Overall Illuminance Uniformity of IRED Lighting in Nighttime CCTV

LEDs (light-emitting diodes) are superior to the conventional light bulbs and florescent lights in terms of low power dissipation, high efficient and longer lifetime. Therefore, conventional light bulbs are gradually being replaced by LED light bulbs as the next generation lighting source. LEDs are also playing more important roles in industrial applications, such as backlighting in LCD displays, street lights and interior illumination. However, LED light bulbs have some problems. Shorter lifetimes and degraded light efficiency occur due to increased heat radiation. This is caused by 70~80% of the supplied power being converted into heat energy. The high heat radiated from a LED light is transferred directly to the LED modules, and light emission efficiency is reduced by this heat generation. As a result, its lifetime is shortened. Furthermore the size of the LED module becomes much larger, and the manufacturing cost is higher, owing to the required installation of a heat sink in the LED module. In a closed circuit TV (CCTV) system installed outdoors in the dark or inside dark buildings, its CCD(charge coupled device) camera cannot recognize objects like the human eye can in low illuminance situations. In order to compensate for this, an IRED (infrared emitting diode) is installed or a CCTV system with built in IRED is used. However, since infrared illuminance is too low, or the overall illuminance is not uniform, infrared light is irradiated in one region or the center of the CCTV screen. As a result, image center is bright, but the outer part of the image is dark. This makes it difficult to distinguish some objects. This is the reason for the ‘white spot’ (saturation of light) caused by the difference of luminous flux intensity between its center portion and outer frame [1]. Other distorted images may be caused by blooming and smearing that degrades image quality [2]. Also, most LED light patterns are circularly symmetric with non-uniform illuminance distribution. This makes it hard to meet the requirement of illumination for nighttime CCTV systems. In order to solve these problems, this study designs a freeform surface lens to obtain better uniform illuminance over a prescribed TRANSACTIONS ON ELECTRICAL AND ELECTRONIC MATERIALS Vol. 18, No. 1, pp. 35-41, February 25, 2017