1. Field of the Invention
The present invention relates to optical imaging devices, preferably devices operating in the infrared band, that are not cooled. This kind of imaging device is more particularly intended for use in a video camera or a telescope to form an image outside the visible band. This kind of device is sometimes referred to as an infrared focal plane.
2. Description of the Prior Art
As with any sensor, one of the main qualities required of an optical imaging device is to be able to deliver a signal of sufficient amplitude compared to the inherent noise of the device. Referring more particularly to infrared imaging devices, the most serious form of noise is usually the inherent thermal noise of the device, caused by the ambient temperature to which the device is exposed. To illustrate the importance of this effect, note that the normal human temperature of 37° C. corresponds to an infrared wavelength of substantially 10 micrometers.
The solution most widely used for reducing this noise consists in cooling the imaging device to a temperature corresponding to a wavelength outside the spectral domain to be imaged.
The devices used to obtain such cooling are bulky and expensive. They also introduce serious limitations on the dimensions of the imaging device and/or the cryogenic device and in some cases have a prohibitive power consumption. The simplest solution, which consists in using cooling by expansion of a gas contained in a pressurized cylinder, although it simplifies the production of the cryogenic device, leads to a considerable consumption of gas and is in practice essentially reserved for single-use devices, such as imaging devices in missiles.
In the case of a video camera generating successive frames, dispensing with a cooling device by using an array of thermocouples or an array of bolometers in combination with a device for chopping the signal between two successive frames has been envisaged. This produces the wanted signal, or at least a signal that is less degraded, by subtracting the noise from the total signal. However, this method has the drawback of not providing a reference signal, which degrades the accuracy of the measurements as a function of the temperature of the device.
Also known in the art are bolometer devices that dispense with chopping of the signal. However, these devices have a temperature drift such that they cannot be used for radiometric measurements.