TEXT AND DIAGRAMS BY SANJAY JOSHI AND DAVE MORGAN

Our initial goal was to track the changes of the same lamps over a period of time, in order to get an idea of how the spectral distribution changes over time. But, due to the difficulties of obtaining the data on the same lamp at various points in its life, getting a large enough sample, the length of time such a study would take and so on, we decided to take a sample of used lamps of varying ages, rather than track the same lamp through its lifetime.

We asked aquarists all over the country to send in any used lamps they were willing to loan for this study. Along with the lamps, aquarists provided us with the hours of usage for each lamp (according to their best estimates). Spectral analysis of was then performed using the same setup as described in “Part 1.”

We realize that not knowing the initial spectral distribution of each lamp precludes us from making strong statements about the deterioration of lamps over time. However, we feel that just knowing the range of spectral distributions of the lamps of different ages may provide some knowledge of how the spectral distribution looks over time to aquarists, and possibly some indicators on a replacement policy for the lamps.

Iwasaki 6500 K Lamp

In addition to the spectral distribution, the “photon flux density” (PPFD) and the “correlated color temperature” (CCT) of these lamps were also calculated. The results are listed in Table I. As can be seen in the table, as expected, there is a drop in PPFD, as well as a shift in the CCT as the lamps age.

There is some variation in the data where you will notice that a lamp of older age has, in fact, “better” output than a lamp with less usage. This could have been due to higher initial values or a slower decay of the lamp halides. Because the initial spectral distribution of the lamps is not known this cannot be ascertained with any degree of certainty.

Although it can be visually determined from the graph in Figure 1, the lamps tend to have a higher drop in PPFD in the violet and blue ranges. Quantitative data showing the drops are shown in Table II.

Age graphs of the various lamps tested. Clockwise from upper left: Figure 1 — 6500 K Iwasaki lamps; Figure 2 — 10,000 K Aqualine Buschke lamps; Figure 3 — 20,000 K Osram/Radium lamps; and Figure 4 —6000 K GE lamps.

The spectral output of each lamp was compared to the new lamp. Lamps with about a year of life at 10 to 12 hours of use per day seem to lose about 17 to 22 percent of their intensity in the “photosynthetically available radiation” (PAR) range (400 to 700 nanometers; nm). Interestingly, the largest contributor to this drop is in the violet range, almost 40 percent for year-old lamps.

A negative number indicates that the lamp output in that range was higher than that of the new lamps used for comparison. It does not necessarily indicate that the lamp output in these ranges tends to increase. It could very well be that these lamps had higher outputs in the respective ranges to begin with, than the new lamp.

A conclusion that could be drawn from this data is that the rate of decrease in PPFD during the second year is less than that during the first. The worst two-year lamp tested only had a 29.4-percent decrease in PPFD. If we assume that, similar to the one-year lamps, 17 to 22 percent of this occurred during the first year, then only 7 to 12 percent of decrease occurred during the second year. One implication of this to the aquarist may be that instead of changing these lamps every year, they could be used for at least 18 months.

Aqualine Buschke 10,000 K Lamps

The 700-hours lamp seems to be an anomaly. As seen from the data, the initial PPFD output is significantly less than the 6500 K lamps, and the output tends to drop 25 to 33 percent within the span of a year. There were no two-year lamps available for testing, as most aquarists tended to replace them on a yearly basis. The output in the violet and blue range tends to fall more rapidly than the 6500 K Iwasaki lamps.

Osram/Radium 20,000 K Lamps

GE 6000 K Lamps

Figure 4 shows the spectral distribution of the GE 6000 Ks we tested and Table VII presents the PPFD of the various. Table VIII presents the drop in PPFD output when compared to new lamp. Interestingly, the new lamp tested was “worse” than the three-month-old lamps.

Conclusion

The answers to the questions involve many aspects and are far beyond the scope of this article. One large factor in answering these questions involves the specific needs of the corals themselves. Many corals are known to show some photoadaptive ability.

Furthermore, by changing the distance between the lamp and the water surface we can increase the amount of light that enters the tank. So, as the lamps age and drop in intensity, we can counteract this drop by reducing the distance between the lamp and the water surface, thereby extracting more life out of the lamp as long as we are convinced that the spectral distribution is still acceptable. It is not hard see how this can complicate the issue of setting rigid guidelines as to when a lamp is good or bad.

Acknowledgements: We would like to thank several hobbyists, without whose help this study would not have been possible. These aquarists were kind enough to loan us their new and used 400-watt lamps and help reduce the financial burden of having to acquire them all. Thanks to: Richard Harker, Dana Riddle, Wayne Shang, Mike Fontana, Jack Chernega, Eric Borneman, James Wiseman, Dan Wohls, Ed Roan and Kevin Carpenter. Finally, we would like to thank Dr. Paul Walker of Penn State University and LiCor Inc., for the use of their spectroradiometers and the darkrooms for testing of the lamps.