Sodium-vapor lamp
[2] Low-pressure sodium lamps give only monochromatic yellow light, inhibiting color vision at night.
These operated at pressures of less than 1 Pa and produced a near monochromatic light spectrum around the sodium emission lines at 589.0 and 589.56 nanometres wavelength.
LPS lamps have an outer glass vacuum envelope around the inner discharge tube for thermal insulation, which improves their efficiency.
[12] Further improvement was attained by coating the glass envelope with an infrared reflecting layer of indium tin oxide, resulting in SOX lamps.
[14] This high efficiency is partly due to the light emitted being at a wavelength near the peak sensitivity of the human eye.
They do not exhibit a bright arc as do high-intensity discharge (HID) lamps; they emit a softer luminous glow, resulting in less glare.
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.
[15] 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 December 31, 2019.
[16] 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).
The yellow color of low-pressure sodium lamps also 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.
Where sodium vapor lights are the source of urban illumination, the night sky is tinged with orange.
Sodium vapor process (occasionally referred to as yellowscreen) is a film technique that relies on narrowband characteristics of LPS lamp.
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.
Later advancements in blue- and green-screen techniques and computer imagery closed that gap, leaving SVP economically impractical.
Understanding the change in human color vision sensitivity from photopic to mesopic and scotopic is essential for proper planning when designing lighting for roadways.
This construction led the General Electric Company to use the tradename "Lucalox" for its line of high-pressure sodium lamps.
This line is extremely pressure (resonance) broadened and is also self-reversed because of absorption in the cooler outer layers of the arc, giving the lamp its improved color rendering characteristics.
In addition, the red wing of the D-line emission is further pressure broadened by the Van der Waals forces from the mercury atoms in the arc.
In another failure mode, a tiny puncture of the arc tube leaks some of the sodium vapor into the outer vacuum bulb.
