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Different wavelengths of light are absorbed at different rates by water. The red and infrared (IR) energy which have the longest wavelength and therefore the least amount of energy in the visible spectrum do not penetrate the water very far and are quickly absorbed at the surface. A red fish swimming near the surface
will appear red and vibrant but that same fish at lower depths will appear black since there is no red light to reflect off of it. Next is orange, yellow, green, and then the blue region around 450nm which reaches the furthest.

Although it has the shortest wavelength and highest energy, ultraviolet light is also quickly absorbed from water. The smaller wavelengths of light are easily scattered by particles in the water.

The difference between natural light at a few meters below the surface and artificial light can be quite dramatic. Professional dive photographers use xenon strobe lamps at 6,000K daylight color temperature to best show off the colors of coral and marine life as we would prefer to see it. You can see how quickly the color of the corals and fish enhanced by a xenon strobe fall off and everything turns blue in the background of professional underwater photographs.

Colors of the reef and fish that are enhanced under the artificial light would normally have a monotone bluish cast to it under natural light at that depth. This is why artificial light in reef aquariums can be very subjective when it comes to color rendering. The choice of lamp color temperature is tied to the individual aquarist's eye when other factors such as PAR ratings and coral growth are ruled out.


Spectral Distribution
Wavelength theory provides a graphical representation of radiant energy and the electromagnetic spectrum. The preferred unit of wavelength for the visible and ultraviolet (UV) regions of the spectrum is the nanometer (nm).

PAR (Photosynthetically Active/Available Radiation)
The first 200 meters of the ocean depth is termed the photic zone which is penetrated by sufficient sunlight for photosynthesis to occur and plants thrive.

PAR is a measurement used to help determine the photosynthetic amount of light needed by corals and plant life. Photosynthesis in corals utilizes energy between the blue 400nm wavelengths and red 700nm wavelengths.

What about the UV?
UVC and UVB in excessive amounts can be detrimental to fish and corals; However, UV light does occur naturally in sunlight which in balanced amounts is not necessarily harmful. Studies have found that the majority of coral reef fish produce mucus that absorbs harmful UVB rays. Corals also have developed a natural pigmentation as a protection from UV. Metal halide lamps produce UV light which can be significantly filtered by fixture glass and water depth. USHIO's Aqualite™ metal halide lamps are balanced to reduce excessive amounts of UV light for your reef system.

Aqualite™ 10,000K lamps produce the ideal balance of blue, white and red light in the spectrum to simulate daylight in the water. Our Aqualite™ lamps reduce the green light, a spectrum which can promote the growth of the "bad" algae that grows on corals.

UVC = 100280nm: Most harmful and used in sterilization to kill biological organisms.
UVB = 280315nm: Harmful and causes sunburn, skin cancer, and eye damage.
UVA = 315400nm: Longer wavelength and less energy than other UV. It is the least harmful but still can be damaging to DNA and the human eye.
Black Light Blue Lamps = 368-371nm
Actinic Lamps = 420nm Peak: Although actinic lamps peak at 420nm they do produce wavelengths in the UVA range so UV protective safety rules apply.
Visible Light = 380780nm
Photosynthetic Light = 400-700nm: The more peaks across this broad spectrum will give you the best photosynthetic performance.

Wavelength (nm)

Correlated Color Temperature
Measured in degrees of kelvin (K), color temperature is the absolute temperature of a blackbody radiator resembling that of the light source. This black body curve can be seen on the Color Chromaticity Chart (see PDF spec. sheet). Color temperature can be used as a general rule of thumb to measure the appearance of "warmth" or "coolness" of a light source. It does have its limitations, since lamps with the same color temperature rating will not often look the same between manufacturers. This is due to the different gas/metal mixtures in metal halide lamps and different phosphors used in fluorescent lamps. Other measurements such as CRI, PAR values, spectral distribution and lumen depreciation should also be considered when choosing a light source for your reef system.

Outdoor daylight is approximately 5,600-6,000K. Aqualite™ 10,000K lamps approximate equatorial daylight at 5 meters of ocean depth. Aqualite™ 14,000K lamps have a bluer tone to enhance blue corals but maintain higher color rendering for white rock and sand. Aqualite™ 20,000K+ lamps are blue in appearance, simulating deeper water environments. Blue lamps are typically not given a color temperature just as we would not assign a color temperature for magenta or green lamps. For simplifying lamp choice selection, USHIO uses 20,000K+ as a marketing term for our blue lamp.

Luminous Flux
Measured in lumens is an industry standard for measuring the visible light output from the lamp. Luminous flux measure the photopic vision of the human eye from approximately 300-700nm. Lumens come into play in a marine aquarium, since a dim tank does not show off the marine life as expected. It will make a difference on what is seen at the bottom of the tank and how the tank is illuminated in comparison to the ambient light of the surrounding room. However, lumens alone cannot be a measurement of perceived brightness because other factors including color temperature and color rendering come into play. All metal halide lamps depreciate in lumen value over time. Because the metal halides used in the 10,000K lamp are more stable, the 10,000K lamps hold their luminous flux values over a much longer period than the 14,000K and 20,000K+ lamps and thus require less lamp changes over time. Out of the box, the 10,000K and 14,000K lamps have over twice the luminous flux values as a 20,000K+ lamp and thus you would have to double your wattage of a 20,000K+ lamp to get the same visible light output of a 10,000K or 14,000K lamp.



Color Shift
USHIO Aqualite™ lamps use a proprietary mixture of salts and metals to ensure that color is stable from lamp to lamp out of the box. Metal halides break down over the life of the lamp and thus all metal halide lamps will experience a color and Kelvin shift near the end of life.

Lamp Life
The median life of metal halide lamps is statistically determined under controlled conditions on a 11 hours on, 1 hour off, cycle. Environmental factors including the lamp housing, reflector, power supply, and lamp cooling will affect lamp life considerably.

All metal halide lamps degrade in light output and may shift in color over time. Inferior lamps are easy to spot since they significantly shift in color and drop off rapidly in output. The lamp life rating between different manufacturers may be the same on paper but performance over the life of the lamp will tell you your true cost of ownership.

USHIO Aqualite™ metal halide lamps utilize proprietary rare-earth mixtures to provide the most consistent lamp color over the entire lifetime of that lamp. Our arc tube forming process and coatings ensure that light degradation levels are kept to a minimum. For the best health of your aquarium and reef system, it is recommended that you schedule regular lamp changes depending upon your timing cycles and rated
lamp life. It is normal for metal halide lamps to stabilize in color and output within 100 hours of "burn in" operation.

Brand new lamps will always produce more light than lamps near their end of life. Your corals and fish will need time to adjust to the higher light levels. It is recommended that when a lamp is first replaced, that you first raise the light fixture and then lower it as the lamp ages.

CRI or Color Rendering Index
CRI is an internationally accepted system to measure the capability of a light source to render color naturally. The closer the number is to 100 the closer that light source is rendering color like natural daylight. Fish, coral and plant life will appear much more true to life under higher CRI light sources. The CRI of the Aqualite™ 10,000K metal halide is higher than the 14,000K and the 20,000K+. We do not measure the CRI of the 20,000K+ lamp because it is blue. Although blue lamps like the 20,000K+ would have a poor CRI and low luminous flux rating, they are appealing to some in the marine aquarium hobby. That is why when it comes to marine aquarium use, all measurements of lighting must be taken into consideration, color temperature, color rendering, luminous flux, and most importantly, spectral distribution and PAR.

Operating/Burn Position
The operating position of the lamp is specified to provide the proper light output and color. Changing the lamp operating position can change the thermal properties of the arc tube during operation causing some metals or salts to drop out of the arc stream and thus changing the color of the lamp.

Timing Cycles
For the health of your fish and reef systems the lighting system should be turned on at least 12 hours per day. Check with marine biology sources to determine the proper amount of daylight hours needed for your specific species.

Ballasts and Power Supplies
USHIO always recommends ballasts that are UL recognized for safety and adhere to ANSI standards. It is extremely critical to match the proper ANSI coded lamp to the ANSI coded ballast. Failure to do so will cause improper lamp ignition, poor color and spectral performance, and short life. Check that the operating current of the ballast and ignition voltage matches the lamp. This information should be readily available from any reputable ballast manufacturer. Some ballasts intentionally overdrive the current to the lamp in order to push up luminous flux values on initial tests. The drawback to this intentional "overdriving" is a faster lumen depreciation and color shift.

For example: USHIO's 175W Aqualite™ is a pulse start lamp. Pulse start type lamps may ignite and work just fine on a probe start ballast for a few months; however, as the lamp ages, the electrodes erode and higher voltage is needed to start the lamp. The open circuit voltage provided by probe start ballasts is not sufficient to ignite a pulse start lamp. Pulse start lamps like our 175W Aqualite™ always require an ignitor to keep the lamp starting throughout its life cycle.

Use Safety Fixture Glass!
Double Ended metal halide lamps are made of quartz which allows the transmission of UV wavelengths from the arc tube. It is mandatory that tempered safety glass be used with double ended metal halide type
lamps. UV protective glass is required for any architectural use of Aqualite™ metal halide lamps where people are directly exposed to the light.

Single Ended metal halide lamps use a hard outer glass jacket which reduces UV. USHIO mandates the use of tempered safety glass on any fixtures using our Aqualite™ metal halide lamps. The safety glass not only reduces UV, but also extends the life of your lamps and sockets by protecting them from salt water corrosion. If the outer glass jacket of the single ended lamps is cracked or damaged in any way this will permit UV light exposure and the lamp should be replaced immediately.

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