The present invention relates to imaging apparatuses of the kind having a detector constituted by a two-dimensional matrix of opto-electronic sensors.  It has a particularly important, although not exclusive, application to projectiles homing to a target and optionally self-propelled, and in particular to air-to-ground missiles. 
At present, apparatuses for imaging a scene as seen by a missile generally comprise a detector constituted by a two-dimensional matrix of opto-electronic sensors of small dimensions, and an inlet optical system incorporating a steerable mirror or xe2x80x9cheliostatxe2x80x9d which performs two functions.  Firstly it enables a target situated in the scene to be kept within the field of view of the detector.  Secondly, given that the characteristics of the opto-electronic sensors always present a certain amount of dispersion, it enables them to be calibrated by forming the image of a common source on all of the sites or sensors of the matrix. 
In general, the correction means for reducing the effect of dispersion in the characteristics operate on the assumption that the response curve of each of the sensors can be caused to coincide with that of a reference photosensitive site or sensor by means of a linear transformation of the form:
yn=anxc2x7xn+bn
where an and bn are a gain and an offset assigned to the sensor of order n, enabling a corrected value yn to be obtained from each output signal xn. 
Conventionally, the gains an are determined in the factory for each matrix for scenes at a plurality of different temperatures.  In operation, the set of gains corresponding to the temperature that is closest to the mean temperature of the scene is selected.  Offset bn is determined in the missile itself when it is put into operation by calculation performed by forming an image of the source on the sensors. 
Inlet optical systems having a steerable mirror require apparatus to servo-control the attitude of the mirror.  Such apparatus is bulky and expensive. 
It is an object of the present invention to provide an imaging apparatus enabling offset to be corrected on implementation, without requiring a deflector mirror. 
To this end, the invention provides, in particular, imaging apparatus comprising a detector constituted by a two-dimensional matrix of opto-electronic sensors and means for correcting the output signals from the detector in order to compensate for the differences between the response characteristics of the various sensors by applying a stored gain and an offset.  The apparatus comprises means for temporarily interposing an optical plate on the inlet beam coming from an observed scene and going towards the matrix, the plate being sufficiently diffusing to make the illumination received from the observed scene uniform over all of the sensors, and means for deducing the offset values from the output signals of the sensors receiving the uniform illumination and from the stored gains. 
The need, in order to implement the invention, to have a detector with a number of sensors that is sufficient to accommodate deviations of the target from the axis of the system while conserving sufficient resolution (which would have been a drawback a few years ago), does not present any difficulty at present given that detectors are available with a large number of elements at acceptable price and with dispersion that can be compensated.  This need will be even more acceptable in the future since detectors with even higher numbers of photosensitive sites are under development. 
The above characteristics and others will appear more clearly on reading the following description of a particular embodiment given by way of example.  The description refers to the accompanying drawings.