Part of the family of High Intensity Discharge (HID) lamps, metal halide lamps are the best in this family in terms of rendering colors correctly (as measured in CRI). While low pressure sodium lamps have a negative score on a traditional CRI scale of 0-100, and mercury vapor and high pressure sodium lamps typically score under 50 on this scale (well below 50 if they're not coated), metal halides score in the 60s and above (even into the 90s).
Though they don't generally produce as many lumens per watt as high and low pressure sodium bulbs, metal halides still have a high efficacy rating ... approximately equal to LED lamps (although this is a little misleading, as we'll explain below). So sodium lamps may be chosen to light up areas where light is needed for safety but not visual acuity (like along a park trail or at a construction site where no work is happening at night). Metal halide lamps are preferred when both the quality and quantity of light are important. Common uses include parking lots, streets, warehouses, and athletic fields.
400 Watt Metal Halide Bulbs and Ballasts
Like any gas discharge lamp, metal halides require a lighting ballast to run, and they need to be matched according to their lamp type and wattage. For instance, you might have a 400 watt probe start metal halide lamp, and you would need a metal halide ballast specifically designed to run that.
In the case of metal halide lamps, you can look at the ANSI code on the lamp and ballast and match these to verify compatibility. 400 watt probe start metal halide lamps and ballasts typically have an M59 ANSI code; and you typically have an M135 ANSI code on 400 watt pulse start metal halide lamps and ballasts.
(Probe start lamps use an older technology with a starting probe inside the lamp. These are not as efficient, they lose more efficacy over the course of their lives, and they may not be rated to last as long as pulse start lamps.)
Metal Halide vs LED Lighting Efficacy
1) A 400 watt metal halide lamp requires more than 400 watts to run because of the ballast. The lamp and ballast system together will take roughly 458 watts to run. So you need to take this into account when assessing how much light you get per watt.
2) LEDs are directional lights, which means they shine all their light where it's needed. A metal halide lamp shines its light in all directions, some of which is reflected by the fixture it's in. This means you lose some of the light that's being produced. So when you upgrade metal halide lamps to LEDs, you can typically use LEDs producing fewer lumens than the metal halides did.
LEDs also retain more of their light output and last longer, though the initial lamp cost is substantially more at the time of this writing. Still, in settings where the lamp is used a good portion of the day, the payback period can be two years or less. This is why many business are now upgrading their metal halides to corn cob LEDs.
Likewise, fluorescent lights have come a long way since the days of early T12 lamps, which could not have been used as high bay lighting. Today, for indoor settings, high output T5 lamps can be used in place of metal halides, though this would require an entire relamping since fluorescent tubes require different fixtures. And of course you couldn't use those in outdoor settings like street or parking lot lights; nor do fluorescent lamps work well in cold weather.
In all cases, when a 400 watt metal halide lamp has seen its final days, the easiest option is direct replacement with another metal halide. These bulbs are affordable and long lasting, and while modern technology provides more efficient and higher quality lighting, metal halides offer a good balance between up-front affordability and quality.
Warm-Up and Restrike Periods
We already mentioned several disadvantages of probe start metal halides compared to lamps with the newer pulse start technology. This is another area where pulse start lamps excel, as you can see in this diagram from the Lighting Research Center. According to their data, probe start metal halides can take 2 to 4 times longer to reach full brightness and easily twice as long to restrike after being shut off.
Overall, while metal halide lighting upgrades exist, relamping with fluorescent or LED technologies requires a substantial initial investment. And since sticking with metal halide lamps means purchasing fairly inexpensive bulbs that last a long time, are efficient, and produce a good quality of light, many businesses continue to find metal halides an attractive option. Of course as LED replacements continue to fall in price, at some point they will be a difficult upgrade to ignore. Until then, businesses can choose to rely on metal halide lamps that have stood the test of time.