The present invention relates to an elevator car lighting system as defined in the preamble of claim 1.

Elevator car lighting is typically implemented with fluorescent or incandescent lamps. Fluorescent bulbs consist of a tube of glass with one electrode on each end. The tube is filled with argon and a drop of mercury, and light is produced by a discharge, while a low-pressure gap between the two electrodes becomes electrically energized. In incandescent light bulbs, electricity is conducted to a glow filament inside the bulb, which leads to generation of heat and visible light.

Over the last years LEDs, that are light emitting di- odes, have developed from a glowing indicator lamp to a legitimate light source. LEDs are semiconductor devices, which, like transistors, and other diodes, are made out of silicon. What make LEDs give off light are the small amounts of chemical impurities that are added to the silicon, such as gallium, arsenide, indium, and nitride. LEDs are formed by bringing two slightly different materials next to each other to form a PN junction. When a forward voltage is applied to the semi-conducting element forming the PN junc- tion, electrons move from the N area toward the P area and holes move toward the N area. Near the junction, the electrons and holes combine. As this occurs, energy is released in the form of ligh't that is emitted by the LED. The material used in the semi-conducting element of an LED determines its colour. The two main types of LEDs presently used for lighting system are aluminium gallium indium phosphide, AlGaInP, alloys for red, orange and yellow LEDs, and indium gallium

nitride (InGaN) alloys for green, blue and white LEDs. Slight changes in the composition of these alloys change the colour of the emitted light.

LEDs are low-voltage devices, typical operating volt- ages being around 3 to 4 volts. The brightness of the LED is dependent on the amount of DC current supplied to the LED chip. When current passes through the LED, it emits photons as a by-product. Because LEDs produce photons directly, not via heat, they are more effi- cient than incandescent light bulbs. The light efficacy of modern LEDs has also become close to that of fluorescent light bulbs, i.e. about 50 Im / W. Unlike the conventional light sources that emit light in all directions, the light emitted from an LED is direc- tional, which makes the fixture efficiency of LEDs higher than that of conventional light sources. With omni-directional lamps, luminaire losses may range from 30 % to 60 % of initial lamp lumens, whereas the directed nature of LEDs can result in fixture effi- ciencies of 80 % - 95 % requiring fewer total lumens to provide the same level of illuminance. This makes the total lighting efficiency of LEDs fairly comparable to that of fluorescent lights.

As the LED technology has evolved from using LEDs for signalisation to their use in illumination purposes, also the lifetime of LEDs has increased. In 2004, there are high power LEDs commercially available, with 70% average lumen maintenance over 50,000 hours. This compares favourably to the lumen depreciation of some conventional light sources at the end of their rated life. The lifetime of LEDs is generally very long compared to conventional light sources, as the lifetime of incandescent light bulbs is typically around 1000 hours and that of fluorescent lamp around 10000 hours.

The brightness of the light emitted by the LED increases, as more current is supplied to the chip. However, increased current increases the juncture temperature of the LED, which may reduce the efficiency and the lifetime of the LED. For example AlInGaP LEDs, typical in generating white light, typically exhibit a 1 percent increase in luminous flux for every 1 ⁰ C reduction in temperature in equilibrium. Also, the reliability of the electronic components, like LEDs, improves drastically with operation temperature reduction. A heat sink, capable of absorbing heat generated by the LED, allows the LEDs to be driven at greater current levels and emit more light, as well as make LEDs lifetime longer.

The thermal design of the LED lighting systems differ from those of conventional light sources. Within a LED, about 15% of the energy is emitted as light and the remaining 85% as heat. Unlike conventional light sources, which dissipate heat by radiation, convection and conduction, all heat from the LEDs must be conducted away by the luminaire and dissipated via convection to the surrounding atmosphere, as the LED radiates negligible amount of IR. Thus, a heat sink is an essential part of a LED based lighting system. A typical construction of a power LED device comprise a heat sink, onto which the LED chip is mounted, the layers between the two parts being thermally conductive. The device also comprise a case, cathode and anode leads, connected to the chip by gold wires and a plastic lens through which the light is emitted, the space between the chip and the lens being typically filled with silicone encapsulent . The physical size of the heat sink in commercially available power LED lighting devices is typically fairly big compared to the size of the lumi- naire.

Lighting systems using LEDs can be thought of as having light sources, typically the individual LEDs sources, a ballast, which for LEDs is often called a driver, and a luminaire, i.e. the surrounding materials for optical control of the emitted light and thermal control of the overall system. The driver performs a function similar to ballast for discharge lamps: it controls the current flowing through the LED. Most LED drivers are designed to provide current to a specific device or array. Since LED packages and arrays are not presently standardized, it is very important that a suitable driver is selected to the specific device or array to be illuminated.

The object of this invention is to provide a new type of lighting system for an elevator car, said lighting system being based on the use of LEDs, utilizing the elevator car structure in making the system efficient and not demand a lot of space.

As for the features of the invention, reference is made to the claims .

The lighting system of the invention comprises one or more lighting elements mounted on a mounting tray and devices to control the operation of the lighting elements, where the lighting elements comprise light emitting diodes. According to the invention, an elevator car roof panel is used both as a mounting tray for the lighting elements and as a heat sink to absorb heat generated by the lighting elements.

The lighting system of the invention provides an efficient, low-maintenance solution for lighting an eleva- tor car. No extra heat is radiated towards passengers from the lighting elements, and the system can be constructed so that the demand of space for the lighting

system is very small. Operation at low voltage makes the system safe for both the passengers and the service staff. Due to the LEDs long lifetime, high energy efficiency and environmentally friendly composition, com- pared to e.g. mercury-bearing fluorescent lamps, use the system can be seen promoting sustainable development .

Further benefits of the system of the invention include reduced maintenance costs for the elevator car lighting system and reduced running costs due to the higher efficiency, as LEDs may be used with up to 80 % less energy than conventional light sources. Further, as LEDs are solid state devices, there are no moving parts or glass or filaments which could be broken, which in part makes the system reliable, robust and vibration proof. Also, the illuminating properties of the system are appreciable, as the light of the LEDs is fully dimmable without colour variation, and the light can be instantly turned on with full colour properties and lighting power.

In the following the invention will be described in more detail with reference to the attached drawings, wherein

Fig. 1 is an assembly drawing of an embodiment of the LED lighting system according to the invention, integrated into elevator car roof panel, and

Fig. 2 is an assembly drawing of another embodiment of the invention where a LED lighting system is integrated into elevator car roof panel .

In figures 1 and 2, a lighting system construction according to the invention is presented. A LED device 1, like a packaged power LED intended for illuminating purposes, is fixed to the elevator car roof panel 2. The LED device comprises LED chip 11, optics 12 and a case 13, which may here comprise electronics, driver and / or a heat sink, built into the device. In the embodiment presented in figure 1, the LED device is fixed onto the upper sheet 3 of the car roof panel 2. In the embodiment presented in figure 2, the LED device 1 is fixed to the lower sheet 4 of the car roof panel 2. In both figures 1 and 2, the paths for heat transmission are presented with arrows. Within the scope of the invention, the car roof panel can also be of other form, and the fixing can be done to other parts of the car roof panel or to another distinct part attached to the panel . According to the invention, heat transition between the LED device and the roof panel is enabled, such that the roof panel may conduct further the heat generated by the LED device. It is clear that the amount of LEDs installed into the car roof panel may be also much bigger that only one single LED.

Heat transfer from LED emitter takes place through conduction, and it is thus important to allow heat to be transferred to ambient by having an effective heat sink and making good thermal connection from LED board to heat sink. Good thermal design gives higher luminous flux at typical temperature, less lumen degrada- tion over lifetime and possibility for higher ambient temperatures . By effectively transferring the heat generated by LEDs, the following benefits will be received: the system will be reliable, it will be possible to use fewer LEDs for same illumination level, and the lifetime of the system components will be long.

By the use of elevator car roof panel as a heat sink the geometrical dimensions of lighting system can be made very small, and the lighting system depth towards the car interior can be very small, even less than 20 mm. The system does not require any changes to be made to a conventional elevator car ceiling structure.

In one embodiment of the invention the elevator car roof panel, used as a heat sink, is made of aluminium, which is light material, having good thermal conduc- tion properties. In another embodiment of the invention, the elevator car roof panel is made of aluminium composite panels, where aluminium honeycomb core endows the composite panel with high bending strength at an extremely low weight, using bonding system that is tough and resilient yet features a high degree of elasticity. In yet another embodiment of the invention the car roof panel comprises two plain sheets, having a corrugated sheet bonded between them.

The inventive step of the invention comprises the pos- sibility to use structures like the one described above on other surfaces of the elevator car, for example on upper parts of elevator car wall, or also in other parts of the elevator system.

In one embodiment of the lighting system of the inven- tion the LED driver is integrated on the circuit board, which offers a possibility to implement the system with minimal amount of single components. It is also possible to integrate the driver into elevator electrification. The above mentioned features simplify the system installation, and may contribute to low power dissipation.

In the foregoing, the invention has been described by way of example while different embodiments of the invention are possible within the scope of the inventive idea defined in the claims.