The invention relates to an optical head used in such equipment ensures a missile launch detector (MSD). A vehicle, an airplane or a helicopter for example, which is equipped with a DDM is thus alerted when the missile threat.

A DDM is conventionally provided with an optical head which collects photons and converts them into an electric signal and a processing unit that analyzes the electrical signal to detect a possible missile, threatening the wearer.
In general, the operational need four main parameters: probability of detection (vs. range), false alarm rate, reporting time and spatial coverage. The first three parameters depend essentially on the processing and the last of the opto-mechanical configuration. The treatment comprises three steps: a step of detecting which role is to distinguish the point sources of resolved sources and extract them from the bottom, a tracking step in which each of the hot spots is the subject of a spatio temporal treatment evaluate its characteristics and classifying step based on the results of tracking. The threat is classified as dangerous or not depending on a comparison with a database of criteria and typical false alarm. Furthermore, taking an image of the scene and its treatment have to be fast. The time statement DDM represents the maximum time between the firing of the missile and the reporting time by the DDM as 'threat'. This constraint is important to the ability to respond to the threat.
DDM performance result of a compromise between these four criteria. This compromise depends on the optimization of the optical head and treatments.
There are various types of optical heads.
It is known to use an optical head in a single spectral IR detector with a wide stripe of about 2 μ and therefore not selective.

The IR detector may be a linear detector such as a strip, or an array detector.
With a linear detector operating at a scan rate of about 1 Hz, the false alarm rate is high, particularly in an urban environment, which makes difficult its operational use.

An array detector makes it possible to obtain an image in a better rate, for example greater than 20 frames per second; but typically, the field of view is narrow. Then you must use multiple optical heads for a large spatial coverage, which has consequences in terms of cost and reliability.
The object of the invention is to achieve reliable standby equipment at reduced cost, to detect missiles with a false alarm rate acceptable.

The invention relates to an optical head to a standby equipment intended to be installed on board a vehicle and comprising a matrix detector. It is mainly characterized in that this detector is sensitive in the spectral band IR2 with a bandwidth less than 1 .mu.m and centered on a predetermined value XO and is adapted to operate at a rate greater than 20 Hz, and in that it comprises an optical device of the type "Fish-Eye" which has a greater angular coverage 140 ° x 140 °.
This optical head thus comprises a matrix detector sensitive in a narrow spectral band centered in XO, associated with an optical device such as "Fish-Eye" (FE) having a large spatial coverage such as a hemispherical angular coverage.
An optical head is then obtained spectrally selective acquiring the signal with a sufficiently fast rate to detect targets with high probability and low false alarm rate and an angular coverage avoids the use of several heads.
The invention also relates to a missile launch detector equipped with an optical head as described.

Other features and advantages of the invention will appear on reading the following detailed description, given by way of nonlimiting example and with reference to the accompanying drawings in which:
Figure 1 schematically shows an optical device of the type "Fish -Eye "
Figure 2 schematically shows an example of distribution due to the sun spurious images,
FIG 3 schematically shows the normalized illumination on the detector taking into account the image of the sun and residual images.

The optical head according to the invention comprises a matrix detector sensitive in the IR2 spectral band between 3 and 5 μ, with a narrow bandwidth centered in XO; it is associated with an optical device such as "Fish-Eye" (FE) having a large spatial coverage such as a hemispherical angular coverage. As shown in Figure 1, the FE device 10 disposed in front of a matrix detector 5 comprises an optical combination, based on lenses including a negative lens 11 input hemispherical shape. Given the need to cover operational up to 4 π steradians, sun, in the case of a daytime mission is present continuously in the field of an optical head. Internally induced reflections are comparable in intensity to signals from the missile. As a result, the total transmission of the FE device is high, greater than 70% and the reflectance of the structure housing the components is low, less than 10%, so as to reduce these internal reflections and thus obtain a low rate of 'ghosting.
In addition to separate an image missile solar environment, it is important that the combined strengths of all solar residual images are maintained at a low level regardless of the sun's position in the field of view. These parasites images are mainly caused by parasitic reflections on lens surfaces, the dissemination of the metal structure and on the surfaces of diamond lenses.

The surfaces of the lenses are optimized to minimize ghosting and flare. Moreover all the ghost images do not degrade the performance of the optical head. The direct image of the sun creates an exclusion zone on the detector 5 as shown in Figure 2. A parasitic image 51 in the vicinity of the sun's image 50 has no impact on performance, but a spurious image 52 remote from the image of the sun impairs performance by creating another exclusion zone. The exclusion zone 55 shown in Figure 3, created by the direct image of the sun 50 is calculated from the diffraction image of the sun 53: it is circular in shape, having a diameter D. A spurious image located in this area is not a problem, even when it has a high intensity. We define a threshold
The diameter d of the entrance pupil 12 is determined so that, from a given source point, enough signal can be focused on the corresponding pixels of the matrix detector. It is also determined depending on the F / d, where F is the overall focal length of the FE device. The quality of the optics (depending on the spatial resolution and aberrations) is selected to focus at least 80% of the energy on a pixel.
To avoid regularly replace the input lens which is subjected to harsh environmental conditions, there is provided a protective structure for the optical head.
To protect the input lens, the protection structure includes a hemispherical dome made of a material to withstand adverse environmental conditions. Silicon or sapphire may be selected. A diamond-like carbon coating having high qualities of hardness, transparency and wear in temperature, can be deposited on the outer side of the dome to resist erosion by rain.
Variations in temperature and pressure degrade the optical quality of the FE device. The optical combination of lenses and materials are chosen to obtain a focal length of the system almost constant despite variations of temperature and pressure capable of covering a wide temperature range [-6O 0 C; + 6O 0C] and pressure [27hPa; 1050 hPa].

Similarly, the optical combination is studied in order to have a constant optical quality in the field.

The matrix detector, in this case an IR detector is included in an infrared detection unit. The IR detector which has for example a size of 640 x 512 pixels 2 (1000 x 1000 pixels or 2 ) is assigned in the block to a dewar, a linear type cooling machine and an electronic card of the detector control. It is recalled that the dewar is a vacuum envelope to about -200 ° C which contains the detector.
The array detector comprises in known manner a transducer that converts received photons into an electrical signal and a read circuit which transmits the electrical signal to means for processing the electrical signal.
Preferably, the matrix detector further comprises a near processing element connected to the reading circuit and the signal processing means. It enables a correction of non-uniformities in the matrix according to the conditions of employment. This element also allows the integration of an image in two integration time choice. We can thus integrate both images in low light at high brightness: the detector then has a high dynamic lighting. A short integration time of 50 microseconds is chosen for example for very bright image, then a longer 1ms integration time, for example is selected for low-light images. Such a matrix detector is designated matrix detector combined mode.
In the case of a conventional detector, forming two successive images each with a different integration time.
The optical head according to the invention is intended to equip a standby equipment installed on board a vehicle such as an airplane, a helicopter, a land or sea vehicle.

WE CLAIMS

1. Optical Head standby equipment intended to be installed on board a vehicle and comprising a matrix detector (5), characterized in that this detector is sensitive in the IR2 spectral band, with a bandwidth less than 1 μ, centered on a predetermined value XO and is operable with a rate greater than 20 Hz, and in that it comprises an optical device (10) of type "Fish-Eye" which has an angular coverage greater than 140 ° x 140 °.

2. An optical head according to the preceding claim, characterized in that the device of "Fish-Eye" has a 70% higher total transmission and is included in a structure whose reflection coefficient is less than 10%, so as to get a low-noise images.

3. An optical head according to the preceding claim, characterized in that the device (10) of "Fish-Eye" type comprises an optical combination, based on lenses manufactured in materials determined so as to obtain a substantially constant focal length whatever variations in pressure and temperature to obtain a constant optical quality in the angular coverage.

4. An optical head according to any one of the preceding claims, characterized in that the device (10) FE comprises an input lens (11) and in that the head comprises a protective structure which comprises a hemispherical dome intended to protect the input lens FE device.

5. An optical head according to any one of the preceding claims, characterized in that the matrix detector is combined mode.
6. Missile departure detector equipped with an optical head according to any preceding claim.