Selective yellow

Selective yellow (desaturated approximation)
Colour coordinates
Hex triplet	#FFBA00
sRGBB  (r, g, b)	(255, 186, 0)
CMYKH   (c, m, y, k)	(0, 27, 100, 0)
HSV       (h, s, v)	(44°, 100%, 100%)
Source	CIECD
B: Normalized to [0–255] (byte) H: Normalized to [0–100] (hundred)

One of the first optic headlamp lenses, the Corning Conaphore made of selective yellow "Noviol" glass.

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.

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.

The intent of selective yellow is to improve vision by removing short, blue to violet wavelengths from the projected light. These wavelengths are difficult for the human visual system to process properly, and they cause perceived dazzle and glare effects in rain, fog and snow. Removing the blue-violet portion of a lamp's output to obtain selective yellow light can entail filter losses of around 15%, though the effect of this reduction is mitigated or countervailed by the increased visual acuity available with yellow rather than white light in bad weather.

Formal definition

The UNECE Regulations formally define selective yellow in terms of the CIE 1931 colour space as follows:

Limit towards red	$y \ge 0.138 + 0.580 x$
Limit towards green	$y \le 1.290 x - 0.100$
Limit towards white	$y \ge 0.966 - x$
Limit towards spectral value	$y \le 0.992 - x$

For front fog lamps, the limit towards white is extended:

Limit towards white	$y \ge 0.940 - x$
	$y \ge 0.440$

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).

Here are images of a car equipped with selective yellow headlamps and driving lamps, a set of fog lamps designed to produce selective yellow light, and of the light beam emitted from the same lamps:

• 

  Selective yellow foglights

• Fogson.jpg

  The beam produced by selective yellow lights

• Porsche 911 with selective yellow headlights.jpg

  Porsche 911 with selective yellow lights