question: Why are most street lights and car lights orangish yellow and not white?
comment: Many street lights were/are sodium vapor lights. It’s a highly efficient light that just produces a warmer light as the byproduct. URL_0
passages: Selective yellow
  
  Selective yellow is a colour for automotive lamps, particularly headlamps and other road-illumination lamps such as fog lamps. Under ECE regulations, headlamps were formerly permitted to be either white or selective yellow—in France, selective yellow was mandatory for all vehicles' road-illumination lamps until 1993.
   Safety yellow is one of the standard high-visibility safety colors defined by ANSI standard Z535, which specifies standards for safety and accident prevention information. It is often used on hazard symbols, warning signs, guard rails, dangerous equipment, and some high-visibility clothing and personal protective equipment. The definition is mirrored in British Standard BS 381C and Australian Standard AS2700 (where it is known as golden yellow). In 1937, it was determined that safety yellow was the best color to be noticed by the human brain; as a result, the paint color of all United States school buses was changed from orange to safety yellow (see also school bus yellow).
   Previous ECE regulations also permitted selective yellow light. A research experiment done in the UK in 1968 using tungsten (non-halogen) lamps found that visual acuity is about 3% better with selective yellow headlamps than with white ones of equal intensity. Research done in the Netherlands in 1976 concluded that yellow and white headlamps are equivalent as regards traffic safety, though yellow light causes less discomfort glare than white light. Researchers note that tungsten filament lamps emit only a small amount of the blue light blocked by a selective-yellow filter, so such filtration makes only a small difference in the characteristics of the light output, and suggest that headlamps using newer kinds of sources such as metal halide (HID) bulbs may, through filtration, give off less visually distracting light while still having greater light output than halogen ones.
   Section::::Colour of light emitted.
  
  The colour of light emitted by vehicle lights is largely standardised by longstanding convention. It was first codified in the 1949 Geneva Convention on Road Traffic and later specified in the 1968 United Nations Vienna Convention on Road Traffic. With some regional exceptions, lamps facing rearward must emit red light, lamps facing sideward and all turn signals must emit amber light, while lamps facing frontward must emit white or selective yellow light. No other colours are permitted except on emergency vehicles.
  
  Section::::Forward illumination.
   Yellow was particularly valued in the 20th century because of its high visibility. Because of its ability to be seen well from greater distances and at high speeds, yellow makes for the ideal color to be viewed from moving automobiles. It often replaced red as the color of fire trucks and other emergency vehicles, and was popular in neon signs, especially in Las Vegas and in China, where yellow was the most esteemed color.
   A research experiment done in the UK in 1968 using tungsten (non-halogen) lamps found that visual acuity is about 3% better with selective yellow headlamps than with white ones of equal intensity. Research done in the Netherlands in 1976 concluded that yellow and white headlamps are equivalent as regards traffic safety, though yellow light causes less discomfort glare than white light. Researchers note that tungsten filament lamps emit only a small amount of the blue light blocked by a selective-yellow filter, so such filtration makes only a small difference in the characteristics of the light output, and suggest that headlamps using newer kinds of sources such as metal halide (HID) bulbs may, through filtration, give off less visually distracting light while still having greater light output than halogen ones.
   Selective yellow headlamps are no longer common, but are permitted in various countries throughout Europe as well as in non-European locales such as South Korea, Japan and New Zealand. In Iceland, yellow headlamps are allowed and the vehicle regulations in Monaco still officially require selective yellow light from all vehicles' low beam and high beam headlamps, and fog lamps if present.
   In the US, daytime running lamps may emit amber or white light, and may produce up to 7,000 candela. This has provoked a large number of complaints about glare.
  
  Section::::Conspicuity, signal and identification lights.:Front.:Dim-dip lamps.
   BULLET::::- Adjusting the "type" of lights used, so that the light waves emitted are those that are less likely to cause severe light pollution problems. Mercury, metal halide and above all first generation of blue-light LED road luminaires are much more polluting than sodium lamps: Earth's atmosphere scatters and transmits blue light better than yellow or red light. It is a common experience observing "glare" and "fog" around and below LED road luminaires as soon as air humidity increases, while orange sodium lamp luminaires are less prone to showing this phenomenon.
   BULLET::::- In the rules of the road, yellow (called "amber" in Britain) is a traffic light signal meaning "slow down", "caution", or "slow speed ahead". It is intermediate between green (go) and red (stop). In railway signaling, yellow is often the color for warning, slow down, such as with distant signals.
  
  BULLET::::- Selective yellow is used in some automotive headlamps and fog lights to reduce the dazzling effects of rain, snow, and fog.
  
  Section::::Symbolism and associations.:Maritime signaling.
   For countries driving on the left, the convention stipulates that the traffic signs should be mirror images of those used in countries driving on the right. This practice, however, is not systematically followed in the four European countries driving on the left, Cyprus, Malta, the Republic of Ireland, and the United Kingdom. The convention permits the use of two background colours for danger and prohibition signs: white or yellow. Most countries use white, with a few – such as Finland, Iceland, Poland, and Sweden – opting for yellow as this tends to improve the winter-time visibility of signs in areas where snow is prevalent. In some countries, such as France, white is the normal background colour for such signs, but yellow is used for temporary signage (as, for example, at road works).
   White LEDs can be used as white holiday lights or to create any other color through the use of colored refractors and lenses similar to those used with incandescent bulbs. Color fading may occur due to the exposure of colored plastics to sunlight or heat, as with ordinary holiday lights. Yellowing may also occur in the epoxy body in which the LED is encased if left in the sun consistently.
   Section::::Formal definition.
  
  The UNECE Regulations formally define selective yellow in terms of the CIE 1931 colour space as follows:
  
  For front fog lamps, the limit towards white is extended:
  
  The entirety of the basic selective yellow definition lies outside the gamut of the sRGB colour space—such a pure yellow cannot be represented using RGB primaries. The colour swatch above is a desaturated approximation, created by taking the centroid of the standard selective yellow definition at (0.502, 0.477) and moving it towards the D65 white point, until it meets the sRGB gamut triangle at (0.478, 0.458).
   LPS lights are even more efficient than HPS, but produce only a single wavelength of yellow light, resulting in a CRI of zero, meaning colors cannot be differentiated. LPS lamp tubes are also significantly longer with a less intense light output than HPS tubes, so they are suited for low mounting height applications, such as under bridge decks and inside tunnels, where the limited light control is less of a liability and the glare of an intense HPS lamp could be objectionable. Many LPS lights are also being changed out to HPS using FCO fixtures, or going to LED.
   In Memphis, Tennessee most city maintenance vehicles (including MLG&W utility trucks) with flashing lights use yellow lights; however, some vehicles with the traffic engineering department use red flashing lights, especially vehicles equipped with cherry pickers used to repair traffic signals.
   Many photosensitive materials used in technical and industrial applications, such as photoresist, are sensitive only to blue, violet, and ultraviolet light and may be handled under a brighter yellow safelight. Low-pressure sodium vapour lamps sometimes are used in larger industrial darkrooms. They emit nearly monochromatic light at 589 nm (yellow), to which the materials are insensitive; as a result they can be extremely bright while still "safe".
  
  The word "safe" in "safelight" is relative, as in most cases, a sensitised material eventually will be affected by its safelight if exposed to it for an extended length of time.
  
  Section::::See also.
   The French yellow-light mandate was based on observations by the French Academy of Sciences in 1934, when the Academy recorded that the selective yellow light was less dazzling than white light and that the light diffused less in fog than green or blue lights. Yellow light was obtained by dint of yellow glass for the headlight bulb or lens, a yellow coating on a colourless bulb, lens, or reflector, or a yellow filter between the bulb and the lens. Filtration losses reduced the emitted light intensity by about 18 percent, which might have contributed to the reduced glare.
   Incandescent light bulbs have been commonly used in holiday lights until recently. These lights produce a broad-spectrum white light, and are colored by coating the glass envelope with a translucent paint which acts as a color filter. Some early Japanese-made lamps used colored glass.
  
  Section::::LEDs.
  
  Light-emitting diode (LED) holiday lights are quickly gaining popularity in many places due to their low energy usage, long lifetime, and associated low maintenance. Colored LEDs are far more efficient at producing light than their colored incandescent counterparts.
   BULLET::::- In Canada and the United States, school buses are almost uniformly painted a yellow color (often referred to as "school bus yellow") for purposes of visibility and safety, and British bus operators such as FirstGroup are attempting to introduce the concept there.
  
  BULLET::::- "Caterpillar yellow" and "high-visibility yellow" are used for highway construction equipment.
   Initial research into fire appliance visibility was conducted by the Lanchester College of Technology and the Fire Brigade in Coventry, in the UK in c. 1965. It concluded that under the range of artificial street lighting in common use at the time, yellow better retained its conspicuity than red. Yellow was also more conspicuous in general road conditions in the daytime and during inclement weather. Research conducted by Stephen Solomon, a New York optometrist, promoted the use of "lime yellow" in the United States from the mid-1970s. Solomon conducted studies of the rate of vehicle accidents involving fire apparatus, concluding that the more conspicuously colored fire apparatus suffered a lower accident rate than the less conspicuous red used by the same fire department.
   Many astronomers request that nearby communities use low pressure sodium lights or amber Aluminium gallium indium phosphide LED as much as possible, because the principal wavelength emitted is comparably easy to work around or in rare cases filter out. The low cost of operating sodium lights is another feature. In 1980, for example, San Jose, California, replaced all street lamps with low pressure sodium lamps, whose light is easier for nearby Lick Observatory to filter out. Similar programs are now in place in Arizona and Hawaii. Such yellow light sources also have significantly less visual skyglow impact, so reduce visual sky brightness and improve star visibility for everyone.
   Section::::Specifications.:Colour of light.
  
  Under ECE regulations, H1 lamps are required to emit white or selective yellow light. U.S. regulations require H1 lamps to emit white light. Under both ECE and U.S. specifications, the allowable range of white light is quite large; some H1 lamps have a slight blue or yellow tint to the glass yet still produce light legally acceptable under the requirement for white light.
  
  Section::::Variants.
  
  Section::::Variants.:Compliant.
   Section::::On service vehicles.:Taxi displays.
  
  Taxicabs are distinguished by special lights according to local regulations.
  
  They may have an illuminated "Taxi" sign, a light to signal that they are ready to take passengers or off duty, or an emergency panic light the driver can activate in the event of a robbery to alert a passersby to call the police.
  
  Section::::Light sources.
  
  Section::::Light sources.:Incandescent lamps.
   The international regulation on motor vehicle bulbs requires manufacturers to test bulbs for colour endurance. However, no test protocol or colour durability requirement is specified. Discussion is ongoing within the "Groupe des Rapporteurs d'Éclairage", the UNECE working group on vehicular lighting regulation, to develop and implement a colour durability standard. Rather than using an amber bulb, some signal lamps contain an inner amber plastic enclosure between a colourless bulb and the colourless outer lens.
  
  Section::::Conspicuity, signal and identification lights.:Rear.
  
  Section::::Conspicuity, signal and identification lights.:Rear.:Rear position lamps (tail lamps).
   Both the internationalized European ECE Regulation 19 and North American SAE standard J583 permit selective yellow front fog lamps. ECE Regulation 48 currently requires new vehicles to be equipped with headlamps emitting white light. However, selective yellow headlamps remain permitted throughout Europe on vehicles already so equipped, as well as in non-European locales such as Japan and New Zealand.
  