In the field of information displaying instruments such as timepieces, hand held calculators, vehicle-boards and the like there has been a tendency in recent years to adopt passive type displays, particularly those involving the by now well-known liquid crystals. Displays of this type are passive, that is to say, they do not emit light of themselves and therefore, in darkness they become totally invisible and unreadable. To remedy this state of affairs there have been proposed types of illuminating arrangements in which for example there may be a simple incandescent lamp placed in proximity to the display, or in other known methods there has been proposed the so-called Beta light using for example tritium acting on a phosphorescent material.
In the case of small portable instruments, particularly in the case of wrist-watches it becomes essential to conserve energy and thus where a display is illuminated for instance by a lamp such as an incandescent lamp or a light emitting diode it becomes most important to assure maximum efficiency in providing the illumination to the display. This is to assure maximum life of the energy cell utilized to drive the instrument and to illuminate the display.
Known optical illuminating systems especially where used in connexion with watch displays exhibit two principal defects which the present invention seeks to remedy. Thus, known optical systems as shown schematically for example in FIGS. 1 and 2 are formed utilizing a luminous source 40 which may be for instance a microlamp having an incandescent filament, or a microtube containing tritium and giving off visible light by means of a luminescent substance or still further a light emitting diode, and of a panel 50 comprising transparent material which serves as an optical wave guide and reflector. The light source 40 is applied against an end face 51 of panel 50 or placed in an opening arranged in a portion of the latter. The known optical systems are all defective by reason of a poor optical coupling between the light source 40 and panel 50 such that the major portion of the luminous energy emitted by the source in the cylindrical sector 41 of peak S does not arrive within the panel 50 and from this fact is lost and only the portion admitted in the hached cylindrical portion 42 of peak S passes into the panel.
On the other hand the light captured by panel 50 is propagated by a total internal reflection or reflection on surfaces treated to be reflecting. If the end face 52 of panel 50 is uncovered, light such as ray L.sub.7 may exit by this face, but is useless in respect of illuminating the display. Should end face 52 be covered with a reflecting layer the light may not exit from panel 50 other than by the entry surface 51. To get around this problem the major face 53 of panel 50 adjacent the display is generally treated in order to obtain a roughened surface. However, because of the type of optical coupling between source 40 and panel 50 and the mode of propagation of light within panel 50 the angles .theta. of internal reflection on faces 53 and 54 are large. Light may thus only exit by refraction along the non-smooth face 53 and thus tangentially in an angle .delta. which is limited and thus becomes useless for illuminating the display. However, a certain rather weak illumination may be obtained using only a small portion of the luminous energy which propagates within panel 50: it comprises light such as the ray L.sub.8 which has run through a complex trajectory comprising an internal reflection on the non-smooth surface 53 followed by a reflection on the opposite face 54 if this is provided with a reflecting coating, or on a diffusing reflector 60 located behind face 54 if the latter is uncovered, thereafter a refraction at surface 53 which gives place to a ray L.sub.8 ' or L.sub.8 " according to the arrangement previously indicated. The known arrangement thus provides only a small portion of the luminous energy produced by source 40, for lighting the display, the remainder of the light being lost.
The present invention provides an improved arrangement in which the major portion of the light emitted from the source is captured and diffused in a more effective manner whereby the display is better illuminated and at the same time a weaker source may be utilized thereby conserving power.