Why electroluminescent lamps?
Original article date: October 1998
A new light emitting technology based electroluminescence (EL) is a thin flexible light source that can be produced in sizes from about 600mm down to a few millimetres. Where should they be used?
Innovel lamps can be made from a single element for backlighting LCD displays for example. Light is emitted evenly over the whole area of each defined segment. Equally multi-segment lamps can be produced for eye-catching graphical displays or for selectively illuminating keypad buttons. Multi-segment lamps can be selectively sequenced on and off or even gently faded up or down. These techniques allow sophisticated eye-catching standalone displays. Low power consumption allows the use of batteries in many applications.
Innovel is constructed using a thin flat polymer substrate with a clear conductive coating. This is used as a base and the clear conductor allows the light generated in the phosphor layer to be seen. This sheet is usually 0.175mm thick. A very thin layer of phosphor is then applied followed by a very thin opaque conductor layer. The whole lamp is therefore very thin flexible and light.
When an electric field is applied across a thin layer of phosphor particles light is emitted. The phosphor layer is a good insulator so very little power is consumed. The electric field is generated by applying an AC voltage across the phosphor layer. Voltages between tens and hundreds of volts can be used at frequencies between tens of Hz to many kHz. Typically though somewhere between 50V and 120V is used at frequencies between 400 and 2000Hz.
Current flow is very small so driving voltages can be generated in a very cost-effective manner by small electronic circuits running from batteries.
Light output rises as voltage and frequency are increased . For a fixed driving voltage and frequency. For a fixed driving voltage and frequency – light output lowers with time – the rate of change will vary according to the level of driving. Acceptable variations will vary considerably according to particular applications. Some driving schemes use known ageing characteristics to alter driving voltages to compensate for loss of lamp brightness (up to certain limits).
As with other lamps such as incandescent and fluorescent light output lowers with use but unlike these other sources Innovel does not suddenly fail. Light output continues to fall gradually .
The initial brightness of the lamp can be within a wide range. However the higher this initial brightness the faster the drop in brightness will be. High temperature and high humidity are often used for accelerated life testing.
- RH Technical Industries
- 01264 363451
October 1998