The 15th century: and as far back as 1417, lanterns with candles were used on the streets of London on winter nights, with a colour temperature of around 1800K.
The 18th century: fast forward to gas and oil lighting, with a CCT of around 2800K.
The 19th century: and electric street lighting arrived, from warm sodium (1700-2000K CCT), to whiter lamps (2700K+ CCT) and finally, LED, with a far broader range, but typically sitting between 3000-5000K CCT. However when LED first arrived it was common to have colour temperatures of in excess of 5000k+. This was the only way to get enough light out of the fledgling technology, but did result in some less than perfect lighting installations, with many users complaining of the harsh cold new luminaires.
This does mean, for the majority of time, exterior lighting has been warm!
As lamp and LED technology has improved we are able to produce white light with greater and greater efficiency and so the need for cold CCTs dissipated.
Our research and understanding of the physical and psychological effects of artificial light on the human body and the environment around us also developed. This has led to greater consideration of how we interact with the light spectrum, for example how the eye works in different light conditions and what we need to be able to see in low-level lighting environments - including to walk and drive. We are also now aware that light has an impact on how we feel - for example safe and secure or alert and awake. And more recently, we have investigated the impact on sleep cycles (our circadian rhythm) and health. These technological developments have come with significant improvements in energy, capital and maintenance costs and, of course, in the quality of light being offered.
White light and street lighting
The use of white light for street lighting has increased as the efficacies and colour properties of LED’s have improved, providing comparable light output to a traditional high-pressure sodium scheme – for a reduced amount of energy consumption. With the advent of white lamp sources replacing sodium, the British Standard for lighting, BS5489 2003, acknowledged that whiter light created a brighter looking environment and so, suggested that lighting designers, using a white light source, CDO-TT or CosmoPolis for example, on a pedestrian focused lighting scheme (S, or P classes as they are known now), drop a whole lighting class. “Interesting” you say.
Why only P class roads?
Our two classifications of roads in the British Standard and European Normative are M and P.
M class roads as generally used by vehicles at speed (motorways, trunk roads, A roads etc.) These are calculated using a luminance method in cd/m square. Luminance measures the light reflected from the road surface and objects are seen in silhouette against this bright background. Colours and edges are less important in this context as you need the contrast.
On the other hand, P class roads are defined as mixed pedestrian and vehicular traffic (residential roads), calculated using an illuminance method measured in lux. Illuminance is considered as the amount of light “falling” onto an object and its surroundings. Here is where a higher S/P ratio can make a difference. Colours become more distinguishable, outlines more vivid and brighter.
Is this the way towards energy savings?
Energy saved is excellent news BUT, sometimes dropping a lighting class and halving your light levels without considering other implications, is not always the most appropriate thing to do.
As the use of LED became more prevalent, and with a deeper understanding of human interaction with certain light spectrums, this “drop a class” method was replaced in the current 2013 version of the British Standard with S/P ratios.
Street lighting energy bills can be expensive and if you make your purchase decision based only on photopic lumens, you may end up overlighting, consuming much more power than is necessary. A lower lumen LED luminaire could save you money, operate more efficiently, while providing exactly the right amount of light, well this is the theory.
Now we are getting the benefits of the S/P ratio
Having covered the background and where it all started, in short, based on the spectral output of the light source, a lamp or LED is assigned an S/P ratio. This is partly why you don’t need as many LED lumens to compete with lamp lumens; the quality of the light dictates how we interpret the brightness of the lit scheme, letting us lower the lighting levels needed to achieve the same effect. This in turn allows you to adjust the levels for that class and deliver major energy and capital cost savings! So, good news on all fronts! Isn’t it?
|S/P Ratio and Illuminance when Ra ≥60
| P class
||Photopic illuminance (lux) for Ra ≥60 according to S/P ratio of lamp
Reference: PLG03 Lighting for Subsidiary Roads: Using White Light Sources to Balance Energy Efficiency and Visual Amenity, ILP publication
Applying the theory