The field of integral images has developed fast during the last years. Integral image devices are today used for creating eye-catching visual effects for many different purposes, e.g. as security markings, tamper indications or simply as esthetic images. Usually, the integral image device is intended to be provided as a label or as an integrated part in another device. Many different optical effects have been discovered and used and often different optical effects are combined to give a certain requested visual appearance. At the same time, it is a general request to keep the integral image device as thin as possible, in order to facilitate the integration into e.g. other sheet materials.
The typical approach for an integral image device is to provide an array of small focusing element. The focusing element may be different kinds of lenses, apertures or reflectors. An image fragment plane is provided with image fragment structures. The image fragment plane is provided relative to the array of focusing elements such that when the device is viewed from different angles, different parts of the image fragments structures are enlarged by the focusing elements and together form an integral image. Depending on the design of the image fragment structures, the integral image can change in different ways when the viewing conditions, e.g. viewing angles, are changed.
A typical realization of an integral image device is a thin polymer foil, where focusing elements and image fragment structures are created in different planes.
In the published European patent application EP2335943, the use of a reflective layer within the foil of an integral image device is disclosed. The image fragment structures are provided directly below the array of micro-lenses and the reflective layer faces the image fragment structures and the micro-lenses from below. The image fragment structures are seen by a viewer as an image reflected in the reflective layer and refracted in the micro-lenses. The optical length between the micro-lenses and the image fragment structures should typically be in the vicinity of a focal length of the micro-lenses in order to give an integral image. In such a design, the optical path is about double the distance between the bottom part of the micro-lenses and the reflective layer. In this way, the total thickness of the integral image device can be reduced.
However, since more elaborate optical designs typically requires additional layers and thereby increase the thickness of the integral image device, there is still a need to further reduce the device thickness.