The study of lighting science is fascinating stuff, but it needs to be framed from a reefkeepers practical perspective. You need to balance coral health, aesthetics, and maintenance when selecting lighting.
Maintaining high PAR values is the best way to assure photosynthetic activity for your corals. Water clarity, lighting intensity, and the presence of viable zooxanthellae are the other chief governing factors. A healthy colony of symbiotic algae within the corals tissue will supply all of the carbohydrates the coral needs; however, corals cannot live by carbohydrates alone. A steady diet of sugar isn't good for any living organism (a fact I need to practice as I preach).
Feeding your corals a diet of zooplankton and phytoplankton will allow them to rely less on the suns energy for nutrients, thus receiving the varied diet they have evolved with, and require. Wild coral populations have a constant supply of marine snow (plankton) for filter and direct (tentacle) feeding. The absence of these planktonic foods in captive reefs, causes the corals to shift to concentrated, photosynthetic algae farming.
The feeding method of a coral dictates the shape and form it assumes. Corals that are able to find nutrients in the water column will grow evenly to effectively trap these food stuffs. Corals that have little available in the way of free-floating nutrients, will grow at strange angles as they struggle to find, and utilize water flow patterns. Corals that rely heavily on photosynthetic activity, will either stretch to collect more intense light, or grow horizontally to expand their "farm".
Needless to say, photosynthesis is the crux of all algae growth, including bacterial algae like hair and slime varieties (cyanobacteria). A high PAR lamp will encourage nuisance algae growth. Aquarium maintenance companies use high kelvin, low PAR lamps to inhibit the growth of nuisance algae.
Reef aquarists all have their own personal agenda of what they expect from the tank. While some push for excelled growth rates, intense colouration is a more common goal. The presence of symbiotic algae in coral tissue, masks the vivid UV blocking and UV induced pigmentation. Zooxanthellae is drab light brown to green, and overshadows these beautiful colours.
Another factor to consider, is that lighting with high PAR values is often lacking in high colour temperature. High kelvin (blue) bulbs don't look realistic, but they do cause corals to fluoresce. When considering the aesthetics of lighting, one needs to showcase the intense colours while respecting the natural colour and appearance of fish and invertebrates. The catch 22, is that the lighting that shows these vivid colours, may not be the type of lighting that promotes their growth, or encourages brightly coloured UV blocking agents to exist. For thee reasons, many aquarists opt to go with a mix of lamps of different parameters.
One method is to use high PAR, 6500K Iwasaki lamps with a bevy of actinics to "drown it out" ("it", being the yellow colour that 6500 K lamps give water as it refracts off of larger pigmented organics and phenols). Another method is to go with a "patchy look" with a hodgepodge of bulbs. This second option fails to provide a uniform look and homogenous light quality throughout the reef. Aqua-Medic has a six foot light that uses 3 x 250 watt HQI, and 2 x 150 watt HQI. This fixture works well with three 250 watt, 14,000 K lamps and two 150 watt, 20,000 K lamps (the latter being of a super blue variety).
Not all 20,000 K lamps are created equally. The colour temperature only indicates the blue appearance of the bulb. Many companies market bulbs with false or estimated colour temperature values. To complicate things more, bulbs have a colour shift that can happen as soon as a month into the bulbs service.
At one point in time (ten years ago) Iwasaki 6500 K lamps produced more light in the 460-480 nanometer range than any so-called 20,000 k "blue" bulbs. This was possible as the blue light produced by the Iwasaki bulb was masked by the higher 630-680 nanometer (red light) produced.
I have found that Giesemann 20,000 K bulbs with no actinic supplementation have a very natural look, while giving enough PAR for good coral growth. Aqualine Buschke 20,000 k bulbs are a nice blue colour without the eerie appearance of most "blue" bulbs. I have noticed that coral growth is slower under AB bulbs, but colours show better, and nuisance algae is inhibited. AB 10,000 K bulbs have a higher PAR and growth is notably better, but you have to clean the glass more often and actinics help bring out colours.
The parameters of bulbs vary even within a manufacturers line. Four hundred watt XM, 20,000 K, single end (mogul) bulbs provide a perfect mix of parameters; however, the 250 watt version has proven to give corals a brown appearance.
Lighting discussions often lead to more questions than answers. I wish there was a definitive answer I could give as to which lighting to use, but I guess that would take the sport out of it. The best reef lighting resource is Sanjays' page here at RC.
http://www.reeflightinginfo.arvixe.com/
