reflector tubes have a reflective coating covering 235 (or 135) degrees of the interior. Over that they have a phosphor covering the entire inside of the bulb. They are available in a number of sizes in Cool White, while one is available in Gro-Lux in a R/GRO/VHO 215 Watt 96" lamp.
Aperture lamps have a 330 or 300 degree reflective coating. They have a phosphor coating covering 330 or 300 degrees of the lamp. There is a 30 or 60 degree clear glass opening or "aperture".
The aperture lamp has a lower light output that standard fluorescent lamps, because some of the phosphor, which converts ultra violet to visible light, has been removed. But when these lamps are used with reflectors or lenses, they provide a very concentrated beam closely projected in one direction. This allows more light to be delivered to a small area.
"Applications of the lamp are bridge lighting from the rails, aircraft landing strips, highways and approach ramps, billboards and sign lighting, sport areas and marina lighting."
The aperture lamps are only available in 3 models: 4 foot 30 degree aperture cool white, 4 foot 60 degree cool white, and 8 foot HO 30 degree cool white.
HO/VHOHO refers to High Output, and VHO is Very High Output. These tubes output more (and a lot more) light by drawing more (and a lot more) current. They are more expensive tubes to buy, require larger more expensive ballasts and don't last as long. The conventional wisdom about these tubes is that if you need a lot of light then it's okay to use an HO, but the VHO's are more bother than they're worth. Neither last as long as regular tubes. A ballast for an 8 foot VHO tube is an enormous black box that draws a lot of current, and gets very hot. Even the tubes themselves get hot. If you need this much light you should probably be thinking about HID lamps. HO and VHO tubes come in many sizes and types, such as cool white, warm white, daylight, Gro-Lux and Gro-Lux wide spectrum
LongevityStandard T12 four foot fluorescent tubes have about a 10,000 hour lifespan, but as stated earlier, their usable life is much shorted because of decreased light output over time. All other tubes are less (by about half) than this, but again, it's a moot point as they should be replaced every six months.
Now we're getting seriousHID or High Intensity Discharge are the big bright lamps you see in grocery stores, street lighting and industrial lighting. They can be very large and draw a lot of power. Indeed 2000 watt and 6000 watt lamps exist, however small ones, down to 70 watts are available.
TradeoffsThese lamps produce a lot of light output quite efficiently, however they can be quite expensive to install initially and may require a fan for cooling in the housing/reflector as they can produce phenomenal amounts of heat. These lamps are used by growers who need lots of light.
HID lamps requite a ballast, and almost every bulb requires it's own type of ballast. The ballasts are expensive and bulky and are not something you trot on down to the corner hardware store to pick up, although larger hardware stores may have some; they are usually reasonably priced. You'll have to go to a lighting supplier for most of them however.
HID lamps are built like halogen bulbs. A small capsule contains the vapour that an arc is sent through. This capsule is in turn encased in the much larger outer bulb body. There is quite a bit of UV generated by the inner capsule that is filtered by the outer capsule. All these bulbs carry warnings not to operate them if the outer capsule is broken.
TypesThere are three basic types of HID lamps: mercury vapour, sodium vapour and metal halide.
Mercury vapourWhen you see a bright light illuminating some industrial building and it has a decided bluish cast - that's mercury vapour. Mercury vapour lamps have an output spectrum that is almost entirely blue-white, with very little red. Worse, the spectrum is not continuous, there are spectral peaks at certain wavelengths. These lamps, although not useless - there is no doubt very good results can be obtained with them - are equivalent to cool white fluorescents. Yes they work, but why bother going to this expense and trouble when other bulbs will yield much greater success?
One interesting variation on this theme is the self ballasted bulb. These bulbs (around 250 watts) require no ballast, they just screw into a standard medium base (ie. incandescent) fixture and voila, light. These lamps have a duo-spectrum for colors, besides it emitting light on the blue/white end of the spectrum, it does emit reddish/yellow light (from the filaments), therefore, it does have more of a light spectrum that plants depend on. The downside is these bulbs are not as efficient as regular mercury vapour lamps because they use the resistive properties of the large filaments as a ballast, and these bulbs are expensive, around $50 for 250 watts. Of course with mercury vapour lamps having a 10,000 hour lifespan the high cost of the bulb must be considered in view of the lack of expense for a ballast.
Sodium vapour lampsThese lamps come in two varieties, high pressure sodium and low pressure sodium, although this is rather a moot point, as the light they output is monochromatic (pure) yellow, and are generally used in conjunction with sunlight or metal halide lights. They are a full ten times more efficient then incandescent bulbs, in fact these are the most efficient bulbs made, and have a 24,000+ hour lifespan. These are one of the cheapest HID bulbs to purchase, and can be found in most hardware stores for around $80 for bulb and ballast. Spare bulbs are around $30.
Metal HalideLike sodium vapour, these lamps come in two versions, regular and colour corrected (HQI) versions. The HQI versions have a uniform, sunlight-like output spectra, whereas the standard halide bulb has a lot of yellow, some blue and not much red. Unlike sodium vapour, these lamps are very useful to the grower needing a lot of light. They can be found nominally in 250, 400, and 1000 watt sizes, from most manufacturers, but Osram also makes a 70 watt and a 150 watt size. The 70 watt bulb is only 2 x 3 inches, although is unfortunately a 3000K colour temperature bulb. You have to go to a 250 watt bulb to get 5400K colour temperature.
These bulbs range in life from 6000 to 10,000 hours. Bulbs are around $50, ballasts are around $100.
Some sample setupsObviously with a plethora of different type of lighting systems to choose from, trying to figure out what tube to use can be a nightmare. Largely it depends on what you are trying to illuminate, and what your budget it. It also depends on what size area you are trying to illuminate.
Many small plantstands have a small plastic or metal hood that has one or two tube shaped incandescent bulbs. For the bulbs to provide enough light to grow plants they need to be of such high wattage that there can be an excessive amount of heat being given off from the bulbs.
Incandescent illumination, although inexpensive in initial setup cost is not recommended. The heat generated by these light bulbs almost always overheats the plants. The cost to operate is fairly high, and the quantity of light is low compared to the amount of heat produced. Some of the smaller halogen bulbs are useful for supplementing fluorescent lights, as the halogens, because they are still incandescent, put out quite a bit of red light. Not only does this help to balance the spectrum, but it has a more pleasant aesthetic appearance.
Theoretically a 300 or 500 watt halogen lamp could be used but 500 watts is a lot of energy; a 175 watt metal halide bulb will provide the same amount of light for a lot less energy. The only practical use for incandescent lights would be in a setup that was primarily fluorescent. A couple of small halogen bulbs, if well shielded from water splashes would provide the red light so needed by plants.
Fluorescent lights are the most economical way of lighting your plants in the long run. Once the initial purchase of the fixture is made the low cost of operation and long life of the tubes makes fluorescent light very attractive. For a beginner that has an incandescent fixture the new compact fluorescent bulbs with integrated ballasts will, in many cases, screw right into the existing socket. Bulbs for these are available from 2700K to 5000K colour temperatures, although as of this writing only Osram makes 5000K compact fluorescents.
The absolute cheapest setup is to buy whatever fluorescent tubes are on sale at the local hardware store. Usually cool white. This is far from the best, but it will work. One cool white and one warm white is a little better, although one plant growth light and one daylight bulb is still a fairly cheap setup, (both are well under $10) with quite good light quality.
For growing plants, a setup consisting of one plant light, two wide spectrum plant lights and one chroma 75 (or equivalent) will provide the right amount of the correct type of light. Triton (or equivalent) tubes could be used of cost is no object. If the pinkish colour is objectionable, two Ultralume 3500 and two Ultralume 5000 can be used instead of the wide spectrum plant lights.
Low light plants will do ok under two Gro-Lux or Gro-Lux wide spectrum tubes.
For growing high light plants, two (or four, depending on preferences) chroma 75's can be used. Or an HID lamp would probably be the most appropriate. Rather than a large number of fluorescent tubes to supply enough light, it would have been cheaper to install a halide lamp in the first place.
The cost of the HID lamps is pretty large, and even worse, the more useful lamps to growers of plants are even more expensive. Usually mercury vapour or sodium vapour lamps are available at semi-reasonable rates from hardware stores where they are sold as security light; especially in rural areas. I have heard of people trying sodium vapour lamps, but have never heard of any success with them. People have had some mixed success with mercury vapour lamps. Metal halide lamps give very good results, but are the most expensive and difficult to obtain of all the HID lamps.
For applications requiring a REALLY BRIGHT light, the current GE lighting catalog lists a 10,000 watt carbon arc lamp used for lighthouses.