Analysis of an optical image

Abstract

In an electro-optical instrument, whether radiometer or imager, the objective has the role of projecting the image of an outside scene onto the plane of the instrument detector. The following stages consist of converting this optical image into an electrical signal. The next chapter will show how detector effectively convert the radiation into electrical voltage. We will consider here just the operation which consists of sequentially analysing each element of a two-dimensional optical image (or, amounting to the same thing, each region of the observed scene) to extract the temporal signal. This operation is called analysis. Many methods enable analysis of the image, the choice depends to a large extent on the type of detector used and the application envisaged. Thus we have the following cases:

• with surface sensitive tubes, the readout is made by deflection of a beam of electrons; the variations in the current which results are proportional to the energy distribution in the image formed on the sensitive surface: it is electronic image analysis with scanning by an electron beam;

• with single element photon detectors which generally remain fixed within the instrument, the image is explored sequentially by virtue of an optomechanical device placed between the objective and the detector: it is optomechanical analysis by deflection of the optical beam;

• with arrays of photon detectors able to cover the whole of the focal plane, electromechanical devices are not necessary; the readout is effected sequentially, pixel by pixel, by gathering the electrical charges generated in the elemental detectors under the influence of thermal radiation: it is a new form of electronic analysis but this time by charge transfer.