Over the years, reef tank lighting has progressed from Fluorescent to VHO to Metal Halide to Compact Fluorescent to, (finally) Light Emitting Diodes (LEDs). Whenever a new product hits the market, they are normally relatively expensive (someone has to pay for the R&D). Prices come down when the product develops a demand and competition kicks in as more companies begin to offer the new product. To begin with, the LED lights were offered only as lunar lights added to existing fixtures to be used only at night. As their popularity increased, and their cost came down manufacturers began to offer fixtures in a wide variety of sizes and shapes.
The first LEDs weren't anywhere near what a saltwater reef tank would require as far as lumens, Correlated Color Temperature (CCT) and/or Color Rendering Index (CRI). LED development has grown at a remarkable rate in the past few years, both in colors available and light intensity. There are now 8000K Daylight and Actinic LEDs being used in aquarium lighting. There will be 14000K LEDs available soon. Compare prices on saltwater aquarium LED lights.
How LEDs Work
The left side marked P has positive bubbles (protons); the right side marked N has negative bubbles (electrons); the center area shows where protons and electrons meet, with arrows leaving the box at this point, marked Light. Junction (depletion region) is marked above the N section. Electrons flow through a wire connecting the P section to the positive terminal of a battery, and through another wire connecting the battery's negative terminal to the N section.
LEDs differ from traditional light sources in the way they produce light. In an incandescent lamp, a tungsten filament is heated by electric current until it glows or emits light. In a fluorescent lamp, an electric arc excites mercury atoms, which emit ultraviolet (UV) radiation. After striking the phosphor coating on the inside of glass tubes, the UV radiation is converted and emitted as visible light.
An LED, in contrast, is a semiconductor diode. It consists of a chip of semiconducting material treated to create a structure called a p-n (positive-negative) junction. When connected to a power source, current flows from the p-side or anode to the n-side, or cathode, but not in the reverse direction. Charge-carriers (electrons and electron holes) flow into the junction from electrodes. When an electron meets a hole, it falls into a lower energy level and releases energy in the form of a photon (light).
The specific wavelength or color emitted by the LED depends on the materials used to make the diode.
Red LEDs are based on aluminum gallium arsenide (AlGaAs). Blue LEDs are made from indium gallium nitride (InGaN) and green from aluminum gallium phosphide (AlGaP). "White" light is created by combining the light from red, green, and blue (RGB) LEDs or by coating a blue LED with yellow phosphor.
*Description of How LEDs Work courtesy of the U.S. Dept. of Energy. Compare prices on saltwater aquarium LED lights.
Typical Luminous Efficacy and Color Characteristics of Current White LEDs
How do currently available white LEDs compare to traditional light sources in terms of color characteristics and luminous efficacy? Standard incandescent A-lamps provide about 15 lumens per watt (lm/W), with Correlated Color Temperature (CCT) of around 2700 K and Color Rendering Index (CRI) close to 100. ENERGY STAR-qualified compact fluorescent lamps (CFLs) produce about 50 lm/W at 2700-3000 K with a CRI of at least 80. Typical efficacies of currently available LED devices from the leading manufacturers are shown above. Improvements are announced by the industry regularly. Please note the efficacies listed below do not include driver or thermal losses.
* The chart above is a bit out of date now, as LEDs in the 8000K and above range are now available.
* Sources: Manufacturer data sheets including Cree XLamp XR-E, Philips Lumileds Rebel, Philips Lumileds K2. Compare prices on saltwater aquarium LED lights.
Advantages of LEDs
So, what are the advantages of LED reef tank lighting? To be sure, the low wattage and long life of the LED bulbs (some manufacturers claim up to 50,000 hours) are definite selling points. LED users have also raved about the low amount of heat produced, which helps to keep tank water temperatures down as well as the "shimmering" effect produced by the LEDs.
What are the disadvantages? One possibility might be the purchase price, but if you factor in the long life of the bulbs and the low amount of electricity required to drive the lights, you might even be saving money over the long haul. The light intensity may be another drawback for a reef tank with hard corals.
An comparative example would be the 24" Marineland Pro Lighting 24-Hour Lighting System with a total of 246 watts (1 150W 14,000K HQI lamp, 4 24 Watt T-5 lamps) and 2 1 watt lunar LEDs. Compare Prices for about $560 and the 23" ecoxotic Panorama LED with 54 White 8,000°K LEDs and 18 Actinic Blue 460nm LEDs with a total of 73 watts Compare Prices for about $760.
The LEDs last up to 50,000 hours) (11 years at 12 hours lighting/day), while the HQI's and T-5s last about 1 year. The replacement cost for the HQI replacement bulbs is about $60 and the replacement cost for the T-5's is about $16 each ($64 /yr). Even if you don't take into consideration the cost of the power savings, the LED package is just about break even with the HQI & T-5 fixture over three years.
Many LED light users have noted an almost immediate reduction in nuisance algae growth in their tanks.
The Future for LEDs
Presently, there aren't too many LED bulbs that are designed specifically for reef tank use. What normally happens is: a bulb with specific properties (CRI, CCT) is developed for another use, than are adopted for aquarium use. At the time of this writing, there are LED bulbs in the 8000K range which are being used for reef tank lighting. There is no doubt that higher-powered bulbs will be developed in the near future which will work even better for aquariums. Compare prices on saltwater aquarium LED lights.