Part 1 – Introduction:
High-Intensity Discharge (HID) lighting has formed the core of lighting technology in the horticulture industry for the last 20 or so years. Metal Halide (MH) and High-Pressure Sodium (HPS) lights are the most commonly used variants of the HID family. This is no surprise as HID lights offer the most lumens per watt in practical lighting systems. So what about fluorescent lamp (FL) technology? Well, in our opinion (also supported by most high-yield professional growers) FLs just don’t cut it when used alone in complete cycle growing. Although their spectrum is fat and broad and can be tailored specifically to maximize lumen power in the most sensitive areas of the photosynthetic curve (i.e. most PAR watts) their overall light output and price make them less than ideal replacements for HID lights. That being said, FLs, especially Compact Fluorescent Lamps (CFLs) definitely have a place in the modern high-yield garden and used properly they are an exceptional supplement to traditional high-lumen HID lights. FLs, when used in-conjunction with traditional bulbs, have the capability to dramatically boost photosynthesis, improve plant health, enhance fruit quality and size and increase overall yields.
High Intensity Discharge (HID) Lamp Technology
Modern horticultural lighting is divided into to main families- High Intensity Discharge (HID) and Low-Intensity Discharge (LID), namely fluorescent technology.
The HID light family contains both HPS and MH lights and is based on a structurally strong, double envelope light with a compact arc tube operating under relatively high-pressure that emits light. Fluorescent lights, on the other hand, have a long arc tube that operates at a relatively low pressure approximately equal to one atmosphere.
HID lamps were designed for street, stadium, and industrial lighting. More recently, they have become popular in horticultural lighting due to their ability to produce the most lumens (light power) per watt of electricity. All HID lights share the common requirement for external ballast.
The most popular type of ballast is a magnetic ballast (large, heavy and cheap) and is technically referred to as a CWA (constant wattage auto-transformer) type that and uses a magnetic auto-transformer to provide a voltage boost upon startup and in conjunction with a capacitor, to provide current limiting to the bulb once ignition has taken place. Without the ballast, the HID bulb could not ignite – and if it were to ignite, the bulb would burnout or explode without the current limiting capabilities provided by the ballast as HID bulbs tend to draw as much power as they can, even if this leads to self destruction (a phenomenon known as negative resistance). Therefore, it is critical that the ballast power (wattage) rating match the HID bulb rating and be designed for the HID bulb type i.e. a 400W HPS ballast must only be used with a 400W HPS bulb and cannot be used with a 400 W MH bulb or a 250W bulb or a 1000W bulb etc. This is due to two factors:
Popular MH bulbs have an internal starter and do not require an external igniter, as is the case with HPS bulbs. As a result the electrical circuitry in the HPS and MH ballast is different.
HPS bulbs generally have a much higher initial ignition and starting voltage that remains relatively constant regardless of their wattage. The electrical values for HPS bulbs are approximately 2,000 to 5,000 volts for ignition and 350 volts for initial startup. MH bulbs on the other hand vary greatly from approximately 55 volts for 50W bulbs to 340 Volts for a 1,000 W unit. MH ignition voltages are also correspondingly lower. Exceptions to the interchangeability rule are electronic and “switchable” ballasts.
Electronic (Digital) Ballasts:
These are a recent introduction to the HID world and consist of a computerized digital circuit accompanied by associated support components. They are more expensive than magnetic ballasts and offer automatic detection of HPS and MH bulbs so that both bulb types can be accommodated. In general, they also offer improved light output as the bulb ages, extend bulb life due to more accurate control over current, the appearance of a “flicker-free” light output due to the electronic ballasts high-switching frequency (generally 15,000 Hz as opposed to 60Hz for the standard magnetic ballast) and lastly, slightly improved efficiency (up to 8%) that leads to slightly reduced electrical costs. Although electronic ballasts offer technical improvements over magnetic ballasts, their cost is approximately three to five times higher than that of the magnetic ballast and is seldom used in any significant quantity. Additionally, electronic ballasts are generally limited to a power rating of 400 watts with some (expensive) 600-watt models on the horizon.
Switchable HPS/MH Ballasts:
Recently introduced, these ballasts allow the use of both HPS and MH bulbs. They operate by simply switching the HPS igniter out of the circuit. Generally, the switchable ballast consists of an HPS ballast with a switchable igniter. When an HPS bulb is used, the igniter is switched into the circuit. When an MH bulb is used, the igniter is switched off or out of the circuit. Although switchable ballast technology is simple, the user should exercise extreme caution because of the different voltages that HPS and MH bulbs operate on.
We do not recommend the use of simple, home-brewed and simple switchable ballasts that only take the HPS igniter out of circuit when in MH mode. Some readers may have the inclination to “make” switchable ballasts using a 1,000 watt HPS ballast core. Remember, HPS and MH bulbs have different operating voltages and electrical characteristics, this makes them incompatible with the idea of a simple switchable ballast. On certain occasions the MH bulbs have been known to rupture and explode. Although you may know someone who has successfully deployed simple switchable 1,000-watt ballast, the stress that this system places on the MH bulb can cause rapid breakdown, bulb integrity failure and fire. If you must use a single ballast for powering both MH and HPS bulbs, choose an HPS ballast with HPS main lights (Bloom/Flower) and purchase a few MH conversion bulbs to cover your vegetative cycle. Additionally with our new CFL and HPS balanced spectrum technology, there is a reduced requirement for MH lights if the vegetative cycle is short, i.e. in sea-of-green applications.
HID Lamp Types and Seasonal Applications:
The following section covers some useful basics of MH and HPS bulb technology.
Common Core: The simplest form of HID lighting is the High-Pressure Mercury Vapor (HPMV) bulb that was developed circa 1934. HPMV lamps consist of an internal arc tube made of quartz encased in a glass outer envelope. The sealed internal arc tube contains inert (non-reactive) argon gas accompanied by a small amount of liquid mercury. Upon ignition, the arc emits both visible and ultra-violet light that later becomes a bluish-white light once most of the liquid mercury becomes vapor (three to five minutes). The HPMV bulb is the granddaddy of all HID lights and forms the basis of both the MH and HPS described below. For the horticulturalist, each lamp type has a specific application, and used properly, provide promising results.
MH Lamps and the Vegetative Stage:
As noted, MH lamp design is derived from HPMV technology. However, in addition to straight mercury and argon gas, the internal arc tube ‘s mix is doped with a variety of metal impurities known as metal halides. These impurities include combinations of sodium iodide, scandium iodide, lithium iodide, thallium iodide, and indium iodide. The purpose of adding these extra compounds is to increase the lumen per watt output and to change the spectrum of emitted light to bring it closer to natural daylight. That being said, MH lights are still less than ideal for horticultural applications because their light is predominantly greenish/pinkish in quality and thus produces less photosynthetic power (PAR watts or plant grow power) than the lamps actual electrical wattage or lumen (visible light power) output would indicate.
For reference, most 1,000-watt “horticultural” MH bulbs only produce about 360 PAR watts (usable plant-grow power). Additionally, most experienced horticulturalists only use MH lights for the vegetative growth cycle of plants as their spectrum is more suited to simulating the “spring” seasonal cycle due to the virtually non-existent red spectrum. Crops grown on a diet of only MH derived light tend to have more elongated or “stretched” stems with larger internodes and generally less yield and relatively lower fruit quality. It is commonly believed that the primary reason for the plant ‘s response is virtual lack of appropriate red spectrum light that many crop varieties appear to require in building “heft”. It is interesting to note however that the MH ‘s light spectrum provides very healthy, lush, green growth making it ideal for vegetative and cloning applications.
HPS Lamps and the Bloom/Flower Stage:
Like HPMV and MH lamps, HPS bulbs also contain an internal arc tube. However, the HPS light is more technically sophisticated and has a longer internal arc tube made of a structurally stronger translucent ceramic material known as polycrystalline alumina. With arc tube temperatures running in excess of 1,200 degrees centigrade coupled with the fact that hot sodium tends to chemically attack glass and quartz, the design of the arc tube differs quite radically from that of HPMV and MH bulbs. Aside from arc tube construction, HPS lights still have an outer glass envelope and operate in a similar manner to the rest of the HID family. The light emission from HPS bulbs tends to be predominantly orange in spectrum and HPS lamps produce the most lumens per watt visible light power and the highest PAR watt plant grow power of all classes of common discharge lights.
For reference, most common 1,000 watt HPS lamps produce upwards of 400 PAR watts of grow power with “horticultural” grade HPS lights (6% enhanced blue light) producing upwards of 430 PAR watts. Due to their high quantity of near-red and relatively high ultraviolet (UV) light output, these lights are well suited for mimicking the summer and fall seasons. Crops grown under these lights tend to be stockier with tight internodes and solid well-developed fruit showing increased yields over MH lights. Additionally, due to the increased UV light output, certain fruit varieties tends to ripen better producing more sugar; the sugar is believed to be the plants defense against increased UV levels as melanin pigmentation (tanning) is in humans. However, it is interesting to note that overall plant health and photosynthetic capabilities appear to be somewhat compromised by the extremely low levels of blue light. In order to correct this situation somewhat, some horticulturalists place one open halide bulb of equal power in the middle of four sodium bulbs. This method of supplementing the light spectrum tends to produce improved results on all fronts. Some commercial vented hood manufacturers have taken this concept one step further and have offered integrated HPS/MH products. I believe that this is a step in the right direction as the enhanced spectrum light is concentrated directly at the plant site where it is needed as opposed to scattered blue light that is less effective because it is so far removed from the plant site. Mixed spectrum lighting is excellent for improved results.
Long-Tube Florescent Technology:
Florescent-lighting dates back to 1938, and consists of a long arc tube that operates at a very low pressure (also known as LID technology). Although quite a contrast to HID lighting, florescent lights do share the common concept of an arc tube filled with an inert gas that contains a liquid metal (mercury) powered through an external ballast. However, unlike HID technology, LID lights operate under completely different parameters. The gas/metal mixture in the long arc tube ignites almost immediately and then produces a cool arc that travels the length of the tube. The arc emits predominantly short-wave UV radiation. The internal surface of the arc tube is coated with a UV sensitized phosphor that converts the arc ‘s UV radiation into visible light. The composition of the coating is directly responsible for the “color” of this emitted light. Without this coating, the UV radiation would appear almost invisible and be very harmful to virtually all-living organisms. In fact UV based Ozone generators use modified, uncoated fluorescent bulbs in order to generate charged particles for the purpose of air scrubbing. Since the coating determines the spectrum of visible emitted light, it is possible to specifically design this coating to generate the desired spectrum of light. Practically however, integrating a four foot or larger florescent tube that is rated at only 40 Watts into a garden is nightmarish. This is where CFL technology comes in.
Developed circa 1980, CFL lights have the same physical (low-heat) and broad spectrum benefits as long-tube fluorescent technology with the added benefits of increased power, greatly reduced size and internal electronic ballasts that ensure log-life and tight control over brightness. They are ideal light sources for a variety of applications and lend themselves well to adoption into horticulture-when used properly. Fig 3 depicts a sample schematic of a common CFL internal power supply.
CFLs entered the horticultural market recently and as any recent technology have been somewhat misunderstood and misapplied. As a result, CFLs have earned a relatively mixed and perhaps poor reputation. Due to their relatively high cost and low lumen output, CFLs are not an ideal replacement for HID lighting in horticultural applications. Aside from cloning, for which they are ideally and naturally suited due to their broad-spectrum light and low heat contribution, the “killer application” for CFLs is supplementary lighting. That is, to use the best characteristics of CFLs in conjunction with the best characteristics of the best HID lights, namely HPS lamps. CFLs offer the broadest light spectrum of any class of practical lighting technology. When equipped with a premium tri-phosphor coating that broadens the light emission even further, CFLs produce an almost flat spectral “curve” that can be designed to compensate for specific spectral deficiencies of the high-lumen HPS bulbs that form the heart of most successful high-yield gardens. Additionally, because CFLs are relatively cool burning, they are ideal for 3-D lighting whereby the CFLs are physically inserted into the plant canopy in order to light the sides of plants so that more complete fruit development can take place on the lower fruiting sites as shown in Figures 4 and 5- 3-D lighting applications. The result is a garden that produces the most.
This article covers an introduction to lighting technology and provides some common horticultural applications that suggest modern variations on “classic” time-tested, high-performance growing techniques. Article two in this series will provide practical real-world examples for enhancing your garden lighting system and maximizing your crop yield and fruit quality by using the best that both HPS and CFL technology have to offer.