Various small optical imaging systems exist, especially for imaging in the infrared spectral domain.
There are two categories of optical systems in this field:                uncooled systems; and        cooled systems.        
In both these categories, it is desirable to decrease the bulk and mass of the imaging optics.
The prior art envisions three solutions, which may be combined:                1. limiting the number of lenses;        2. using thinner lenses such as Fresnel lenses or multichannel lenses; and/or        3. using lenses manufactured from a lighter material.        
Regarding the category of uncooled systems, the first solution is generally employed i.e. the number of lenses is limited to two. Regarding more particularly the uncooled imaging systems used in the infrared, denoted IR, spectral domain, this minimum of two lenses is most often used to form a Petzval architecture consisting of a focusing first lens and a field-flattening lens allowing field curvature to be corrected.
Patent JP 2001/141993 A adds to this first solution, consisting in limiting the number of lenses to two, the replacement of conventional lenses with thin Fresnel lenses.
Patent U.S. 2007/0002467 A1 also adds a third solution to these first two solutions, this third solution consisting in using low-cost materials such as polyethylene, which can be used only in the context of thin lenses because of its high absorption. This United States patent especially describes uncooled infrared detectors of the microbolometer type.
The main drawbacks of these architectures are their low optical range, their low sensitivity, and the low resolution of the images obtained. This is especially due to the Fresnel lenses being used in diffractive mode, in which mode they have the drawback of being very chromatic.
Regarding the category of cooled systems, U.S. patent application FR 2 936 878 describes an optical imaging system comprising a dark chamber containing a detection chip, a single lens, which is not necessarily thin, and a diaphragm placed upstream of this lens. In the case where said lens is not thin, this system is required to cool an additional mass of optics relative to a system using a thin lens as its single lens, thus this would be a drawback because this additional mass to be cooled would increase the cool-down time of the optical system.
Regarding both categories of systems, i.e. both cooled and uncooled systems, another solution for limiting the thickness of the lenses is to use a multichannel system based on a matrix of microlenses. This solution has the drawback of requiring microlenses with large sags, which are difficult to produce.