Per @GoVols suggestion: Lighting for reef aquaria can be as simple or complex as you wish to make it. If you’re really serious about lighting, these are some of the terms you might want to be familiar with: Accessory Pigment: A photopigment that absorbs light and transfers it to chlorophyll a and hence reaction centers. Accessory pigments in zooxanthellae include chlorophyll c2, peridinin, beta-carotene and a few others. Chlorophyll: An organic compound with a magnesium cluster that is capable of absorbing light. There are a few chlorophylls. Zooxanthellae in corals contain chlorophylls a and c2 (not chlorophyll b!) Compensation Point: When light intensity is sufficient to produce enough oxygen to meet the respiratory needs of the organism. Compensation points of zooxanthellae are usually less than 50 microMol/m2/sec. DLI (Daily Light Integral): The total number of photons that promote photosynthesis falling upon an area of one square meter per second. If we think of PPFD as the number of raindrops falling upon 1 square meter per second, DLI would be the inches of rainfall per photoperiod. ETR (Electron Transport Rate): The rate of photosynthesis and is the flow of energy between Photosystem II and Photosystem I. Interruption of this flow can create destructive pressures that can destroy a zooxanthella cell. ETRs are measured using a PAM fluorometer. Nanometer: One billionth of a meter. PAM Fluorometer: Pulsed Amplitude Modulation Fluorometer or ‘photosynthesis meter.’ PAR (Photosynthetically Active Radiation): The bandwidth of radiation that promotes photosynthesis. It is generally those wavelengths falling between 400 and 700 nanometers, although some ultraviolet-a wavelengths will make photosynthesis happen. It is reported in units of microMol photons per square meter per second (µmol·m2·sec.) PUR (Photosynthetically Usable Radiation): That portion of PAR that is highly usable in photosynthesis. PUR varies among organisms. Zooxanthellae can utilize some portions of green radiation (up to ~550nm) while green plnts can use it to a much lesser degree. Photoinhibition: The act of regulating photosynthesis under conditions of high light. There are two types of photoinhibition: Dynamic and Chronic. Dynamic Photoinhibition is reversible response to high light (involving the xanthophylls diadinoxanthin and diatoxanthin. Energy that could be used in photosynthesis converts diadinoxanthin (DD) to diatoxanthin (DT.) In darkness, DT reverts to DD. In case the DD/DT Cycle fails to provide adequate protection, photoinhibition can become chronic. This occurs when excess light energy damages the mechanisms of photosynthesis and energy that could be directed towards growth or reproduction is channeled to repair. Long-term chronic photoinhibition can be fatal. Photon: A discreet particle of light. Photoperiod: For our purposes, the number of hours the aquarium receives light, either natural and/or artificial. Photosynthesis: A complex reaction where light energy converts a carbon source (carbon dioxide or carbonates) into oxygen and simple sugar. PPFD (Photosynthetic Photon Flux Density): The number of photons that promote photosynthesis falling upon a surface area of one square meter per second. Reaction Centers: Where photosynthetic reactions occur. Saturation: Saturation occurs when increasing light intensity no longer increases the rate of photosynthesis. Zooxanthellae saturation generally occurs at a PAR value of 300 to 400. Wavelength: A measure of a wave from crest-to-crest (or trough-to-trough.) Light wavelengths are measured in nanometers. Xanthophylls: Literally ‘yellow leaf.’ Organic compounds that, for our purposes, protect zooxanthellae/coral host from excessive light by shunting light energy away from the photosynthesis process. In corals, these include diadinoxanthin and diatoxanthin. These absorb blue light.