BY RICHARD HARKER
It’s easy to find the reefkeeper in any neighborhood. It’s usually the house with the strange intense light streaming from it into the street. It is a rare reefkeeper who is able to illuminate a reef tank without brightly lighting the room, a large part of the house and often the neighbors’ houses as well. Unfortunately, while the light falling outside the tank may be cutting down on crime in the neighborhood, it is not serving the purpose for which it was intended — to benefit the photosynthetic animals in the tank. Because of the loss of light, the high cost of reef tank lighting, from the bulbs, fixtures and ballasts to the electricity, ends up even higher than if the hobbyist could direct all the light into the tank. Reflectors play a central role in directing this light into the tank.
While there are occasional discussions regarding reflectors, the subject rarely generates the passionate debate that other aspects of light typically do. Put a power compact advocate in the same room with a metal halide fan and the fur will fly. However, if one proudly announces that his reflector is made of aluminum flashing bought at the hardware store, the comment will hardly cause a murmur. This is unfortunate, because in several respects, the reflector used over the tank has a greater impact on the ultimate intensity of lighting than the type of lighting itself. It is the combination of bulb and reflector — not the type of bulb alone — that determines the effectiveness of the lighting, and the wrong reflector design can negate much of the effort the hobbyist has made to create intense lighting.
A reflector does one thing — it redirects light — but it serves two purposes in a reef tank. First, it reflects the light that would be lost back into the tank. Both metal halide and fluorescent bulbs radiate light in 360 degrees. Depending on the height of the bulb, only about 25 percent of the light generated reaches the tank directly. The only way to get the rest of the light into the tank is to bounce the light off some surface. A reflector that “returns†the otherwise lost light to the tank increases the intensity of light in the tank, and therefore maximizes the efficiency of the lighting arrangement by increasing the amount of light in the tank without increasing the power consumption.
The second purpose of a reflector is to redirect the light to distribute it to as much of the tank as possible. Metal halide and power compact fluorescent bulbs generate intense light in a small area. Most hobbyists want to have more broadly distributed lighting. Reflectors can spread the light from a small light source so that it covers a wider area than it would without a reflector. Focusing light into the tank, while also distributing it over the tank, creates design compromises. The best material and design to focus light is not necessarily the best material and design to diffuse light.
In the early days of reefkeeping, most tanks were lighted with fluorescent tubes. Standard fixtures for these tubes typically used a white reflector behind the tube. More expensive fixtures might have had a wider, somewhat curved, reflector, but the differences were minor.
Small reef tanks with standard hoods continue to use fluorescent bulbs with white reflectors and most freshwater planted tanks use standard white reflector fluorescent lighting. In contrast, as reef tanks grew in size, manufacturers began building custom hoods with specialized lighting that generally featured very high output (VHO) fluorescent tubes. These hoods were often made of wood, sometimes with integrated reflectors, but often with only the inside lid painted white.
In search of brighter light, hobbyists began switching to metal halide bulbs. Metal halide bulbs were able to generate more intense lighting by concentrating the light over a smaller area. A 110-watt VHO fluorescent tube distributes light over a 4-foot distance. Compared to the VHO tube, a 70-watt metal halide bulb generates more intense light immediately near the bulb, but less than the fluorescent 4 feet away. Heat distribution follows the same pattern, with the metal halide generating greater heat in the immediate area surrounding the bulb.
These differences created a few engineering challenges for designers of metal halide fixtures that the fluorescent designers never faced. First, because of the high temperatures involved, metal halide reflectors had to be much more heat resistant. Secondly, the reflector had to be designed to take a concentrated light source and distribute it so the light could cover a larger area. These engineering challenges were met by switching to polished aluminum for the reflector. Bare aluminum could handle the intense heat, and a flat reflector could spread out the light reasonably well. This became the standard metal halide fixture, and most metal halide kits sold today are unchanged from the earliest designs.
The only variation on this theme was the occasional use of a “dimpled†polished aluminum reflector called Hammertone. The small dimples were said to distribute light more evenly and eliminate the hot-spots of light that a flat reflector can create.
Digital Ocean changed the status quo in 1996 with the introduction of the Spiderlight reflector. It created quite a stir because of several new innovations in the Spiderlight. First, it used a highly polished “specular,†mirror-like aluminum reflective material made by Alcoa called Everbrite. Second, the reflector was bent into a narrow ellipse that was said to focus the light better than standard flat reflectors. In tests conducted in 1998, I confirmed the company’s claims and found that it did indeed generate considerably more light over a small area than a standard flat reflector.
Digital Ocean’s efforts ignited a flurry of innovation and now hobbyists have a growing range of options in reflectors. Questions remain, however. Is there a single “best†reflector material? Should all tanks use the same type of reflector? Are the same reflective materials appropriate for both fluorescent and metal halide lighted tanks? In this article I will address these questions and provide some data that suggests there are yet unresolved issues in reef tank lighting, and that much more needs to be learned before we can confidently say we have optimized lighting over a reef tank.
What is reflectivity?
To reflect is to give back an image. Reflectivity, however, is a little more complex than the dictionary definition would suggest. Reflectance is the ratio of reflected flux to incident flux. Reflectivity of 100 percent suggests that all the light striking a reflector is reflected back, while a reflectance of 0 percent suggests that all of the incident light is absorbed. The data in given here suggests that reflectivity of some reef tank reflectors can reach 96 percent.
There are different types of reflectivity. Specular reflectivity is the type that occurs in a mirror or any other highly polished surface. In the case of specular reflectivity, the angle of the reflection is equal to the angle of incidence. When we look into a mirror, the objects we see are at the same angle to the mirror as the angle at which we are looking at the mirror. Spread reflectivity is similar to specular reflectivity, but the reflected light is more scattered about the reflected angle. Diffuse reflectivity, as the name implies, is reflectivity where the reflected light is spread in all directions. An example of a diffuse reflector is a white painted reflector.
“Mixed reflection†refers to a reflector that exhibits some characteristics of all three forms of reflection. Real world reef tank reflectors fall into this category. There may be some degree of specular reflectivity, but at the same time, the reflection may be spread or diffuse. When comparing reflectors, we are essentially comparing the degree to which a reflector exhibits specular, spread, and diffuse reflectivity.
Measuring reflectivity
Given that reflectivity is simply the ratio of reflected flux to incident flux, it might seem like a simple matter to measure it. One need only generate a beam of light, bounce the beam off a reflector and then measure the light intensity of the incident and reflected beam. In fact, measuring reflectivity is complicated by the different types of reflectivity.
Specular reflectivity is straightforward. One need only measure the reflected light at the angle equal to the incident angle. Specular reflectivity is typically the reflectivity quoted in reflector specifications. Spread and diffused reflectance, however, are important for hobbyists, but much more difficult to measure. Spread and diffuse reflectance by definition produce reflected light that is distributed over a wide area. One needs to measure the entire reflected light field if one is to determine the total reflected light flux.
I experimented with several different arrangements to address the difficulties of measuring reflectance. Normal tank lighting, whether it is generated by metal halide or fluorescent bulbs, is impractical as a light source because both produce light from too broad a surface. What is needed is a point source and a coherent beam of light.
A laser light source was tried, but a laser is extremely coherent over the distances normally encountered in reef tank lighting, so applicability was a concern. After several attempts with other light sources proved too unwieldy, I settled on a quartz halogen light source focused with a lens and a very narrow aperture to produce a light beam approximately 1 centimeter in diameter at a distance of 1 meter from the light source. The intensity of the beam at 1 meter was measured with a Li-Cor Datalogger and PAR sensor, and noted. The reflector under evaluation was then placed approximately 40 centimeters from the light source at a 45-degree angle to the source and the sensor moved to a position 60 centimeters from the reflector at 90 degrees to the light source to again place the sensor at a total distance of 1 meter from the light source.
Measuring the light intensity at a single point proved sufficient for the reflectors that exhibited primarily specular reflectivity. However, many reflectors produced very little specular reflectivity. For these reflectors, the intensity of light over a wide area had to be taken into account.
Several common reflective materials were evaluated. They included the polished aluminum found in standard retro-fit kits, a glass mirror, dimpled polished aluminum (Hammertone), common aluminum flashing sold in hardware stores and glossy white spray painted aluminum. In addition, three different specular aluminum materials were evaluated: Everbrite, the material used in Spiderlight reflector, a reflecting material used in the PFO Lighting specular reflectors and the “Synthetic Sun†specular reflector. Each flat piece of material was rigidly mounted in an apparatus that kept the angle constant from test to test. The light source was placed on a heavy tripod that kept it at a constant distance from the reflector, and the light sensor was attached to a stand that could be moved horizontally and vertically, while maintaining a constant distance from the reflector.
Results
Taking into account only specular reflectivity, the specular reflectors performed measurably better than the other reflectors. The PFO Lighting reflector, Everbrite and Synthetic Sun reflectors were very close in reflectivity to call a winner between the three. As Figure 1 illustrates, the specular material reflected back nearly all the flux in a narrow beam. The glass mirror was a close second to the specular reflectors, followed by the polished aluminum found in a standard retro-kit. Dimpled aluminum, aluminum flashing and white paint scored very low in specular reflectivity. In the case of the last three reflectors, less than 10 percent of the incident light was reflected back as a narrow beam.
Aluminum roofing flashing has been frequently recommended to budget-oriented hobbyists as a viable alternative to more expensive aluminum reflectors. Some advocates of aluminum flashing have argued that one need not spend money on a reflector. One need only “polish†aluminum flashing to create a specular reflector.
To test this suggestion, sections of aluminum flashing were hand polished with Brasso, Turtlewax Polishing Compound, and 3M Marine Aluminum Restorer and Polish. None of the polishes produced greater reflectivity than the untreated flashing. Perhaps mechanical sanding of the surface might produce better results, but the “grain†of the aluminum appears to be too deep to respond to anything short of a significant amount of grinding.
While there are clear winners and losers when only specular reflectivity is considered, specular reflectivity is only one measure of the performance of a reef tank reflector. One could argue that under some circumstances, it may not even be the best. If one goal of a reflector is to distribute light over the tank, then the degree of diffusion needs to be considered.
Light diffusion
To determine diffusion, reflected light surrounding the target was measured. Diffusion was calculated by dividing the total flux surrounding the center by the flux at the center of the reflected beam. The resulting number is a dimension-less ratio. A number greater than one indicates that diffuse reflectance exceeds specular reflectance, as the light intensity outside the reflected beam is greater than the beam itself, while a number less than one indicates that the specular reflectance is greater than the diffuse reflectance.
The mirror produced the lowest diffusion. The diffuse to specular reflectivity was .04, indicating that nearly all of the reflected light was contained within the reflected beam. White paint produced a diffusion ratio of 2.72, indicating a significant amount of diffusion. Reflectivity at angles other than the incident angle was nearly three times that of specular reflectivity. The greatest diffusion was created by the dimpled aluminum. It had a diffusion ratio of 5.06, indicating that the light intensity surrounding the beam angle was five times greater than reflected beam.
I took photographs to show the dispersion of the light beam as it is reflected off the different materials. The light sensor was replaced by a screen and the beam of light photographed from behind the screen. This is how the light field over a reef tank would appear. All photographs were taken at the same distance. The smallest circular target shown is approximately 10 centimeters in diameter. No attempt was made to reproduce relative light intensities in the photographs since intensities varied too much. The same aperture was maintained throughout the tests, while the exposure time was varied so that the light field would be visible in all of the photographs.
The specular aluminum and mirror accurately reproduced the narrow beam of light, which was indicated by well-defined circles of light. The retro-fit polished aluminum produced a reasonably well-defined, but slightly more diffused, circular spot of light. The white paint reflector produced a much weaker central spot of light with an evenly distributed weaker outer circle of light. The dimpled aluminum reflector produced a pronouncedly skewed light field with light on one side of the target much brighter than the other side. The skewed distribution of light may have been an artifact of the testing, but it raises the possibility that the dimples are not symmetrical and that the orientation of the material could make a significant difference in the distribution of reflected light. The most unexpected result was the light distribution of aluminum flashing material, which produced a very well-defined narrow line of light that varied depending on the orientation of the sheet.
Clearly, aluminum flashing is a poor reflector for a reef tank. It reflects much less light than the other materials and the reflected light is oddly distributed. Those who council hobbyists to save their money and buy aluminum flashing are not helping them. A modest investment in a real reflector will more than pay for itself in increasing the light reaching the tank.
So, which reflector is better?
In order to judge the relative value of a reflector, one must first determine by which criteria reflectors should be judged. Is specular reflectivity the primary concern? Is the goal to reflect as much light as possible in the smallest area? If that is the case, specular polished aluminum reflectors are clear winners. These materials reflect nearly all of the incident light.
On the other hand, if even distribution of light over a large area is the primary concern, then other materials may be competitive with specular aluminum. Used in a flat or nearly flat reflector, dimpled aluminum or white gloss paint may be better solution. These materials are less likely to produce hot-spots areas of intense light, by diffusing light better.
Further complicating matters is the fact that the light source plays a significant role in determining the best reflector criteria. A tank lighted by a quasi-point source light like a metal halide bulb requires greater diffusion than a tank lighted by a source that already diffuses the light, such as a fluorescent bulb. This is one irony of the evolution of reef tank lighting. Fluorescent lighting has traditionally relied on white reflectors that end up further diffusing already diffused light. Metal halide lighting has generally used polished aluminum, which focuses an already focused light source. It would appear that perhaps the hobby has not optimized either light source.
Of course, the geometry of a reflector must be factored into the decision. The shape of a reflector ultimately plays as important a role as the reflecting material in determining how light is distributed over the tank. Reflector geometry is a complex subject, and one that will be addressed in a future article. Based on the above findings, however, we can speculate about the relationship between reflector reflectivity and reflector geometry.
The traditional angular painted fluorescent light reflector disperses light rather than focusing it. An efficient parabolic polished aluminum reflector might be just as effective in fluorescent lighting as it is for metal halide lighting. On the other hand, the need to diffuse metal halide light might be best accomplished with a material, such as dimpled aluminum, rather than specular aluminum. At this point, these are really just questions that I’m hopeful will be answered as we continue to develop more efficient lighting for the reef tank.
In conclusion, there are significant differences between reflective materials and these differences can have a dramatic impact on the efficiency of reef tank lighting. The reflective material used should be chosen with specific design goals in mind. By designing reflectors with specific lamps and specific goals in mind, a hobbyist can deliver more of the dearly paid for light into the tank and direct it where it will do the most good. Your photosynthetic animals will thank you. Your neighbors will thank you.
