|
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, sodium light is almost totally made up from deep yellow wavelengths at 589.0 and 589.6nm, so no colour rendering is possible While this does not cause a problem on main 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 place 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 4.
Figure 4 - Spectral Power Distributions of Various Light Sources
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 5, thus the images received by the human eye are always sharp and clear under low pressure sodium.
Figure 5 - SOX light cannot be dispersed into multiple colours, thus images are sharper
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 does not penetrate the atmosphere well and is scattered around 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 often be enhanced simply by removing blue wavelengths of light so that the pupil opens wider and allows the eye to see a brighter image 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 6 below shows the appearance of a piece of black cloth and white paper illuminated half with a 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 made much more vivid with this light source It is for this reason that warning signs around railway premises, 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
Figure 6 - Blacks appear blacker under SOX light - contrast is enhanced
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 here 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 totally No other light source offers such easy filtering of night time glow in the sky Cities which are home to major astronomical sites, most famously Tucson in Arizona, 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 normal photopic region and its colour sensitivity shifts to the scotopic conditions Thus the curve shown in Figure 2 may not always be valid, and its peak is shifted towards the blue end of the spectrum This is known as the Purkinje shift and the dashed line in Figure 7 shows the scotopic night-time eye sensitivity Thus although SOX produces many lumens per watt and is efficient when the lighting level is high, it performs very poorly and the eye finds it hard to see under this light if the illumination level is extremely low Extensive studies in USA have shown that at the very low lighting levels often found on their streets, sodium lamps are particularly difficult for the eye to see clearly with and due to our enhanced blue sensitivity under such conditions, the relatively inefficient mercury and metal halide lamps actually perform better Roads in Europe are customarily illuminated much better than this and the problem is not so severe here.
Figure 7 - SOX is not so efficient when the eye's sensitivity shifts to Scotopic Vision
No recent work has been conducted on whether or not the advantages of reducing scattering and penetrating rain/fog better, 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!
|
