By Drs. Foster & Smith Aquatic Veterinary Staff Aquarium lighting not only enhances the appearance of your aquarium, it also helps boost the general health of your fish, plants, and invertebrates. When designing a lighting system for your aquarium, you should try to duplicate - as closely as possible - the conditions in which your aquarium pets would live naturally. Thankfully, technological advancements in aquarium lighting over the past decade have significantly simplified the task of simulating natural conditions. This two-part article will explain the characteristics of light and discuss the duplication of natural light in fish-only, freshwater planted, and saltwater reef aquariums. Several factors including water depth, water clarity, weather, and air clarity affect the color and intensity of light in natural aquatic environments. These factors, combined with varied overall water conditions between individual aquatic habitats, cause light spectrum and intensity to differ from one aquatic environment to another. Understanding how aquarium lighting duplicates natural underwater lighting conditions requires an understanding of how these conditions are measured. Light Spectrum - Measured with the Kelvin Scale The spectrum or "temperature" of light is measured in degrees Kelvin (K). The Kelvin scale describes the color of a light source when compared to a theoretical "blackbody." Think of a piece of steel that changes color as it is heated. The Kelvin scale does not measure the light the steel gives off as it is heated; instead it measures the color of the light being given off. Color temperature does not represent the temperature of the light source. It merely defines the color given off in comparison to a blackbody radiating at the same temperature. At 0 degrees Kelvin (equivalent to -273° Celsius), the theoretical blackbody emits no light. As the blackbody warms, it begins emitting red light. As the blackbody continues to increase in temperature, light wavelengths become more yellow, then green, blue, and finally violet. A candle flame on the Kelvin scale has a rating of 1800 K. Sunlight at noon, with a Kelvin rating of 5500 K, is typically referred to as full spectrum because it contains a blend of all colors throughout the spectrum. Reddish light has a lower K rating and color temperature, while bluer light has a higher K rating and a higher color temperature. In nature, as light enters and passes through the first 15 feet of water, the red and orange wavelengths are absorbed by the water, increasing the light's K rating, and giving the light a bluer appearance. As the light penetrates to 30 feet, the water absorbs the yellow spectrum. And as the light continues past 50 feet, the water filters the green wavelengths, leaving just the blue and violet wavelengths. This results in light with the highest Kelvin rating. Light intensity - Measured in Lux and Watts Light intensity can be measured in two ways: at the surface it impacts (in lux), and at its source (in watts). Lux Light intensity measured at an impacted surface is expressed in lux, an international metric unit of measurement similar to a foot-candle: 1 foot-candle equals 10.7 lux. The intensity of the sun on the water surface above a reef can reach values exceeding 120,000 lux. However, varied weather patterns and air quality causes this measurement to average approximately 75,000 lux. As sunlight enters the water and the different wavelengths are absorbed, the light intensity diminishes. Water clarity determines the speed at which light degrades in different aquatic environments. For example, the level of intensity on a reef with clear water will average about 20,000 lux at a depth of 15 feet, and 10,000 lux at 30 feet. Knowing where an organism lives in nature will give you a good idea of the light intensity required to maintain that organism in your aquarium. Lux meters are relatively inexpensive, and can be used to check your lighting for required intensities, as well as to determine when bulbs in your lighting system need replacing. Watts Light intensity at the source is measured in watts. The higher the watts, the more intense the light, and the more energy required to produce the light. A 100-watt bulb, for example, will give off more light than a 40-watt bulb, and will cost more to use. A watt is actually related to a lux. One lux is equal to 1.46 milliwatts (0.00146 watts) of energy of one specific frequency (555 nm) hitting a surface area of one square meter. However, since bulbs used in aquarium lighting systems emit light of many frequencies (not just 555nm), no exact formula can be used when determining the number of lux produced by a bulb of a specific wattage. Part two of this article discusses duplicating the natural light spectrum and intensity for fish-only aquariums, freshwater planted aquariums, and saltwater reef aquariums.