s under low-pressure sodium lamps. Both appear black, even though the car on the left is bright red, while the car on the right is actually black. Low-pressure sodium (LPS) lamps have a
borosilicate glass gas discharge tube containing solid
sodium and a small amount of
neon and
argon gas in a
Penning mixture (99% Neon, 1% Argon) to start the gas discharge. The discharge tube may be linear (SLI lamp) or U-shaped. When the lamp is first started, it emits a dim red/pink light (from the neon and argon) to warm the sodium metal; within a few minutes as the sodium metal
vaporizes, the emission becomes the common bright
yellow. 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). The colors of objects illuminated by only this narrow bandwidth are difficult to distinguish. LPS lamps have an outer glass vacuum envelope around the inner discharge tube for
thermal insulation, which improves their efficiency. Earlier LPS lamps had a detachable dewar jacket (SO lamps). Lamps with a permanent vacuum envelope (SOI lamps) were developed to improve thermal insulation. Further improvement was attained by coating the glass envelope with an infrared
reflecting layer of
indium tin oxide, resulting in SOX lamps, the current, standard type of Low pressure sodium lamps. Following the development of the Philips Mini-SOX 18W (which became the smallest size of Low pressure sodium lamps and later incorporated into the SOX-E range), some of the design features that contributed to its high efficacy were applied to the existing high power SOX lamps. The result was the SOX-E (Economy) range, that was more energy-efficient and operated on less current than before. They are easily distinguished by their black bayonet caps instead of red on standard SOX lamps. Lamp power was significantly reduced compared to that of the traditional SOX range, lowering discharge current density and increasing luminous efficacy. This was achieved through improved thermal insulation and a modified infrared-reflective coating. Another later development was the SOX-PSG (Philips Solid-state Getter) lamp by Philips, which introduced an drastically different getter (meant to maintain a perfect vacuum) system to prevent leaks that could cause early failures in standard SOX lamps and thus intended to have a more reliable and long-lasting life for the end user. They also employed a refined thermal design for greater efficacy and durability. Some (mainly early-mid production) examples are usually identified by their blue bayonet caps, though later examples retained the normal red ones. SOX-PSG lamps were produced only in the 35 W and 55 W ratings, with the 36 W SOX-E being the only energy efficent/Economy model adapted into a SOX-E-PSG variant that has a green bayonet cap. SOX-Plus/Pro lamps are also known to have been made available that were intended to have double the overall lifespan that of regular SOX/low-pressure sodium lamps but look extremely similar to regular lamps. LPS lamps are among the most
efficient electrical light sources when measured in
photopic lighting conditions, producing above 100 and up to 206
lm/
W. This high efficiency is partly due to the light emitted being at a wavelength near the peak sensitivity of the human eye. They are used mainly for outdoor lighting (such as
street lights and
security lighting) where faithful color rendition is not important. LPS lamps are similar to fluorescent lamps in that 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 to 180 W; longer lamp lengths can, however, suffer design and engineering problems. Modern LPS lamps have a service life of about 18,000 hours and do not decline in lumen output with age, though they do increase in energy consumption by about 10% towards end of life. This property contrasts with mercury vapor HID lamps, which become dimmer towards the end of life to the point of being ineffective, while consuming undiminished electrical power. In 2017 Philips Lighting, a major manufacturer of LPS lamps, announced they were discontinuing production of the lamps due to falling demand. Initially, production was due to be phased out in the course of 2020, but this date was brought forward and the last lamps were produced at the
Hamilton, Scotland factory on December31, 2019. LPS lamps, ballasts, ignitors and sockets are still being manufactured and sold by Qian Shun Lighting Company in Taizhou, China.
Light pollution considerations For locations where
light pollution is a consideration, such as near
astronomical observatories or
sea turtle nesting beaches, low-pressure sodium is preferred (as formerly in
San Jose, California and
Flagstaff, Arizona). Such lamps emit light on just two dominant spectral lines (with other much weaker lines), and therefore have the least spectral interference with astronomical observation. Now that production of LPS lamps has ceased, consideration is being given into the use of narrow-band amber LEDs, which are on a similar color spectrum to LPS. The yellow color of low-pressure sodium lamps leads to the least visual sky glow, due primarily to the
Purkinje shift of dark-adapted human vision, causing the eye to be relatively insensitive to the yellow light scattered at low luminance levels in the clear atmosphere. One consequence of widespread public lighting is that on cloudy nights, cities with enough lighting are illuminated by light reflected off the clouds. Where sodium vapor lights are the source of urban illumination, the night sky is tinged with orange.
Film special effects Sodium vapor process (occasionally referred to as yellowscreen) is a film technique that relies on narrowband characteristics of LPS lamp. Color negative film typically is insensitive to the yellow light of an LPS lamp, but special black-and-white film is able to record it. Using a special camera, scenes are recorded on two spools simultaneously: one with actors (or other foreground objects), and another that becomes a mask for later combination with a different
background. This technique originally yielded results superior to
blue-screen technology, and was used in years 1956 to 1990, mostly by
Disney Studios. Notable examples of films using this technique include
Alfred Hitchcock's
The Birds and the Disney films
Mary Poppins and
Bedknobs and Broomsticks. Later advancements in blue and green-screen techniques and computer imagery closed that gap, leaving SVP economically impractical.
Electrical parameters ==High-pressure sodium==