"All fluorescent and MH bulbs have a discrete spectrum, whereas sunlight has a continuous spectrum. Discrete spectrum is a result of using a discharge in mercury (and other metal) vapors, with several peaks at different wavelengths, mostly in the ultraviolet range. Phosphors on the bulb convert this radiation into narrow bands of visible light. A discrete spectrum vs. continuous is shown in Fig. 18:
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Fig. 18 Continuous (above) and discrete (below) spectrum
The gaps - wavelengths that are missing in a discrete spectrum - mean that certain tints of color cannot be correctly rendered under such illumination and, as a result, the light source will have a low color rendition index (CRI). Of course, light bulb manufacturers try to avoid deep gaps in the spectrum. Look at the spectrums of popular marine MH bulbs: BLV HIT 10000K and BLV HIT 14000K (Fig. 19).
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Fig. 19 The spectrum of Metal Halide bulbs BLV HIT 10000K (a).
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Fig. 19 The spectrum of Metal Halide bulbs BLV HIT 14000K (b).
These bulbs do not have deep gaps in their spectrum, so that the intensity at a certain wavelength would drop to zero, hence both are full-spectrum bulbs and their CRI can be determined. At the same time, they exhibit clear discrete peaks, meaning that when using these bulbs precise color rendition cannot be achieved. Note that bulbs with different CCT: 10,000 Kelvin - 14,000 Kelvin are used in this example. Their main difference is in the significant portion of 400-440nm radiation in the second bulb, whereas the 460nm peak is missing. This is logical and clear: the higher the temperature of an absolutely black body, the more its spectrum would shift into the short wavelength region. Since the 400-450nm range is most important for a reef aquarium, and because, in order to attract the customer, manufacturers often calculate the CCT to satisfy their interests, we can safely state that maximum radiation in the required range is only achieved when a CCT of approximately 20000K is declared. Have a look at the spectrum of a 400W Hamilton Metal Halide bulb with 20000K CCT (Fig. 20):
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Fig. 20 The spectrum of a 400W Hamilton Radium Metal Halide bulb with 20000K " C/P
