<P> HPS lamps have slightly different electrical requirements than do the older MV lamps . Both HPS and MV lamps require a transformer or ballast to change the voltage and regulate the current, however, HPS lamps also require an electrical "starter" circuit--much like older fluorescent lamps in residential use . MV lamps do not require a separate "starter" circuit because they have a special starter element within the bulb used for striking the arc . MV lamps slowly dim over time, and a 20 - year - old lamp may emit a very pleasing, but useless, soft green glow, rather than the powerful blue - white light of a new MV lamp . The yellow - spectrum HPS lamps also slowly dim over time but are known for "cycling," where the lamp cycles on and off when it has reached the end of its life cycle . When cycling, the arc within the lamp extinguishes and the lamp must cool down before the starter circuit initializes a new arc . This has been the most recognized downfall of HPS . Some HPS lamps start to burn a pinkish / reddish color at the end of their life (usually when already cycling), or start to burn a pinkish / white color and go dim, or also burn out at the end of their life cycle whether they cycle or not . HPS fixtures can contain a special photocell or ballast that can sense a cycling lamp and shut off the fixture to prevent damage to the ignitor and ballast . </P> <P> HPS lamps generally have the same rated lifespan as MV lamps, and they give increased light and efficiency at lower wattages . Usually, when an MV light is replaced, it is replaced with a HPS light of a lower wattage, for example, a 175 watt MV fixture will get replaced with a 100 or 150 watt HPS fixture as that will meet or exceed the lumen output of the 175 watt MV fixture . At end of life MV lamps just become dimmer and sometimes color shift towards the green end of the spectrum but continue to consume the same amount of electricity . HPS lamps begin to suffer end - of - life cycling before the amount of useful light becomes visibly diminished, or just burn out . HPS lights come in wattages of 35, 50, 70, 100, 150, 200, 250, 310, 400, 600, 750, and 1,000 watt sizes, while LPS lights come in wattages of 18, 35, 55, 90, 135, and 180 watt sizes </P> <P> In recent years, metal halide lamp (MH) streetlights have illuminated roadways and parking lots . Metal halide has long been popular in business installations and can be found in warehouses, schools, hospitals and office buildings . Unlike the old mercury lights, metal halide casts a true white light . It is not nearly as popular as its sodium counterparts, as it is newer and less efficient than sodium . </P> <P> Metal halide lights have also been used for retrofitting . Virtually all fixtures that are converted to metal halide have previously been lit with high - pressure sodium (HPS). Examples of retrofitted fixtures for metal halide use include the Thomas & Betts Model 25 and Model 327, as well as full - cutoff versions of GE's M400 . MH lamps suffer color shift as they age though this has been improving . Actual life expectancy is about 10,000 to 12,000 hours on average . There has also been a noted issue with the lamps "exploding / shattering" during a failure . Metal Halide light bulbs also tend to dim and / or flicker at the end of their life cycles, and on occasion, cycle . Sometimes, they emit a pinkish glow at or near the end of their life cycle which in this case, the bulb just burns out . High cost and low life hours has kept them from becoming popular municipal lighting sources even though they have a much improved CRI around 85 . Therefore, the use of metal halide is limited mainly to city and high end street lighting . They are available in clear or coated bulbs . Probe start MH lights (which are less efficient and are also soon to be banned, unlike Mercury Vapor lamps) come in wattages of 50, 70, 100, 175, 250, 400, and 1000 watt sizes, while pulse start MH lights come in sizes of 50, 70, 100, 150, 200, 250, 320, 350, 400, 450, 750, and 1,000 watt sizes . The wattages of pulse start metal halide lamps are similar to HPS lamp wattages . </P>

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