Three recent columns provided a brief history of lighting, an overview of fluorescent technology, and a look at the challenges of improving streetlights. Following a side trip into the issue of “passive survivability,” I’m returning this week to illumination with an overview of high-intensity discharge (HID) lighting.

HID lighting involves creating an arc of electricity in a sealed bulb containing mercury vapor and other gases. One of the advantages of HID lighting is long lamp life—which is important in outdoor and hard-to-reach indoor applications. Another advantage is the potential for very high light output from a concentrated light source—useful in illuminating Fenway Park, for example. All HID lamps require a ballast to produce the proper electric current to start and then maintain the electric arc. Because of this ballast, most HID lamps take several minutes to turn on (referred to as restrike delay)—which is one of their drawbacks.

There are four basic types of HID lighting:

Mercury vapor is the oldest type of HID lamp and still the most common outdoor lighting in some areas. When new, mercury vapor lamps produce a bluish light, though it yellows (and dims) as the lamps age. The efficacy of mercury vapor lamps is mediocre (20-60 lumens per watt), which is better than incandescent, but not as good as most fluorescent or other HID lamps. Because of the relatively low efficiency of mercury vapor lamps, the Energy Policy Act of 2005 banned their manufacture and sale in the U.S. beginning in 2008. So, as the older mercury vapor lamps in use today fail, they will have to be replaced with other options. As described a couple weeks ago in the story about West Dummerston Village, replacing older mercury vapor lights can be problematic.

高压钠（HPS）灯会产生淡黄色的橙色光（低“色温”）；这些在公路上很普遍。HPS灯使用高频，高压电流将密封灯泡中的氙气电离。这个过程使钠汞汞齐度蒸发，当电弧穿过该气体混合物时，将从钠原子中释放出电子混合物，从而产生可见光的光。功效非常高（每瓦最高150个流明 - 与白炽灯大约15个，最多可用于最佳线性荧光灯100）。但是，大多数HPS灯的轻质质量非常差，但是，最佳荧光灯灯的颜色渲染指数（CRI）仅为20-25，而白炽灯灯泡的颜色为85，100个（100个较新的HPS灯都更好，更好的HPS灯都更好，但要受到能源绩效的惩罚）。这意味着在HPS光线下，颜色看起来并不自然。随着时间的推移，HPS的光输出也大大减少 - 技术说话中的“管腔贬值”。

当今任何光源的低压钠具有最高的功效（每瓦约200流明），但光质量最差，CRI接近0。实际上，在深橙色低压钠下glow, it’s hard to distinguish a red car from a blue car. Most light sources produce a wide spectrum of light, while low-pressure sodium is limited to a very narrow band—in the orange part of the spectrum. Due to this “monochromatic” nature of the light, astronomers like low-pressure sodium, because they can use filters to block out its light pollution. In towns near astronomical observatories, such as Tucson, Arizona, ordinances are sometimes passed to allow only low-pressure sodium for street lighting.

Metal halide has emerged as the most popular form of HID lighting. Introduced around 1960 as a modification of mercury vapor lamps, metal halide lamps incorporate mercury vapor along with such metal halides as sodium-iodide and thallium-iodide. The result is a reasonable efficacy (50-115 lumens per watt) and much better light quality than other HID light sources, with much whiter light and CRI ratings of 65 to 93. They also will restart more quickly if they are turned off (restrike time of just a few minutes). While the efficacy is lower than that of high-pressure or low-pressure sodium lamps, metal halide light shows off colors much better, so it is often possible to get by with lower total light output, thus achieving savings. The expected lamp life of metal halide lamps (10,000 to 20,000 hours) is lower than that of HPS, some of which are rated at over 30,000 hours.

The light quality of metal halide lamps is good enough to use indoors, and you will commonly see this form of lighting in big-box stores, warehouses, and other buildings with high ceilings—though in these applications, newer T-5 hi-bay fluorescent lighting, which can be turned off and on instantly, is now providing serious competition. Metal halide is also now the outdoor light source of choice along streets, in parking lots, and in sports stadiums.

所有HID的巨大缺点 - 加上荧光 - 产生光所需的汞。汞是一种有效的毒素，汞的采矿和灯的处置都可以将汞释放到环境中。下周，我们将研究LED照明，这是一种避免汞的相对较新的技术。