LED | 12 June 2017
Safe, functional, natural and efficient. White light creates an atmosphere of well-being in the city and provides safety on the streets. White LEDs state their case successfully with high luminous efficacy, good to excellent luminous intensity and high energy efficiency. As a trendsetter in the lighting industry OSRAM places much importance on the development of long-lasting, efficient LEDs. White light in the city In the course of evolution, the human eye has adapted to bright, white sunlight. It is, therefore, not surprising that artificial white light is regarded as particularly natural.
Despite this fact, high-pressure sodium vapour lamps were a preferred solution in street lighting for many years. Compared to the high-pressure mercury vapour lamps which were previously used, high-pressure sodium vapour lamps have a higher luminous efficacy and last longer. Their typically yellowish light, however, makes it difficult to distinguish colours at night and gives objects and buildings an unnatural effect. This is one reason why municipalities and lighting planners increasingly insist on white street lighting.
More and more, the choice is becoming LED. White light provides increased safety: To a great extent, white LED light contributes to safety in the city. The luminous efficacy, ranging from good to excellent, and the subjectively greater brightness of white light contribute to the fact that objects, persons and buildings are more easily recognisable. Thus the number of traffic accidents, for example, can be markedly reduced. Investigations have shown that car drivers detect persons at the edge of the road earlier and from a greater distance away, when there is white street lighting. Pedestrians, too, recognise obstacles more quickly than under the yellowish light of the high-pressure sodium vapour globes frequently used in streetlamps. The following is a further top benefit of white LED light: A decrease in the crime rate in cities is being achieved through the use of white light sources. Under this type of lighting, people are easier to recognise, potential crimes easier to detect and attacks easier for a third party to report. White light also enhances the quality of images from video surveillance systems and thus supports investigations by the authorities. The advantages of white LED: White is not always white. That is because, as is often the case in daylight, various colour temperatures ranging from cold white to warm white are produced by artificial light. In public squares and in pedestrian malls, warm white light produces a comfortable atmosphere of well-being with colour temperatures around 3,000 Kelvin.
On the other hand, neutral white light promotes safety on the streets. As a result of a carefully selected colour temperature, certain materials appear to better advantage in the architecture. For glass and steel, for example, a cooler light is chosen, whereas brick façades look more natural in warm white light. Natural lighting effects In street lighting, why do we perceive white light as brighter than artificially-produced yellow light? During the course of evolution, the human eye has adjusted to natural white light. In the eye, there are two types of sensory cells which are activated for sensory perception at different intensities, according to the level of brightness.
According to a popular saying, "All cats are grey in the dark" [it's impossible to see any details in the dark]. At low levels of brightness, only sensory cells which do not perceive colour are activated within the eye but, in return, they react more sensitively to blue light. The cornflower, for example, still glows a brilliant blue even in strong moonlight, whereas the poppy looks an insipid red. These sensory cells, therefore, react less sensitively to yellow and reddish light. A low level of brightness thus results in the fact that white light (with a greater component of blue) is perceived as brighter in street lighting - as yellow light with a slight component of blue, or without any blue at all, in the case of low-pressure sodium vapor bulbs. Luminaires that are equipped with white LEDs are thus perceived as brighter than luminaires with sodium vapour bulbs, even if the volume of emitted light is identical. Strictly speaking, most outdoor lighting calculations are now evaluated incorrectly, since the sensitivity of the human eye, adapted to brightness, is taken as the starting point. The levels of brightness in street lighting usually correspond more to twilight vision. Many system operators already take this into account in their planning and are switching from yellow to white light. This can lower the level of brightness and thus the power consumed, without citizens noticing any difference. Finally, not only power and costs are saved in this way but there is also an increase in safety and quality of light emitted from lighting systems, as has been proved by numerous studies in past years. Some countries like Great Britain even changed their local regulations using this effect to save money, protect the environment and make their cities more livable (compare British Standard BS5489-1+A2).
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