I see some of you newbies were missing at head count this morning. Too tough for you huh?
I imagine some of you were involved in that HQI stuff. You can't fool the old WaterKeeper. No wonder Freeskier couldn't tell much about his tank. If you are High on quartz and iodine your lucky to know what day it is much less the details on your tank.
I really hope you have been able to follow me so far in this thread. I think lighting is one of the most difficult topics to understand in reefing. Complicating matters most is that the technology here is in constant change with new equipment appearing almost on a monthly basis.
Anyhow, to this point we talked a little about how these lighting systems work. Whether it is fluorescent or metal halide you have a tube filled with an inert gas. A starter circuit creates an electrical arc in the tube between the filaments. A ballast then limits current going to the tube so the lamp doesn't burn out. In a fluorescent this discharge in the tube excites phosphors coated on the glass tube. These emit light along the entire length of the tube. In an MH system a gas cloud inside the tube becomes very hot and emits visible light. The basic difference between the two types of lighting is that fluorescent lighting is a continuum source, spreading the light out over a relatively large area, while MH is a point source with the light mainly emanating from a relatively small area.
I already mentioned that ballasts for MH lighting ignite the bulb using either a starting probe, probe start, or pulse high voltage between two electrodes, pulse start. Fluorescent lights have a couple of different starting systems too. Almost everyone is familiar with the two common type tubes in use, instant start and rapid start. They are pretty much a household staple. In instant start tubes you flip the switch and the light comes on right away. Rapid start usually has a flicker for a second or two. That is because here the ballast looks at the tube and checks to see that it fired. If not it sends another high voltage jolt to the tube. It does so till in detects that it is illuminated. The oldest method is called pre-heat and is common mainly in power compacts. One of the things about rapid and instant start lamps are they do best in nice cozy environments. Put them out in the cold and they may refuse to function. Instant start lamps in a cold area like Ohio pretty much fail to light if outdoors on a cold March day. Rapid starts have some advantage as their repeated attempts to fire create heat and may get them going in close to freezing temps. They usually fail if below zero temperatures are encountered. One of the oldest methods to fire a fluorescent is a preheat start. Here moderate voltage is applied to first warm the filament. This helps warm and vaporize mercury in the tube. After a short preheat the voltage is kicked up igniting the lamp. This type of lamp will work in cold weather. It also can help a lamp burn longer.
Why? Well every time a lamp is fired the high voltage at the filaments sputters?off some of the filament material. That is some of the filament itself is vaporized. You can visually see this effect in a fluorescent. This is the dark area that forms near the ends of the tube. It is material that leaves the filament and is deposited on the relatively cool walls near the ends of the tube. It fortunately doesn't move much further down the tube. The more often a lamp is started the more material leaves the filament and eventually the lamp fails. In a power compact the filament is fairly small compared to its larger cousins so the filament wouldn't last too long. Using a preheat starting system allows less of a kick?to be needed to fire the lamp, thus reducing sputtering and conserving filament life.
The big thing here is if you use magnetic ballasts make sure they match the lamp's firing method. A preheat won't work on a ballast for an Instant-On type fluorescent. These little subtleties are not usually a worry when doing home lighting as people usually buy a complete light system. In reefing many hobbyists like to experiment and purchase individual components. It is a pain to find that the ballasts and lights you buy don't match. Most electronic ballasts are designed to fire any type bulb so they are an advantage if you plan on experimenting with different lighting types.
One more word about that sputtering?effect I mentioned. In a fluorescent the deposited material is confined to the base of the tubes. Sputtering also occurs in MH. Here it is more of a problem. The material that sputters off coats the entire tube surface, not just the ends. Over time it tends to obscure the tube envelope reducing output and causing a shift in the emitted spectrum. Lamps that allow hotter operation of the tube, such as Ski's HQI, decrease this effect as the tube wall allows less condensation of sputtered material.
I want to mention a little more about the HQI lamps. HQI stands for High Quartz Iodide and has become more or less a generic term for double-ended MH lamps. MH lamps come in two varieties, the mogul based, single ended and the double ended. The first type has nothing to do with those snow bumps on a ski slope. They refer to lamps that screw into a socket much like the bulbs in a table lamp. These bulbs have an inner tube, which provides the light, and an outer glass tube that acts as a safety shield. Most people think that this shield guards against breakage of the light tube inside. This it does but it also provides protection against an unseen hazard, short wavelength Ultraviolet. Shortwave UV is what causes you to get a sunburn when outside. You may have noticed if you sit by a window in full sun you don't get a burn, or tan for that matter. That's because standard glass is a strong barrier to shortwave UV and blocks almost all of it. A fluorescent emits UV but the glass tube does not transmit it. The fluorescent tubes in a tanning booth are made of quartz glass to allow UV to pass. In a single ended MH the glass shield contains the UV but in the double ended there is no shield. To make matters worse, the tube is a high quartz glass. This protects it from heat but does not protect you as quartz passes UV.
Double ended (DE) MH needs a glass protective shield on the hood or other fixture! Without this shield serious eye damage can occur!!This shield must be glass or specially coated plastic. Plain acrylic plastic won't stop UV radiation. Please be very careful to see to it this shield is used. Staring up at an unshielded DE tube can damage your eyesight and lead to blindness.
One thing I also haven't mention yet is the T numbers assigned to fluorescents. If you look at a common fluorescent you see a number like F40T12/845. The number following the F?is usually the wattage but sometimes the length. The number that follows the T?is the diameter of the bulb in eighths of an inch. The numbers run up to T17 but only two or three are generally used on aquariums. The first, a T12 is 12/8ths or 1.5 inches in diameter. Most bulbs were T12's in the past but some people did use 1", T8. Lately 5/8?T5's have gotten much attention. We will discuss these smaller tubes shortly.
The last number, here 845, gives two values. The first, 8, is the Color Rendition Index (CRI). This is a value that a standard Observer?sees when comparing colors under the artificial light to true sunlight. A value of 100 means the bulb matches sunlight completely. Here in the number
845 the 8 means that the bulb shows colors at about 80-89% of that as observed in full sunlight. It use to be that the Standard Observer?was Clark W. Griswold of Chicago, Illinois but since it was leaked he was colorblind there has been a replacement. I have no idea who that is.
The last two digits in the 8
45 are the color temperature, K, times one hundred, so in this case the bulb has a K value of 4500. Sometimes there will be letters following those numbers. A HO for instance is high output. Unfortunately, the manufactures often take liberty with the system and don't label their tubes in this fashion.
Well, we are now starting to get into things like CRI so I think it is time to assign some homework. There are many terms used in lighting so look up Color Rendition Index (CRI), Kelvin, lumens, lux, candlepower, foot-candles, lamberts, phot, apostilb, Photosynthetically Available Radiation (PAR), Photosynthetically Available Usable (PUR), Photosynthetic Photon Flux Density (PPFD), Einsteins, and E=(π/4)δ
2Ω(I think the last is the name of a Fraternity).
Well I ready for some corned beef and cabbage. Great chow on a fine day for the Irish. Till next time.