Spectral Analysis of Metal Halide Lamps
Used in the Reef Aquarium Hobby
Part 3 — 250-Watt Metal Halide Lamps

In the first two parts of this series of articles we presented a spectral analysis of new and used 400-watt metal halide lamps. In this article we will analyze the spectral output for the 250-watt lamps.

The 250-watt lamps are not as popular as the 175- or 400-watt lamps because of the limited choices, although some new lamps have recently been introduced. The seven lamps shown in Table I were used for our new lamp analysis. These lamps were burned in for 100 hours in order to stabilize the output.

Brand name Color temperature
Coralife 5500 Kelvin (K)
Iwasaki 6500 K
Coralife (new)1 10,000 K
Coralife 20,000 K
Double-ended HQI 10,000 K
Tungsram2 6000 K
Coralife 6500 K
1 = Coralife has recently introduced this new 10,000 K, 250-watt lamp that is being made in Germany.
2 = The Tungsram lamp was sent to us from Australia.

Analysis of New 250-watt Lamps

The basic setup used for testing was identical to the one used for the 400-watt lamps (for details of the setup used and the basics of lighting measurements the reader is referred to “ Spectral Analysis of Metal Halide Lamps Used in the Reef Aquarium Hobby — Part 1”). Because the same setup and equipment was used, the lighting data for the 250-watt lamps can be directly compared to the 400-watt lamps.

The lamps were placed 18 inches from the LI-1800 spectroradiometer and powered by a MagneTek M58 ballast. The double-ended HQI lamp was run using the recommended HQI ballast.

Spectral change during burn-in phase

Most lamp manufacturers recommend a burn-in period of about 100 hours. However, many hobbyists do not burn in the lamp in a separate fixture outside the aquarium. We were curious as to how the spectrum changed during this burn-in period.

Figure 1 shows the spectral curve of the Iwasaki lamp at various stages of its burn in. As you can see in the chart the lamp shifts in spectral output when it is new. As the lamp reaches about 93 hours it has stabilized and there is almost no change between the 93-hour curve and the 82-hour curve. The change in “photosynthetic photon flux density” (PPFD) and “correlated color temperature” (CCT) is shown in Table II.

PPFD and CCT for the
250-watt Iwasaki 6500 K lamps
during burn in
2 hours 116.1 6697 K
25 hours 120.6 7097 K
48 hours 108.6 7614 K
59 hours 98.1 7488 K
68 hours 104.8 7470 K
82 hours 99.7 7271 K
93 hours 98.6 7302 K

New Lamp analysis

The first analysis done on the seven samples was a direct comparison of the spectral outputs of the lamps, along with the PPFD. The spectral plots are shown in Figure 2 and the PPFD is shown in Figure 3. As seen from the data, the Iwasaki 6500 K and the double ended HQI 10,000 K lamps have higher output than most of the other lamps over most of the 400 to 700 nanometer (nm) range. The Tungsram 6000 K lamp was a little better spectrally than the Coralife 5500 and 6500 K lamps, but it still did not reach the performance of the Iwasaki lamp or the double-ended HQI.

The double-ended HQI lamp is very different from the other lamps tested. It is double ended, while all the others have a single screw-type base. It lacks the outer ultraviolet (UV) filtering glass bulb of the other lamps. This lamp must be used in an enclosed fixture with an UV protective shield. It does have a very nice color and has the best spectral curve of any of the higher CCT lamps tested.

Figure 1 Figure 2
Figure 3 Figure 4
Figure 5

The next comparison we did was to calculate the PPFD over the “photosythetically available radiation” (PAR region; 400 to 700 nm) of the lamp output. This value is a good indicator of a lamp’s ability to support photosynthesis. Table III and Figure 3 show that the double-ended 10000 K lamp has over twice the PPFD of the Coralife 5500 K, 6500 K and 10,000 K lamps and the Tungsram 6000 K lamp, and three times that of the 20,000 K lamp, but is only about 20-percent higher than the Iwasaki 6500 K lamp. The double-ended 10,000 K lamp performs very well, giving off nice blue-white light, and still offers a very respectable PPFD.

The CCT was also computed for each lamp and is also shown in Table III. The CCT for the 6500 K lamp was higher than the rating, whereas for the 5500 K Coralife lamp it was much lower than the lamp designation would suggest. For the 10,000 and 20,000 K Coralife lamps the CCT could not be calculated. These lamps appear too much like a monochromatic source for an accurate CCT calculation. The double-ended HQI 10,000 K lamp has a measured CCT close to 12,000 K, which gives it its nice blue-white appearance.

PPFD and CCT for the
new 250-watt lamps
Lamp Type PPFD CCT
6500 K Iwasaki 104.50 7457
5000 K Coralife 58.34 4585
10,000 K Coralife 51.57 not applicable (na)
20,000 K Coralife 37.24 na
10,000 K HQI 128.80 11,723
6000 K Tungsram 56.00 8152
6500 K Coralife 53.38 5339

Finally, the spectral output of the lamps was divided into color ranges to compare the distribution of their energy output (see Figure 4). Aquarists are often concerned with the quantity of radiation in the violet and blue range — hence the propensity to use 10,000 and 20,000 lamps. P Another misconception of most aquarists is that the higher CCT lamps have more output in the blue range. As the data in Figure 4 show, the Iwasaki 6500 K lamp has a total PPFD in the blue and violet range of 29.56, when compared to the 20.47 for the 10,000 K Coralife lamp, and 18.85 for the 20,000 K lamp. The only higher CCT lamp that we tested that had more blue and violet light than the Iwasaki 6500 K is the HQI lamp, at 52.95. However, it does have a very strong violet output.

The 6500 K lamp is also strong in the yellow and green ranges, which explains the yellowish-green look reported by aquarists using this lamp. In the double-ended HQI lamp this yellowish-green look is compensated for by the large output in the violet range, giving it a very pleasing look.

The UV output of the various lamps is shown in Figure 5. Comparing the UV output in this chart is somewhat misleading. The double-ended lamp has much higher output thaen the others, but remember this lamp requires a UV filter, which would bring this number down.

Figures 6 through 12 show the PPFD in six ranges as a percent of the total PPFD. The 5500 K lamp has over 50 percent of its total output in the yellow and orange range. The 10000 K Coralife lamp has 67 percent of its PPFD in blue and violet range. The 6500 K and the double-ended HQI lamps both show nearly uniform output. These figures show one interesting result in that the 20,000 K Coralife lamp has a more evenly distributed output than the 10,000 K Coralife lamp.

The double-ended HQI lamp is a good example of the way the human eye is fooled by color. The lamp is very strong in the violet and a yellow color, but the light from this lamp looks very blue-white in color.

Figure 6 Figure 7
Figure 8 Figure 9
Figure 10 Figure 11
Figure 12

Used 250-watt lamps

Used lamps were requested from aquarists all over the country in an attempt to analyze the change in spectral output over time. It was a lot easier to get used lamps of ages around one year or more, rather than used lamps in the three-, six- or nine-month range of use. Thus, we currently lack data in the intermediate ranges of the life of the lamps.

The lamps solicited for the used lamp evaluation were the same as the ones solicited for the new lamp evaluation. However, we were very disappointed that we were not able to obtain used samples of the Coralife 6500 K, the double-ended 10,000 K or the Tungsram 6000 K lamps. We did get good samplings of the 6500 K lamp by Iwasaki, the 5500 the 10,000 (old lamps) and the 20000 Ks from Coralife. The spectral characteristics of these lamps were compared to a new lamp in each category.

There are a few limitations that arise due to the testing strategy used:

  • The initial spectral distribution is not known, so it may be difficult to ascertain the role of the variation in the initial spectral distribution of the lamp.
  • Often, the exact number of hours the lamp was used was not known. The age was given by aquarists in the total duration of time the lamp was in use.
  • The operating environment for the lamps may be very different, leading to variation in the lamp output arising from the variation in operating conditions.

    In spite of these limitations we felt it is of some educational value to look at the spectral distributions of the lamps during various points in their life. We’re hopeful some conclusions can be drawn.

    Figure 13 Figure 14
    Figure 15 Figure 16
    Figure 17

    The Iwasaki 6500 K lamps

    Figure 13 shows the spectral plots of three used 6500 K lamps as compared to two new ones. As you can see from the figure, there is good similarity between the curves of the different lamps, aside from the second one-year-old lamp. The curve of this lamp has a spectral curve much different from what is normally seen in a 6500 K lamp, indicating that the lamp may have been defective. As seen with the 400-watt Iwasaki lamps, the 250-watt lamps also tend to lose more intensity in the violet/blue end of the spectrum range, evidenced by the wider spread in the spectral distribution for those wavelengths.

    PPFD and CCT for the
    250-watt Iwasaki 6500 K lamps
    of various ages
    Age PPFD CCT
    New 104.50 7109
    New 92.87 7457
    One year 97.53 6940
    One year* 40.63 6600
    One year, six months 81.46 6656
    * = defective lamp

    Table IV shows some interesting data. The PPFD for some of the used lamps showed up as being slightly higher than one of the new ones tested. This could simply be due to the variation among the initial PPFD of the lamps, or it could be that the new lamp was not a very good lamp. This table also shows that there is actually some variation between new lamps. One of the new ones has about 10-percent higher PPFD than the other.

    The important point here is that even after as long as two years of use the lamps were within 20 percent of each other. One could infer from this that these lamps could easily be used beyond the one-year period.

    Another interesting observation that can be seen in Table IV is that the CCT for the defective lamp is nearly the same as that of a lamp that has twice the PPFD. This shows that the ability of a lamp to provide a good supply of photosynthetic radiation can not be determined by CCT alone. The defective lamp has a PPFD that is less than half of any of the other lamps.

    The Coralife 5500 K lamps

    Figure 14 shows the spectral curves for a group of used 5500 K Coralife lamps. The spectral plots show a strong similarity in the spectral curves with a fairly small spread. There is a sharper drop in the peaks, with small drops over the rest of the spectrum.

    PPFD and CCT for the
    250-watt Coralife 5500 K lamps
    of various ages
    Age PPFD CCT
    New 58.34 4585
    One year 49.90 5347
    One year 53.99 5108
    One year 60.69 4409
    One year 44.41 4282
    One year 65.32 3920
    One year, six months 35.94 3928

    Once again, as seen from the data in Table V, it is difficult to draw any strong conclusions, other than the fact that these lamps could be used longer than a year. The variation in the PPFD after a year of use could well be due to the initial variation in the lamps. This may show some substantial difference from lamp to lamp, but a large group of new lamps would be needed to confirm it.

    The Coralife 10,000 K lamps (old version)

    Coralife has recently introduced a new 10,000 K lamp. The results presented in this section are for the old version of the lamp. The used 250-watt 10,000 K lamps that were tested show a typical spectral curve for higher color temperature lamps. There is a very large spike at 450 nm. Figure 15 shows the nearly monochromatic spectral curve of these lamps.

    Table VI shows the PPFD of each of the 10,000 K lamps. This set of lamps seems to have the widest fluctuation in the output of the used lamps. Unless the quality of this lamp can be established it is almost impossible to make any conclusions about it, other than its inconsistent performance. Because of this inconsistent output you really don’t know what you are buying.

    PPFD and CCT for the
    250-watt Coralife 10,000 K lamps
    of various ages
    Age PPFD CCT
    New 30.94 na
    Nine months 46.45 na
    Nine months 75.73 na
    One year 68.01 na
    One year 43.30 na
    One year 38.12 na
    One year, six months 60.18 na

    The Coralife 20,000 K lamps

    The data for the 20,000 K Coralife lamps (see Figure 16) shows a plot very similar to the Coralife 10,000 K lamps. However, these lamps seem more consistent and predictable in performance. There is a large drop in the peak output at the 450 nm range, as a function of the age of the lamps.

    These lamps tended to lose up to 37 percent of their output within a year. This indicates that these lamps should probably be changed earlier than the lower CCT lamps.

    PPFD and CCT for the
    250-watt Coralife 20,000 K lamps
    of various ages
    Age PPFD CCT
    New 37.24 na
    Eight months 23.51 na
    One year 29.94 na
    One year 27.59 na
    One year 24.91 na

    Tungsram 6000 K lamps

    We were only able to obtain one new and one used Tungsram lamp. The spectral plot for the new one compared with the used one is shown in Figure 17. The PPFD for the new lamp was 56.00 and the used one with 2900 hours of life was 84.33. Without more lamps of this type it is difficult to find a reason for the low PPFD of the new lamp, and the difference in the general shape of the curves.

    Further, because these were lamps from Australia it is quite likely the ballasts we used (M58) do not match the ones specified for these lamps.

    PPFD and CCT for the
    250-watt Tungsram 6000 K lamps
    of various ages
    Age PPFD CCT
    New 56.00 8152
    2900 hours 84.33 5839


    In this article we presented the spectral analysis of 250-watt metal halide lamps used in the hobby. The data presented in this article are intended to provide a better understanding of the light output of the lamps commonly used in the reef hobby. We do hope this data will help you make more informed decisions when choosing which lamp is best for your reef.

    If you subscribe to the “more PAR is better” theory, then obviously the best choice is either the double-ended 10,000 K lamp or the 6500 K Iwasaki lamp. If you subscribe to the “more blue is better because corals are found in water where the higher wavelengths are filtered out” theory, then it’s worth noting that the 6500 K Iwasaki lamp had higher output in the violet/blue range than the 10,000 and 20,000 K Coralife lamps. The double-ended 10,000 K lamp is the best combination, offering both the best PPFD and the more blue color many reef hobbyists are interested in.

    As mentioned in pervious articles, we would like to continue collecting data on metal halide lamps. We would like to appeal to aquarists to help by loaning us the lamps for testing. If you have lamps that you would be willing to provide for the study, please contact Sanjay Joshi or Dave Morgan, in care of Aquarium Frontiers.

    Acknowledgements. We would like to thank several aquarists for lending us their lamps — Richard Harker, Robert Singer, Mike Fontana, John Newton, Dana Riddle, Chris Paris, Dallas Warren, Brian Griffin, Coralife and Hamilton Technologies. We would also like to thank Dr. Paul Walker of Penn State University and LiCor, Inc., for the use of their spectroradiometers and darkrooms for testing the lamps.

    HOME Table Of Contents FEATURE

    (c)Copyright, Aquarium Frontiers and Fancy
    Publications Inc. All rights reserved.