Spectral Properties of the Sodium Discharge

It has already been mentioned that sodium lamps emit their light very near to the peak sensitivity of the human eye under normal viewing conditions, this fact accounting for their remarkably high luminous efficacy. The light emitted is monochromatic, i.e. it is comprised of only one colour. Incandescent lamps produce light of all colours over the entire visible range 400 to 700 nanometres (nm), so these have good colour rendering properties. Other light sources produce light at many different discrete wavelengths and all provide some degree of colour rendering. However, low pressure sodium light is almost totally made up from deep yellow wavelengths at 589.0 and 589.6nm, and no colour rendering is possible. While this does not cause a problem on major roads it may be unacceptable in other areas such as town centres, and it totally precludes the use of this light source for interior lighting or in any other application where colour rendering is required. The spectral power distribution of the low pressure sodium lamp is compared with several other common light sources, as well as Daylight, in Figure S5.

Traditionally, it has been proposed that low pressure sodium light is the safest to drive under. This is due to its monochromatic output which improves the perception of contrast and allows the light to penetrate fog and rain with the minimum of dispersion. Moisture in the atmosphere acts as a kind of prism, and will split light up into its component colours, a phenomenon which is seen naturally whenever the conditions are such that a rainbow becomes visible in the sky. SOX light contains only one colour and cannot be dispersed into blurred multicolour images that other light sources deliver, illustrated in Figure S6, thus the images received by the human eye are always sharp and clear.

The light contains no blue radiation, a colour which is easily scattered in the atmosphere and to a very great extent in fog and mist - indeed, the sky appears blue because this wavelength of light is scattered around in the upper atmosphere, and is less able to penetrate it. Blue light also has an adverse effect on the pupil, in that relatively small amounts of this wavelength cause it to contract and not as much light is then able to enter the eye. Visibility can sometimes be enhanced simply by removing blue wavelengths of light such that the pupil opens wider and allows more light into the eye. Because low pressure sodium does not contain any blue radiation, the eye is able to see very clearly under this light.

In addition, we can perceive the contrasts between moving and stationary objects more quickly under monochromatic light and this is of paramount importance in night-time driving. Figure S7 shows the appearance of a piece of black cloth and white paper illuminated half with a low pressure sodium (SOX) lamp and half with high pressure sodium (SON) type. The black appears much blacker when lit with SOX light, succinctly demonstrating how contrasts are enhanced with this light source. It is for this reason that warning signs, road side chevrons etc. are often printed in deep yellow and black, and even in nature many insects adopt this colour combination because it stands out better than any other

The large physical size of low pressure sodium sources can make the light more difficult to control efficiently, and upward sky glare is sometimes more of a problem. Despite this, astronomers are actually fully in favour of sky glow originating from this light source. Because the light is monochromatic, it can be filtered out by removing its single wavelength and a dark sky is then restored. No other light source offers such easy filtering of night time glow in the sky. Several cities which are home to major astronomical sites have almost completely converted their night-time illumination to low pressure sodium for this reason.

However, recent studies show that at the low light levels often employed in street lighting, the human eye does not function fully in the photopic state, and its colour sensitivity shifts to the scotopic (night vision) conditions. Thus the curve shown in Figure S3 may not always be valid, because its peak is shifted towards the blue end of the spectrum, as illustrated by the dashed line in Figure S8. This phenomenon is known as the Purkinje shift. Herein lies a deficiency of our measure of light - the lumen - because it is based on the normal daylight sensitivity curve of the eye. At very low light levels, one really ought to instead rate lamps in terms of their scotopic luminous flux, instead of the universally published photopic luminous flux. Due to the Purkinje shift, low pressure sodium lighting becomes less and less efficient at low illuminance levels. Extensive studies in USA, a country where streelighting levels are considerably lower than in other parts of the world, have clearly demonstrated that light sources having a greater proportion of green-blue light in their spectra are in fact more efficient under these circumstances. Roads in other parts of the world, however, are lighted to much higher standards such that the effect of the Purkinje shift are much less significant.

No recent work has been conducted on whether or not the advantages of reducing scattering and penetrating rain/fog, improving contrast or making moving objects stand out more clearly is outweighed by the poor scotopic performance. As a result, SOX lamps continue to go into many new installations and this is expected to carry on until more evidence is gathered - which may either kill off the SOX market totally or further reinforce its apparent strengths!