Optical Systems For Light-Emitting Diodes
All over the world the light-emitting diode industry has become one of the fast-growing economic sectors over the recent years. Every six months the parameters of light-emitting diodes (LED) and lighting devices based on them move to the new quantitative level by luminous efficiency, radiation power and light flux, color characteristics. According to the evaluation made by Philips , the reduction of LED cost is anticipated, and it is forecasted that by 2020 the light sources and lighting devices based on light-emitting diodes will occupy 75% of lighting market.
Tasks and Development Prospects of Light-Emitting Diodes
Light-emitting diodes (Figure 1) started to be used as light sources in 60s of the 20th century. The operation principle of light-emitting diodes as semiconductor devices is based on the transformation of electrical energy directly into light radiation. The mechanical construction of light-emitting diode determines the light distribution in space. Only the light-emitting diodes with the supply current of more than 100 mA, the light flux of which is more than 10 lm and which radiate white light, are of interest for the common illumination.
The main properties of light-emitting diodes, which will make them the most economical light sources in the nearest future in comparison with other light sources:
High luminous efficiency (100–150 lm/W);
Low energy consumption (units of watts);
High values of efficiency coefficients of lighting fixtures and coefficients of use of light flux in lighting systems;
Small size (capability to use them in point-light or flat devices);
High longevity (tens of thousands of hours);
Absence of light flux pulsation;
Capability to obtain the radiation with different spectral composition;
Capability to decrease the coefficient of reserve of lighting devices due to the stability of characteristics and long service life;
Capability to use them for illumination of fading objects (works of art, printed products, products of textile industry);
High environmental resistance (in case of exposure to temperature, vibration, impact, humidity);
Electrical safety and explosion safety;
Opportunity of fast decrease in the size, materials and labor consumption of production of lighting devices;
Opportunity of design of maintenance-free lighting fixtures;
High degree of control (capability to construct the systems of multilevel illumination control);
High manufacturability in case of mass production;
Low costs connected with packaging and transportation.
Wide application of light-emitting diodes in illumination systems, lighting and indication systems makes the estimation and design of light-emitting diodes topical in relation to the optical systems, which have high luminous efficiency and ample opportunities for the control of energy characteristics of radiation.
Use of light-emitting diodes in lighting systems requires the application of secondary optics, the intended purpose of which is to direct the light flux radiated by light-emitting diode into the set space area and provide the formation of required illumination distribution in this area [2–4]. Over the recent years, the interest of leading international manufacturers and consumers of light sources has been rapidly growing in the replacement of traditional incandescent lamps and daylight fluorescent lamps by the lighting devices based on semiconductor light-emitting diodes integrated into the light-emitting diode modules (LEDM) (Figure 2). Development in this area is hindered due to the absence of the methods of estimation of LEDM lighting characteristics and systematic information on the luminous efficiency of LEDs, which is also caused by insufficient progress in the international standardization and shortage of special measurement instrumentation. Therefore, the task of modeling of lighting characteristics of LEDs and LEDMs is topical at the stage of design of lighting devices (LD) .
At the present time, the following basic areas of research and development of light-emitting diodes can be mentioned:
Decrease of the level of luminous efficiency drop in case of high current density;
Enhancement of light output ratio and color characteristics of white LEDs;
Decrease of thermal resistance and heat rejection of LEDs;
Study of perception of lighting based on light-emitting diode by human vision;
Production automation of light-emitting diodes and light-emitting diode lamps.
Secondary Optics for Light-Emitting Diodes
It is well known that LDs must solve two basic lighting tasks: redistribution of the light flux of light source in proper manner and restriction of its dazzling effect. LEDs are the light sources radiating into one hemisphere, and this fact requires the special approach to the design of LD based on LEDs.
Light-emitting diode has the cosine light distribution (type of light intensity curve (LIC) – D) (Figure 3). LIC is the graph of dependence of light flux intensity on colatitude and equator angles obtained by the section of its solid of light distribution by plane. But such light distribution is not suitable for the illumination of the premises with high apertures or streets. The specialized optics is required for the design of energy-efficient illumination.
Secondary optics – lens or mirror reflector of plastics, which is mounted on one or several light-emitting diodes; it is represented by individual component which is not the part of light-emitting diode. Use of secondary optics allows solving the following tasks:
Change of LED light distribution, for example, focusing the radiation at needed angle or making it nonsymmetrical;
Redirection of the whole light flux from light-emitting diode into the illuminated area increasing the efficiency of lighting device and reducing its cost;
Formation of required illumination distribution corresponding to all lighting standards.
The first parameter, in accordance to which the selection of secondary optics for the solution of one or another task is performed, refers to the angle of light flux formed by the optical element. Most often, this value corresponds to the half-value angle, which is defined as the angle, at which the light intensity in this direction accounts for 50% of maximum value (Figure 4).
Nowadays, the majority of powerful LEDs are produced with one-two variants of light distribution for different reasons. Task of the optical system used in couple with light-emitting diode is to distribute the light flux in space as rationally as possible. Correctly selected optics allows increasing the density of diode light flux considerably and adjusting its operation for the current technical task in more accurate manner.
Optical systems can be divided into two main types – lens and reflecting systems (Figure 5). All of them create different diagrams of radiation directivity in space. The most common diagrams (Figure 6):
Narrow diagram – angle of efficient radiation 5–20°;
Medium diagram – angle of efficient radiation 20–50°;
Wide diagram – angle of efficient radiation from 50°.
The lens of total internal reflection (TIR) – collimator (Figure 7) operates in accordance with the following scheme. Large angles of light-emitting diode are converged by the sides of lens, which represent the reflector of total internal reflection. Small angles are converged by the lenticular surface located in TIR. Practically all light of LED is efficiently used and not lost. One of the criteria which influence on the optics efficiency and lens, in particular, is the ratio between its size and radiating surface of light-emitting diode. The standard type of collimator is given in Figure 8. Part А of the cavity internal surface corresponds to rotation hyperboloid and provides the collimation of central beams from the source. The beams refracted on the lateral surface of internal cavity experience the total internal reflection on the external parabolic surface B. At the expense of TIR use, the collimator efficiency is equal to 90%.
The material, of which the optics is made, also plays significant role in the increase of transmission coefficient. This material should not blur with time due to the action of environmental factors or radiation. At the present time, the lenses for LED are mainly made of polymethyl methacrylate, optical polycarbonate with the transmission coefficient of 95–98% or silicon-organic compounds (silicone) . LEDiL company offers to produce the lenses of polymethamethylacrilate but coat them with thin layer of silicon on the top . On one hand, it allows using well-tested technology, which is suitable for the lenses with any shape; on the other hand, the lens with silicon coating obtains many useful properties, which are typical for the lenses of one-piece silicon. These properties include high strength and sealing potential. Such lenses can operate without protective dome lamp.
The popularity of lenses can be explained by the high level of convenience and relative simplicity of formation of required light flux because the radiation control is performed by three planes: two refracting surfaces on the radiation input and output and one reflecting surface of lens. However, the design of such collimators is quite complex process. Besides, their application for large-size LEDs turns out to be problematic.
Reflectors (Figure 9) have only one reflecting surface, the task of which is to form the required light flux. Most often, they are used in combination with the light-emitting diodes, which have radiating surface with larger size, or with group of LEDs.
Optical efficiency or capability to transform the light flux of LED with the lowest possible losses upon its spatial transformation is significant characteristic of optics. Converging capacity of the systems (ratio of light flux inside the angle of efficient radiation to the total light flux which passed through the system) is also important parameter. As a rule, the efficiency of lens systems is lower.
The reflecting optics in LD based on LEDs has quite large size in comparison with lens optics. The reflectors are made of aluminum with high reflection coefficient or plastic with the deposition of mirror layer and protection from environmental exposure.
Secondary lenses as well as secondary reflectors require very accurate arrangement in relation to illuminating body. Therefore, the special original holders are usually added to them.
At the present time, lens and reflecting secondary optics with wide, semi-wide, cosine, deep and concentrated LICs is produced . The lenses with asymmetrical light distribution (wide in transverse plane and concentrated lateral in longitudinal plane) are produced for the street lighting fitting based on LEDs.
Problems Connected with Estimation of Secondary Light-Emitting Diode Optics
Design and production of modern optical systems requires the modeling of complex physical phenomena. The models of light propagation in light-scattering elements are the part of optical modeling systems.
Estimation of secondary optics for light-emitting diodes refers to one of the most complicated tasks occurring in lighting engineering. It is connected with large amount of the requirements, which are simultaneously made in relation to such optical elements:
Provision of high luminous efficiency;
Minimum distance from the light-emitting diode to the illuminated area at maximum angular size of illuminated area;
High level of uniformity of the formed illumination distribution.
Secondary optics is estimated for the certain type of light-emitting diode and set light distribution. It will not provide the required width of flux with other light-emitting diodes. Besides, the illuminated area can be non-uniform and occurrence of additional rings is possible. Therefore, with the appearance of new light-emitting diodes at the market the task of estimation and modeling of secondary optics, which is compatible with them, occurs.
Nowadays, several tens of universal software packages for design of optical systems with different intended purposes and number of the systems intended for the design of specialized systems are offered .
Use of light-emitting diodes in illumination systems requires the application of secondary optics – lens and reflecting optics, which allows changing the light distribution of light-emitting diode, enhances efficiency of lighting fixture as a whole, forms the required illumination distribution. The task of estimation and modeling of lighting characteristics of light-emitting diodes and light-emitting diode modules is topical at the stage of design of lighting fixtures.