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A sodium-vapor lamp is a gas-discharge lamp that uses sodium in an excited state to produce light. There are two varieties of such lamps: ''low pressure'' and ''high pressure''. Low-pressure sodium lamps are highly efficient electrical light sources, but their yellow light restricts applications to outdoor lighting such as street lamps. High-pressure sodium lamps have a broader spectrum of light than the low-pressure lamps, but still poorer color rendering than other types of lamps. Low-pressure sodium lamps only give monochromatic yellow light and so inhibit color vision at night. == Low-pressure sodium == Low-pressure sodium (LPS) lamps have a borosilicate glass gas discharge tube (arc tube) containing solid sodium, a small amount of neon, and argon gas in a Penning mixture to start the gas discharge. The discharge tube may be linear (SLI lamp) 〔(【引用サイトリンク】title=SLI/H Sodium )〕 or U-shaped. When the lamp is turned on it emits a dim red/pink light to warm the sodium metal and within a few minutes it turns into the common bright yellow as the sodium metal vaporizes. These lamps produce a virtually monochromatic light averaging a 589.3 nm wavelength (actually two dominant spectral lines very close together at 589.0 and 589.6 nm). As a result, the colors of illuminated objects are not easily distinguished because they are seen almost entirely by their reflection of this narrow bandwidth yellow light. LPS lamps have an outer glass vacuum envelope around the inner discharge tube for thermal insulation, which improves their efficiency. Earlier types of LPS lamps had a detachable dewar jacket (SO lamps).〔(【引用サイトリンク】title=SO/H Sodium )〕 Lamps with a permanent vacuum envelope (SOI lamps) were developed to improve thermal insulation.〔(【引用サイトリンク】title=SOI/H Sodium )〕 Further improvement was attained by coating the glass envelope with an infrared reflecting layer of indium tin oxide, resulting in SOX lamps.〔(【引用サイトリンク】title=SOX Sodium )〕 LPS lamps are some of the most efficient electrically powered light sources when measured in photopic lighting conditions—producing up to 200 lm/W,〔(【引用サイトリンク】 url=http://webexhibits.org/causesofcolor/4.html )〕 partly because the light is at a wavelength near the peak sensitivity of the human eye. They are mainly used for outdoor lighting such as street lights and security lighting where faithful color rendition is considered unimportant. Recently it has been found that under typical nighttime mesopic driving conditions, whiter light can provide better results at a lower level.〔 (Comparison is with HPS and MH lamps)〕 LPS lamps are similar to fluorescent lamps because they are a low-intensity light source with a linear lamp shape. They do not exhibit a bright arc as do High-intensity discharge (HID) lamps; they emit a softer luminous glow, resulting in less glare. Unlike HID lamps, during a voltage dip low-pressure sodium lamps return to full brightness rapidly. LPS lamps are available with power ratings from 10 W up to 180 W; longer bulb lengths can however create design and engineering problems. Another property of LPS lamps is they do not decline in lumen output with age. For example, mercury vapor HID lamps become dimmer towards the end of their lives, to the point of being ineffective, while continuing to consume their rated electrical power. LPS lamps do increase their energy use slightly (by about 10%) towards the end of their life, which is generally about 18,000 hours for modern lamps. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sodium-vapor lamp」の詳細全文を読む スポンサード リンク
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