Backlighting has always been a problem for membrane switch manufacturers, the opaque silver pads preventing full function lighting of the overlay from the rear of the switch
A variety of approaches have been used, often with banks of LED's or reflector boxes. Cut outs have been required in the backing panels.
Markit Graphics offers to forms of effective backlighting, Electroluminescent, and Lumitex Woven Fibre. These are both thin film (less than 200 mic -.008") panels and are laminated within the switch directly below the overlay for full function backlighting.
There are advantages for each product, and also many environmental considerations. For instance, in both products, the inclusion of the backlighting layer has a noticeable impact on the tactile feel - it deadens that "click" when the button is pressed.
EL is manufactured using a printed process. This gives great flexibility - we can print to customise the product to your specific application.
EL is a phosphor based technology, and has a definite life cycle. In a typical application this would be a half life of approximately 3000 hours
The EL panel is driven by a AC source, typically 120VAC @ 400Hz. An inverter derives this from DC supply of 5-24VDC
The higher the frequency, the higher the light output. The lifetime is also inversely correlated to the frequency - increase the light intensity, reduce the life time.
It is common to integrate a light sensor or timer within the circuit to switch the panel on and off to maximise the effective life.
Phosphors are available in a number of different colours - blue-green often the most popular. White is also available.
EL Panels are very sensitive to humidity and moisture, and also to UV light. Special allowances have to be made if they are used in these environments.
LUMITEX WOVEN FIBRE BACKLIGHTING
Lumitex is a woven fibre optic sheet commonly used for the applications such as LCD backlighting, Membrane switches, machine vision applications. It is patented, light emitting panel woven from plastic optical fibres.
The thin panel is heat and EMI free and provides superior brightness, longer life, and significant design flexibility when compared with other backlighting methods.
The light source is LED, or gas filled bulb, or halogen, and can be simply changed