While the drive for efficiency has long been an important issue across many industries the economic turmoil of recent years made it an absolute top priority. With budgets under the microscope more than ever before every penny counts, and in the street lighting sector this has led to suppliers focusing on ensuring that all products and solutions perform at optimum levels.

One way in which energy and cost efficiency can be achieved in our industry is by having tighter control over street lighting stock, in terms of when road lights come on at night and go off during the day. A central management system, such as our Vizion solution, provides local authorities with the means of not only controlling off and on times but also identifying any stock that is not performing as it should be or is in need of repair. Another option that gives councils greater control over road lighting is photocells, which react to natural light levels to switch lanterns on and off.

Earlier automatic switching methods included basic electro-mechanical time clocks, which were then replaced by digital time clocks thanks to the pace of new developments in electronic devices. The disadvantage of both of these was that neither could measure the ambient lighting level where they were installed, so they could not take into account weather conditions during dusk and dawn periods when natural lighting levels might be much darker than expected.

Photocells, or PECUs (Photo Electric Control Units), were the next evolution in switching methods and made it possible to switch lanterns on and off in accordance with actual ambient lighting levels. The critical component of PECUs is the actual light-sensor, which in the majority of current PECUs is either a Photodiode or a Photo IC. These units feature in-built filters to eliminate non-visible light and enable photocell sensors to replicate the spectral sensitivity and photopic response of the human eye.

While the current widely accepted switching on level of 70 lux caters for the lamp types used in road lighting that require time to run up to a fully functional output, there has been a drive to move to a 35 lux switch on level to account for the shorter run up times required by modern lamp types. Switch off levels of the corresponding level of 35 lux allows for the natural ambient lighting level to be established before the artificial lighting is switched off. Again the threshold has been reduced to 18 lux (for 35 lux on) as part of a continued drive to reduce the length of time a lantern is energised for.

However, on and off times for road lighting stock are not purely dictated by concerns over cost and energy use. The safety of roads users is of course also a key consideration. There needs to be a smooth transition between declining natural ambient light levels, which the human eye adapts to slowly, and the production of artificial light levels and a correlation needs to be made between falling natural light levels and lamp run-up times. As the human eye adapts more quickly to rising ambient light levels the PECU can switch off the lantern when daylight is at such a level that it provides for a smooth transition back from artificial to natural light.

Typical run up times vary from 2 minutes for CPO lamps to 10 minutes for LPS lamps. LED lamps however, due to the starting characteristics of light emitting diodes, are considered not to have any discernable run up time but rather can achieve an instant full output. As the 70 lux switch on level was considered the norm to accommodate the relatively long run up times for lamps being used, the increased use of LED technology in road lighting is surely grounds to reconsider switching times specifically for LED lanterns.

With LED light sources 100% flux is generated almost immediately at switch on, therefore a potential lower PECU switch on value could be used for LED installations. The immediate 100% switching for LEDs though could also present potential problems, as in some instances it might actually instantly double a scheme's lighting levels. This could have a number of disturbing visual effects especially for those who are particularly susceptible to sudden changes in lighting. This problem can be overcome by some of the latest LED drivers that have a ramp-up feature on start-up that gradually increases the light levels of LEDs and provides a smoother transition.

Ideally LEDs should have a fade up rate that is equal to the reducing ambient light rate so to avoid problems caused by either 100% lamp flux or general over lighting of a scheme. This would minimise the demand for eye adaptation, making the transition period more comfortable for road users at dusk.

Looking at the situation from an energy, cost and CO2 perspective, retaining higher PECU settings and ratios does not realise the full saving potential and therefore benefits of using LED technology. However, if 'fade up' escalating LED lighting was available within a PECU, part night diming could also be employed, which would offer potential energy savings and increased LED life. So while LED technology has been seen as the way forward within the street lighting industry for a while now, when it comes to road lighting specifically savings will depend on the development of new PECUs.