<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> <P> Sodium vapor process (occasionally referred to as yellowscreen) is a film technique that relies on narrowband characteristics of LPS lamp . Color negative film is typically not sensitive to the yellow light from 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 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 . </P>

Sodium vapour lamp has how much ionisation potential