PRODUCT REVIEW		RICHARD HARKER

The Digital Ocean SpiderLight™ Reflector Kit

In many hobbyist circles, there’s a feeling that any reflecting surface will do, that plain aluminum foil or aluminum flashing material is as good as reflectors sold to the hobby. It doesn’t make that much difference, does it? An examination of a new product, the SpiderLight™ Metal Halide Reflector Kit, suggests that reflectors may be the most overlooked aspect of reef tank lighting today.

The SpiderLight is produced by Digital Oceans Corporation, a manufacturer of several lighting related products. They can reached by phone, at (205) 880-0814 or at their web site. It is a metal halide reflector kit consisting of a mogul metal halide bulb socket, bracket, polished aluminum reflector, and related hardware. It is designed to be mounted in a hood. It measures 13 inches wide by 16 inches long and requires a hood height of a little less than 5 inches. The reflector is made of Alcoa Everbrite, a highly polished aluminum reflector. It appears considerably more reflective than a standard “retro-fit” kit I had purchased from another reef hobby related company.

Digital Ocean claims the reflector is designed to light an area 2 feet by 2 feet, although they also state that the reflector will cover a 3 foot length of tank. The company emphasizes that the reflector should be oriented with the bulb perpendicular to the front of the tank. As my testing confirmed, the orientation of the reflector is important for proper use of the reflector.

RICHARD HARKER

Figure 1

Most aluminum reflectors sold with retro-fit kits are flat. In contrast, the SpiderLight has a series of ridges running the length of the reflector (see Figure 1). The effect of these ridges is to produce a series of reflections that focus the light. A close look at the light at the bottom of the photo shows the multiple reflections the SpiderLight produces.

To determine the difference in performance between the SpiderLight and a standard retro-fit kit, I conducted a series of tests. The light from a 175-watt, 10,000 degree Kelvin (K) Aqualine metal halide bulb was first evaluated using a retro-fit kit reflector, and then the reflector was replaced by the SpiderLight. The light striking an area approximately 2 feet by 2 feet at a distance of approximately 2 feet was measured using a procedure similar to other tests conducted for Aquarium Frontiers (Harker 1997, Harker 1998). A flat 2-pi sensor calibrated for air measurements and cosine-corrected was used.

Ninety-nine data points of “photosythetically available radiation” (PAR) were recorded for each reflector in a 7 x 9 grid covering an area approximately 200 square inches. Each data point was an average of 30 one-second measurements to eliminate any momentary surges. A standard tar ballast included with the retro-fit kit was used with both reflectors. Line voltage to the ballast was adjusted with a variable transformer to 110 volts and a digital power monitor was used to make sure the ballast was consuming the same power with both reflector setups.

The standard flat reflector arrangement provided an average of 28.8 microEinsteins per square meter per second (µE/m²/sec) over the entire grid. In contrast, the SpiderLight provided 37.1 µE/m²/sec over the same area. This translates into an increase of nearly 29 percent in PAR over the area.

RICHARD HARKER

Figure 2

The average light over the area is only part of the story. The SpiderLight produced much higher intensities at specific points in the grid, particularly along the long edge of the reflector. Near the bulb along the outer ridge, light intensity reached 54 µE/m²/sec, 37 percent higher than the brightest points under the flat reflector.

Figure 2 and Figure 3 compare the light fields of the two reflectors. The X and Y dimensions are the grid with the units reflecting the width of the sensor. The lamp inner envelope is at (0,0) on the grid. The Y dimension is PAR. Figure 2 shows the light field looking down the length of the reflector. The flat reflector shows a rather flat field, with intensity slightly higher on either side of the bulb. In contrast, the SpiderLight displays a “dished-out” light field with light intensities comparable to the flat reflector near the bulb and much higher intensities than the flat reflector along the outer edge of the grid.

Figure 3 is from the same set of data, but now looking at the light field as if looking at the tank from the front. The bulb is to the right of the diagram with the inner envelope at (0,0) on the grid. Light falls off for both reflectors as one moves away from the bulb. Light drops off more for the SpiderLight than the flat reflector, reaching a minimum of 17.4 µE/m²/sec compared to 18.8 µE/m²/sec for the flat reflector. While the lowest light levels are at the extreme outside corners for the flat reflector, the lowest level for the SpiderLight is at the center of the short edge furthest from the bulb.

RICHARD HARKER

Figure 3

Given the expense of metal halide lighting, a passive device like the SpiderLight can be an effective tool in increasing the light delivered to a reef tank. Adding a third more light to a reef aquarium is like switching from 175-watt bulbs to 250-watt bulbs, while incurring only a one-time expense rather than an on-going expense as bulb and electricity expenses mount up.

A hobbyist interested in a SpiderLight reflector kit should consider the unique light field it produces. With a flat reflector, the orientation of a metal halide bulb is of little consequence. With the SpiderLight reflector, orientation can have a significant impact on the tank’s light field.

If the hobbyist chooses to ignore the manufacturer’s recommendations and mount the reflector in the traditional fashion with the bulb oriented parallel to the length of the tank, the most intense light will be projected toward the front and back of the tank. If the hobbyist follows the recommendations of the manufacturer, the reflector will produce two intense ridges of light on either side of the bulb running from the front of the tank to the rear. Aquarists can take advantage of this by placing light-demanding animals under these ridges.

With the reflectors oriented perpendicularly to the long side of the tank, the ends of a long tank can also be used more effectively. With the SpiderLight, the ridge of light can be positioned to fall just inside the end of the tank, placing considerably more light in the corners than one would have with a traditional flat reflector in the corners.

The light field diagrams point out one misconception about metal halide lighting with reflectors. Even flat aluminum reflectors create “hot spots” on either side of the bulb. The brightest point in a reef tank may not be directly under a metal halide bulb. It is more likely to be to one side of the bulb. Hobbyists should remember this when they are attempting to acclimate photosynthetic animals to a tank. Placing a new addition directly under a lamp exposes the animal to less light than if you place it a few inches on either side.

The issues of reflectors is a complex one and worthy of greater attention on the part of hobbyists. In a future article, we’ll look at more traditional reflectors comparing aluminum to a painted surface to see if a painted surface can perform as well as traditional reflectors.