<P> 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 . </P> <P> 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 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 . 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 . 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 . </P> <Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2017) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2017) (Learn how and when to remove this template message) </Td> </Tr>

Circuit diagram of high pressure sodium vapour lamp