Which bulb produces more intense light: A double ended Aqualine 150 watt bulb or a single ended Blueline 400 watt bulb? If you’ve followed the on-line discussions of the past few months, you might guess the double ended 150 watt bulb. While this is wrong, the assertion joins a growing list of reefkeeping urban legends. While urban legends such as this are often hard to trace back to their source, in this case it is not difficult. Sanjay Joshi writing in the 2002 Marine Fish and Reef Annual at first glance appeared to suggest this very thing. According to Mr. Joshi the 400 watt Blueline produces 71.6 μE/sec/m2 while the AB 150 watt bulb produces up to 246 μE/sec/m2. Reading the text carefully makes it clear that the 400 watt bulb was measured without reflector while the AB bulb was measured with an integrated reflector. However, anyone just scanning the tables might not understand this. The confusion over this unfortunatejuxtaposition of data on bare bulbs versus a bulb in an enclosure with reflector has gradually been sorted out, but it points out the need to carefully read how data are collected.
The double ended bulb results would have been much more helpful had the bulb been measured in the same manner as the single ended bulbs. Unfortunately, measuring bare double ended bulbs is much more complicated than single ended bulbs for two reasons. First there is a safety issue. Some double ended bulbs lack a UV filtering outer envelope. All high intensity bulbs can burn a person’s cornea, but unshielded bulbs can do much more damage. The second complication is the way double ended bulbs are mounted. While any mogul socket will accommodate a single ended metal halide bulb, a double ended bulb requires a special spring loaded socket that needs to be rigidly mounted in some sort of bracket or enclosure. Most double ended bulbs sold to the hobby are mounted in an enclosure that has an integrated reflector and UV filtering safety glass (figure 1). Mr. Joshi chose to measure the AB bulb in an enclosure with such a reflector, and that is why he measured a much higher intensity thaneither he or I have reported for 400 watt bulbs.
In order to determine the actual direct light output of the AB 150 watt bulb, I disassembled an Aqualine 150 watt fixture replacing the integrated reflector with a flat black reflector (figure 2). This meant that the measured intensity would be comparable to how we have been measuring single ended bulbs. Mr. Joshi has been reporting intensities at 18 inches and my measurements have been at 24 inches, so I measured intensity at both distances. All measurements were taken with a LiCor Datalogger and calibrated LiCor PAR sensor. The sensor was placed directly below the bulb with the distance measured from the inner envelope to the edge of the sensor.
|18 inches||24 inches|
|With reflector||207.6||124.1 μE/sec/m2|
As table one indicates, the reflector significantly increases intensity directly below the bulb. Consider that Mr. Joshi reported an intensity of 135 μE/sec/m2 for the 400 watt AB bulb at 18 inches and I measured the Iwasaki 400 watt bulb at 165.1 μE/sec/m2 at 24 inches. Just glancing at the 150 watt results, one might conclude that we could achieve the same intensity of a 400 watt system with a double ended 150 watt system. In one sense it might be true. A hobbyist using a light canopy without specular reflectors (for example just painting the inside of the canopy) might have no more light in the tank than with a 150 watt fixture with integrated reflectors. Hopefully, however, most hobbyists have learned about the value of a specular reflector, and in that case, the intensity advantage is clearly with 400 watt bulbs. For more about specular reflectors, see my review of the Spiderlight reflector http://www.animalnetwork.com/fish2/aqfm/1999/feb/product/default.asp
Double ended versus single ended bulbs
Now having data on the intensity of an AB double ended 150 watt bulb enables us to compare it to the intensity of the AB single ended 175 watt bulb. In the May/June 1997 issue of Aquarium Frontiers I measured a wide range of 175, 250, and 400 watt bulbs including the AB 175 watt bulb. Table 2 compares the single ended 10,000 Kelvin AB 175 watt bulb using a tar ballast to the 10,000 Kelvin double ended AB 150 watt bulb using the AB fixture and tar ballast. The second column is a measure of the efficiency of each bulb. It is the measured intensity divided by the power consumption of the bulb.
|Single ended 175 watt||54.2 μE/sec/m2||0.31 μE/watt|
|Double ended 150 watt||45.4||0.30|
The 175 watt bulb produces slightly more light, but the increase in intensity can be attributed to the higher wattage of the bulb. It turns out that the efficiency of the two bulbs is virtually identical indicating that double ended bulbs are no more efficient than single ended bulbs with comparable ballasts.
Double ended metal halide bulbs are not inherently better than similar wattage single ended bulbs, but the integration of a reflector into a double ended lighting fixture can give the double ended system an advantage. With double ended systems the reflectors are designed for specific bulbs with known envelope geometries. This means the system designer can create a reflector that maximizes the light getting into the tank and minimize light spillage outside the tank. Too often reflectors for single ended bulbs are poorly designed misdirecting light outside the tank. Next month we will look more closely at the design of the AB reflector and compare the design to another 150 watt system to see how the reflector design impacts the light intensity of double ended systems.