  Chapter 9 USING OF LED LIGHTING TECHNOLOGIES TO SUBSTITUTE TRADITIONAL LIGHTING SYSTEMS IN GREENHOUSES Nuri CAGLAYAN, Can ERTEKIN INTRODUCTION One of the major parameters that influence plant growth is the availability of light. Greenhouse plant production systems have the capability of providing supplemental lighting during plant growth in cases where daylight is insufficient for optimal crop production. Supplemental lighting is provided to increase photosynthesis in plants and is often referred to as ‘‘assimilative lighting’’ because its main purpose is to increase the growth, that is, the assimilation of CO 2 in the crop [Ciolkosz et al. 2001]. Supplemental lighting in greenhouse facilities is provided by specially designed lighting systems, which, in the case of assimilative lighting (as opposed to morphogenetic lighting where light is provided to control the plant form and not growth), usually consist of high intensity discharge lamps in direct reflectors, mounted in a grid pattern above the plants. The performance of these systems is measured in terms of uniformity of the light supplied and average light intensity provided [Deitzeret al. 1994]. These properties are inherent of the design characteristics of the lighting system, the goal of which is to provide a highly uniform light level over the entire growing area in order to facilitate uniform crop production [Ciolkosz et al. 2001]. The structure and operating conditions of greenhouse plant production facilities make design of supplemental lighting systems a complex process. Many interactions exist between lighting systems and plants, such as photosynthesis, photo morphogenesis and thermal effects, and between lighting systems and parts of the greenhouse structure (e.g. reflections of the cover) and mechanisms of the production system (e.g. shading by other mechanisms). In addition, design properties of the lighting system are limited by several factors of the greenhouse production system, like the type of cultivated plant, greenhouse layout, available greenhouse height and desired light intensity level, light distribution of the luminaires and their power consumption and the availability of electric power. Electrical efficiency of the lighting system is the most important parameter in the majority of studies in system design [Bubbenheim et al. 1988; Albright, Both 1994; Sager 1984; Both et al. 1997 ].