Candles inside glasses to William Murdoch’s gas lighting in the late eighteenth century were the concept of streetlighting for over 200 years. It is only in 1880 that Wabash, a small town in Indiana in USA, earned the distinction of becoming the first electrically lighted city in the world. When automobiles started rolling out and flourishing in early twentieth century, a street with electrical lights came to be referred as ‘White Way’ – like New York’s Broadway was nicknamed the ‘Great Whiteway’.Streetlights then used incandescent and low pressure mercury vapour lamps more popularly referred as fluorescent lamps. Streetlight luminaires with High Pressure Mercury Vapour lamps took over from early 1950 and lasted over two decades. It never gained popularity as the bluish green colour made people look like they had the ‘blood drained from them’. The same fate faced Low Pressure Sodium Vapour lamp (LPS) during that time. Although the most efficient discharge lamp in terms of its efficacy, people under LPS looked all dark like ‘ghosts’. The advent of High Pressure Sodium Vapour Lamp (HPS) around 1970 happens to be one of the most desirable and adorable light source for streetlighting. The warm colour (CCT between 2700K to 3000K), quite high CRI, contrast enhancement and retrofitting capability put HPS on advantageous position over its predecessors. Till date its popularity has not waned. Even after over four decades of existence this conventional lamp commands stronghold over major streets across the globe. Apart from assisting motorists, streetlight’s contribution to public lighting plays a major role on psychological behaviour of human beings by inducing warmth, harmony and also sense of security and confidence. So in order to assess the quality of streetlighting, a basic understanding of seeing process is important.
The process of seeing is through optical pathway in a human eye which is connected to the visual cortex (part of the brain which mediates the sense of light) via a nervous system. The nervous system is made two types of receptors like rods and cones. These, in turn, contribute to 3 types of image forming functions, and are important for normal daily function and life quality. The visions are 1) SCOTOPIC (rod), where the field luminances lie between 10-6cd/sqM to 10-2cd/sqM. This is a vision in the darkness where the world is grey and without colour sensation; 2) PHOTOPIC (cone) – coloured vision with luminance above 10 cd/sqM; 3) MESOPIC – Most important vision where the field luminance is raised upwards from 10-2cd/sqM to 10cd/sqM. With luminance moving up the luminosity of red increases more strongly than that of blues due to changing contributions of rods and cones receptors. Known as Purkinje phenomenon, this is an important aspect for designing effective road lighting, as it takes into account the luminance concept as well as neurological aspects of road lighting.
Venice Canal way with all outdoor lights converted to LED
For over four decades, colour corrected HPS streetlights have complied closely with Mesopic vision. Today, despite several advantages of energy efficient LED light source, there are a lot of hurdles to overcome before it is able to take over HPS in streetlighting. One of the major hurdles is retrofitting the HPS streetlighting and replacing the lamps with LEDs of CCT 2700K to 3000K. Once this can be achieved upgradation of existing streetlights will save energy upto 80%, produce better light colour stability and can be dimmed to adjust to the traffic load.
Osram has recently set up a new LED chip factory at Kulim, Malaysia to produce high power chips and light engines to meet the retrofitting requirement. Osram is also expanding its capacity in Wuxi, China for producing LED with housing and partly with primary optics too.
The canal ways and Piazza San Marco in Venice, Italy has been upgraded with LED using the vintage luminaires. The look and feel of bygone era has been retained while reducing energy usage by 80%.Around 7000 luminaires have been retrofitted with LED. This has been completed in 2017.
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