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Fig.1

What is a bin? LED makers sort out their product into several categories, depending on certain characteristics, which they call bins. Color bins contain LEDs with similar wavelengths, or spectrum; luminosity; whereas, efficiency bins consist of LEDs that are sorted out according to the efficiency of converting the electric energy into light. Each manufacturer assigns certain bin codes to these LEDs, and this bin code is attached to LED model number in order to better characterize the properties of the lot that is offered for saleIf we look at the offered color bins, for example, we can see that emitters with different prevailing wavelengths are available for the same LED type. For example, consider the newest generation of Philips Luxeon Rebel ES Royal Blue emitters (Fig 1):

Choice of LEDs The specifications of many reef LED fixtures available today are not clear.Most manufacturers do not specify, the particular LED bins which they used in their fixture.Bins are a very important characteristic which we need to keep in mind when building a LED fixture. This is number 1 important. Are you getting the best bin or generate bin led?

Part One: Planning When planning for a light source for a reef tank, there are many aspects that we needed to take into account. One challenge is the choice of LEDs that would provide for optimum efficiency. The next challenge is achieving uniform color mixing, in order to avoid unsightly color shades (“Disco” effect) in the tank. Another challenge is how to organize efficient heat removal from the LEDs. Last but not least, we need to “fill up” the spectrum, i.e. which supplemental colored LEDs to use for best visual representation of the corals.

We like to create a light source that would not only good for coral growth but would also yield premium results in the visual perception of a reef tank

We now know the theoretical aspects of reef lighting, such as the light spectrum and intensity required for growing corals and achieving their best coloration.We would now like to discuss the various practical challenges one faces when designing a LED light for the reef tank.

Hope this info can help you in selecting the right light for your reef tank.

As we have seen, formal parameters such as CRI and CCT are not very useful for determining whether a particular light fixture is suitable for a reef tank. At the same time we need to point out again that sufficient power in the 400-480nm wavelength range is critically important.We have to admit, unfortunately, that most of the commercially available light fixtures today are only utilizing the 450nm range and above, whereas an ultimately important range between 400 and 440nm is usually left out, or is inadequately represented.

If the reef tank is only illuminated in these wavelength ranges for 12 hours, with short sunrises and sunsets specific to the equatorial zone, we will obtain an average radiation power of 400μmol·photons/m2/s, which is sufficient for optimal production of chromoproteins. Since the light fixture is likely to also include LEDs in other wavelength ranges, we can safely assume that these figures include some power margin.

Marine photosynthetic organisms most efficiently utilize radiation with the wavelengths around 430nm, and this range also stimulates their most intensive coloration. We believe that the most reasonable maximum radiation power should be about 45W/m2 for the 400-440nm range and about 40W/m2 for the 440-480nm range. To determine the number of LEDs required in a fixture and their rated current these figures must be converted into electrical power, which depends on the efficiency of the LEDs actually used.

the 400-500nm range is the most required, since it provides the best coloration and fluorescence in corals; whereas, the longer wavelength radiation in 500-700nm range is poorly utilized by marine photosynthetic organisms. At the same time, the human eye is very sensitive to the 520-600nm range and therefore we do not need very much radiation power in that range: even small amounts of illumination will be sufficient for the eye to perceive the tank as brightly lit. Meanwhile, supplementation of 660nm LEDs can be beneficial for shallow-water organisms. At the same time, this wavelength, in combination with the 400-420nm range, will promote correct rendition of the purple color. As we have shown, the 400-480nm range is most important for marine photosynthetic organisms. In their natural environment corals are getting 52 to 55W/m2 of optical power in the 400-440nm range and 60 to 64W/m2 in the 440-480nm range.

We shall now try to estimate the amounts of light at selected wavelength ranges: 400-440nm, 440-480nm, 480-520nm, and 520-700nm. Each range will correspond to one color channel in a LED fixture and can be achieved by using one type or a combination of several types of LEDs.

Many hobbyists tried to use inexpensive no-brand Chinese LEDs in the pure actinic range. However, their efficiency is low and, as a result, the crystal deteriorates quickly due to overheating. Worst of all, this deterioration is hard to estimate visually, since the eye's sensitivity in 420nm range is very poor

The gap in the 480nm range is properly filled (this fixture uses Cree's blue LEDs). Besides, a small peak in the 660nm range is available. However, any wavelengths in the 400-430nm range, which could promote the fluorescence of many marine organisms, are virtually missing. This range is missing in the majority of reef LED fixtures. Until recently, no LEDs of proper quality were available in the market for the 420nm range. For the few available offerings the prices were quite high, along with short operation time and poor efficiency.

Fig. 23 shows the spectrum of Ecotechmarine Radion, ReefBuilders 2011 LED Fig. 23

We shall now turn to actual implementation of LED fixtures for the reef aquaria.Using just two types of LEDs (white and blue) is not sufficient, because such a fixture will miss a significant amount of light in the 400-450nm range - much less than it is measured in the ocean, at the depth of just a few meters. The 450nm spectral range can be easily scaled up by using Royal Blue LEDs with a corresponding peak.

Adding a LED with the DWL bin code 4, we can flatten the white LED's spectral curve in the 430 to 600nm wavelength range.

Table 1

Radiation of the Blue LED is most suitable to compensate for the required 470-490nm range. Even a better match could be achieved by using a LED with a 475nm peak - fortunately, such LEDs exist!To better explain this, let us consider the term bin, which manufacturers use to characterize their LEDs. A bin is a group of LEDs that have been selected according to a certain parameter. There are efficiency bins, CCT and CRI bins, and dominating wavelength (DWL) bins are available for monochromatic (single color) LEDs. DWL bins for blue LUXEON Rebel color LEDs are shown in Table 1.

Let us now consider the spectrum radiated by various LEDs. The spectrum of a cool-white LED with CCT around 7000K is shown in Fig. 21. Fig.21

The gaps - wavelengths that are missing in a discrete spectrum - mean that certain tints of color cannot be correctly rendered under such illumination and, as a result, the light source will have a low color rendition index (CRI). Meaning that when using these bulbs precise color rendition cannot be achieved.we can safely state that maximum radiation in the required range is only achieved when a CCT of approximately 20000K is declared.High CCT bulbs are often characterized by a significant portion of radiation in the 420-430nm range. Experienced reef aquarists recommend 20000K bulbs for providing the best color for marine organisms. This advice, obtained through years of practice, matches well with the conclusions we derived above.

Fig.18

It is important to know, however, that CRI is calculated for light sources with a particular color temperature. It is not appropriate to compare a 2700K, 82 CRI light source with a 5000K, 85 CRI source.Also note that CCT and CRI are only defined for full-spectrum light sources. The CRI of monochromatic light is close to zero, and its CCT cannot be calculated.All fluorescent and MH bulbs have a discrete spectrum, whereas sunlight has a continuous spectrum. Discrete spectrum is a result of using a discharge in mercury (and other metal) vapors, with several peaks at different wavelengths, mostly in the ultraviolet range. Phosphors on the bulb convert this radiation into narrow bands of visible light. A discrete spectrum vs. continuous is shown in Fig. 18:

Another important characteristic is CRI - the Color Rendition Index. To determine the CRI, a set of 8 standard color samples is illuminated with the source and with the light of a back body with the same color temperature. If none of the samples change their color, CRI is equal to 100.

Today let us consider important characteristics of lightFirst such characteristic is CCT - Correlated Color Temperature. CCT of a given light source characterizes the temperature of an absolutely black body that would radiate a similar spectrum. The hotter the black body, the higher will be the CCT and the more blue or "cold" will be the light. As an illustration, sunlight has a yellow tint, whereas blue giants - huge stars with high temperature of the surface: 10000K and above (Sirius, for example) - seem bluish even to the naked eye