Review and studies on the effect of spectral composition of LED based lighting system over its scotopic-photopic ratio

Abstract

This paper starts with a short introduction followed by the idea of mesopic photometry followed by different road lighting terminologies and concluded with the analysis of the S/P ratio and the SPD of the light sources. The light sources considered here are mainly concerned with the LEDs having variation in their CCTs. The road lighting actually consumes a significant amount of total energy requirement of a country all through the year. But from the point of view of security of road users road lighting is extremely important. With the passage of time role of road lighting has also gradually evolved from providing safety to drivers to providing visual comfort to the road users. Now road lighting also plays a major role in increasing the pedestrian feeling of safety and creating a better ambience. Thus visually acceptable and energy efficient road lighting is desirable. The mesopic luminance region lies between photopic and scotopic. The mesopic region covers luminance between about 0.001cd/m2 to 3cd/m2. It is neither luminosity function nor luminosity function alone are representative of the eye's spectral sensitivity in the mesopic region. Still today all dimensioning is based on the photopic luminosity function, which was established in 1924. Luminosity function describes the sensitivity of the retinal cone receptors and it is applicable to daytime vision. In the mesopic range both the rods and cones on the retina are active and this is believed to cause changes in the spectral sensitivity of the eye. LEDs are said to be the future light source because of their low energy usage and efficiency. The advantages of LEDs are that they are very robust, have a very long lifetime or up to 50,000 hours, they are easily dimmable and fail by dimming over time, rather than burn off like incandescent light bulbs. LEDs cause less glare irritation because of the smaller beam angle of the luminaire. The colour of the light depends on the composition and condition of the semiconducting material used. It can be infrared, visible or ultraviolet. Blue, green and red LEDs can be used to produce most perceptible colours, including white. Today, after many years of development, the LEDs on the market are now emitting white light in different colour temperatures as well as an advanced RGB control to produce coloured light to capture different moods for various aspects. Because of the huge potential of LED technology and the constant improvements in the quality (e.g. colour rendering), it can be predicted that the use of LEDs will become more common in both indoor and outdoor lighting with the advantage of energy savings due to their efficiency and long lifetime. Another advantage of using LED is it does not contain mercury (Hg), which is not an eco-friendly chemical and has adverse effect on human body.