Overall implementation of a keypad for a mobile device, i.e., the final package, including domes, backlight and keyset, should provide the required visual functionality as well as sufficient tactile feedback for the user when pressing an individual key. In view of thin keypads, the tactile feedback can be obtained using elastic light guide, dome pin lead throughs (holes above domes), or dome pins integrated with a plastic light guide (flexible but not elastic). Of these, sufficient optical quality can be obtained using either the first or the last approach.
Spatially selective out-coupling of light from a planar light guide plate for keypad illumination is an important desired feature. It means that simultaneous control of out-coupling efficiency and spectra of the out-coupled light needs to be provided to enable modification of the visual appearance (intensity, colored/white) of the individual keys. In addition, a tactile feedback when pressing an individual key should not be compromised by the illumination system.
Various types of out-coupling means based on surface structuring of the planar light guide exist. Spatial modulation of the out-coupling can be obtained by limiting/modulating the spatial structuring to/within separate regions (key areas) on the light guide.’
Moreover, colored appearance can be typically obtained using white light out-coupling and ‘painted’ keysets on a keymat. In cost sensitive applications, such as mobile phones, the utilized ‘paints’ typically have large variety of optical quality depending on the utilized manufacturer and can cause significant efficiency loss due to poor optical characteristics.
Various difficulties appear in keypad utilizing light guides for illumination and usually requiring several different manufacturing steps in keypad and light guide fabrication. Manufacturing of keypad's dome pins is usually realized by injection molding. The mechanical key function is realized by dome pins through holes in the illumination light guide. This can lead to energy losses in the illumination.