This application claims priority from European patent application No. 17181521.0 filed on Jul. 14, 2017, the entire disclosure of which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a display assembly and more specifically an OLED display assembly intended to light a passive display device, for example in a timepiece such as a watch.

BACKGROUND OF THE INVENTION

In the watch industry, an increasing number of devices combine a conventional mechanical display using hands with a digital display, for example for applications connected to a smart phone, or to any other object provided with communication means.

In such devices, the digital display, when it is a backlit display, is switched on only on demand, in order to reduce the electrical power consumption of the system, whereas the mechanical display is purely passive and therefore cannot be read in the dark without a Superluminova® type coating. These phosphorescent coatings do not perform well enough for some applications (for a date disc, for example) because of the small surface area coated, but also because of the excess thickness created.

In a digital display of the prior art however, the light is emitted forwards and does not light other objects in the same plane or slightly set back from the front face of the display.

Moreover, it is known that the efficiency of light emitting diode (LED) devices, including those with organic active layers (OLED in English or DELo in French) generally used for lighting these displays, is greatly limited by optical efficiency, due to the difficulty in extracting light from a highly refractive medium to a weak refractive medium. This low efficiency is mainly due to the total internal reflection of the light emitted by the active layer. Indeed, unlike lasers where coherent light is highly oriented by the resonant cavity, light is emitted isotropically in LEDs. As a result of this isotropy, light emitted at an angle to the extraction surface normal higher than the critical angle undergoes total reflection, which traps this light in its substrate. This trapped light can represent up to 80% of emitted light for a substrate that has not been optimised.

To enhance the optical efficiency of digital displays, various strategies for texturing the extraction surface have been proposed in the prior art surface depolishing, regular structures of ‘optical’ films optimised for light extraction, semi-cylindrical ‘domes’ above the active zones (pixels).

However, to date, regardless of the extraction device, a non-negligible proportion of the light emitted at the LED junction remains trapped and is lost inside the light guide formed by the substrate and, where appropriate, in the different layers that make up the OLED.

SUMMARY OF THE INVENTION

The present invention concerns a display assembly including an organic light-emitting diode display device (OLED), comprising at least one OLED formed of an emitting stack placed on a transparent (or semi-transparent or translucent) substrate whose back face is placed on the main emitting surface of the OLED and a secondary object to be illuminated, said substrate including a front face and at least one side face, said side face being arranged to light a secondary object.

The display assembly is particularly suited to being incorporated in a watch, in which the main surface, which may include an integrated or added decoration and which can be produced by various known techniques (pad printing, silk screen printing, digital printing, transfer printing, etc. . . . ), forms the face of a watch dial capable of displaying information transmitted to the user, a secondary object arranged in proximity to the side face which can be illuminated by the light trapped between the front faces of the substrate when it exits through the side face of the substrate. Typically, this recovered light can advantageously be used to light a passive analogue display arranged in proximity to the side face. When the OLED display device forms a watch dial, a through opening can be arranged in the thickness of said dial to form an aperture and advantageously the OLED display device can be combined with a passive analogue display device of the date display type, wherein the date disc moves underneath the aperture and wherein the information borne by the date disc seen through the aperture can be illuminated by the light extracted from the side walls of the aperture. The display device can also light a secondary object arranged in proximity to the side face and substantially in the same plane as the substrate.

Preferably, said side face is frosted to produce a diffused light during use.

Advantageously, said side face is at an angle of between 90 and 135° relative to the front face.

Advantageously, the transparent substrate has a higher refractive index than the refractive index of the OLED layer so as to confine the light within the substrate.

Opaque covers are advantageously arranged on the side faces that do not light the secondary object.

