SOLID STATE IMAGING APPARATUS

Abstract: Solid state imaging apparatus comprising a charge-­ coupled frame transfer image sensor including an image-­ sensitive integration region for forming a charge pattern representative of an incident image, first and second storage regions associated with respective opposed ends of said integration region, and charge transfer means for shifting the charge pattern formed in said integration region into a selected one of said storage regions.

Description: This invention relates to solid state imaging apparatus which includes a charge-coupled frame transfer image sensing array. In such sensors, an image is focussed onto a photosensitive area of the device known as the integration region. After a charge integration period comparable with the TV readout field (normally 16.7 or 20 ms), the charge pattern generated by the incident radiation is moved bodily into a storage region by means of parallel column transfer registers and read out line by line via a horizontal readout register. It is known from U.K. Patent No. GB-B-2,083,968 to vary the integration period to provide a variable sensitivity for the device. According to one arrangement disclosed therein, an image sensor includes a charge drain structure in the form of signal sink means accessible from the column registers in the integration region of the array. During the normal field integration period, charge is integrated, but at a predetermined time dependent on the required sensitivity, the charge pattern in the integration region is shifted at high speed away from the storage region so that the charges spill over the ends of the column registers and become dumped into the device substrate. Having lost this charge, charge is allowed to integrate in the integration region for the remainder of the field period. The integrated charge pattern formed during the remainder of the period is then read out in the normal way. Thus the apparent sensitivity of the CCD is proportional to the length of the remainder of the integration period. Variation in sensitivity may also be achieved by an electro-mechanical iris. It will, however, be understood that a typical scene focussed on the charge-coupled device (CCD) may have a very wide dynamic range, i.e. it may include a wide range of light levels. Conventional frame transfer charge-coupled devices can have a dynamic range in excess of 60dB, this being the ratio of the maximum number of charges that may be stored in each picture element to the number of noise charges generated in the device and the readout amplifier. In practice, the useful range is severely curtailed by fixed pattern noise due to variations in sensitivity and dark current between individual picture elements. This fixed pattern noise often limits the practical dynamic range to below 40dB. US-A-3,777,061 discloses a solid state CCD device comprising an image pick-up region with a storage region located at either side thereof. According to this invention there is provided solid state imaging apparatus comprising a charge-coupled frame transfer image sensor including an image-sensitive integration region for forming a charge pattern representative of an incident image, first and second storage regions associated with respective opposed ends of said integration region, and charge transfer means for shifting the charge pattern formed in said integration region into a selected one of said storage regions, said apparatus further comprising control means for defining a continuing series of field time periods each including a long portion and a short portion, said control means being operable for causing said charge transfer means to shift a charge pattern formed during the long portions of said field time period into one of said storage regions and to shift the charge pattern formed during the short portions of said field time period into the other storage region. In one embodiment, during each integration period, charge is integrated for a portion of said period and then shifted to one of said storage regions and the charge integrated during the remainder of the period is transferred to the other storage region. In an embodiment operating at a given TV field rate, a long and a short sensitivity image may be obtained for each TV field. Alternatively, the data output from the device may be reconstructed to give a single image with an increased dynamic range. Furthermore an image of increased dynamic range may be recorded on two conventional video tape recorders of limited dynamic range. The apparatus preferably includes charge drain means in said image sensitive integration region, for example an antiblooming structure, and said control means are preferably operable to cause charge accumulated for a predetermined variable portion of said integration period to become lost into said charge drain means. By this arrangement the exposure periods in each TV field may be controlled independently By way of example only, certain embodiments of this invention will now be described in detail, reference being made to the accompanying drawings in which: Figure 1 is a schematic diagram of a CCD frame transfer imager having dual storage regions and configured to allow construction of images of wide dynamic range; and Figure 2 is a schematic diagram of a CCD frame transfer image with an antiblooming structure. Referring initially to Figure 1, the device illustrated includes a CCD frame transfer imager 10 having two storage regions 11, 12 provided above and below the imaging (charge integration) region 13. Each of the regions 11, 12 and 13 includes a set of gate electrodes (not shown) and channel stops (not shown) which together define a rectangular array of picture elements. Independent three or four phase clocks (not shown) connected to the gate electrodes control or "clock" the movement of image charge between and within the three regions along the columns defined by the channel stops. The direction of charge movement can be reversed in any region by simply reversing the phase of the electrical clock signals. In operation, a charge pattern of an image focussed on the phtotsensitive integration region 13 is integrated for less than the normal 20 ms TV integration period. The image is then transferred by clocking into the upper storage region 11 and a second charge image integrated for the remaining portion of the TV field, this is then transferred by clocking into the lower storage region 12. The two stored images are simultaneously read out line-by-line during the following TV field via parrallel to serial readout registers 14 and 15 for display or recording. The sequence of lines in the TV field readout from store 11 is reversed; this may be rectified on a cathode ray-tube display by reversing the vertical scan coil connections. Alternatively, if the image is stored in a digital frame store, the mode of addressing may be altered to accommodate the reversed line sequence. In and alternative embodiment of the device, the output at A is reconfigured to B and the horizontal register 14 readout in reverse. Two matching images could then be displayed with one TV monitor bodily inverted. In a particular application, the two image integration periods are adjusted to give one long exposure and one very short exposure. This enables images to be reconstructed with a very wide dynamic range. It is desirable for the device to have good antiblooming capabilities, which means that, if excess image charge is generated by over exposure, this charge does not spill over and corrupt image charge in adjacent picture elements. To illustrate a technique according to a preferred embodiment of the invention a particular case will be cited in which two sequential exposures are made on the frame transfer charge-coupled device in the ratio 64:1. This ratio is obtained by choosing the points in each 20 ms TV field that transfers occur into the storage regions 11 and 12. If the time taken to transfer a complete image into one of the storage regions is 0.1 ms then the remaining 19.8 ms should be divided in the ratio 64:1. Thus an integration period of 19.5 ms will be transferred into storage region 11 followed by a further exposure of 0.3 ms which is transferred into storage region 12. The readout from both stores will occur simultaneously in the conventional 19.5 ms active field time of the following TV field. The two output signals may be recorded simultaneously on two video tape recorders 16, 17 which may be replayed in synchronism, either continuously or a single field at a time. It is not feasible to display an image with a very wide dynamic range on a single CRT monitor as both the normal cathode ray tube and the eye are non linear and of limited dynamic range. For the purpose of scientific analysis the two images may be combined in a 12 bit TV frame store 20 as depicted in Figure 1. The video signals are digitised via two 6 bit analogue to digital converters 18, 19 and the signals amalgamated in the 12 bit frame store 20, the 6 bits of data from each element of storage region 12 (the short exposure) forming the most significant bits. In the cases cited, the CCD imager 10 and/or VTR 16, 17 are used in which the dynamic range need only be 36dB to provide an image with a dynamic range of 72dB. If the device is constructed with an antiblooming structure 21 such as depicted in Figure 2, the two exposure periods in each TV field may be controlled independently. An antiblooming structure serves to drain away excess charge which "overflows" from any of the charge integration regions so as to prevent "blooming" of the image in the region of a bright point of the image. The structure may typically comprise charge drains lying between or beneath the charge integration sites. A method of varying the integration period of a conventional frame transfer charge coupled device by purging the integration region of charge is described in U.K. Patent Specification GB-B-2,083,968. The essence of the method is to shorten the exposure period by a precisely controlled amount by reverse clocking the vertical charge transfer registers in the integration region 13 and dumping this charge into a drain diffusion along the periphery of the device. For devices with an integral antiblooming structure 21 within the image charge integration region 13, such as drains lying between or below the charge integration sites, this same technique can be used to shed charge at the interface between the storage and integration regions. Thus in the typical structure depicted in Figure 2, if the charge coupled clocking mechanism is used to move image charge either upwards or downwards to either storage region, and the charge is not moved in the storage regions, then moving charge will meet a barrier at the interface between the storage region and the integration regions. Charge will then flow into the antiblooming drains along this interface; this mechanism can be used for purging charge from the integration region as described in U.K. Specification GB-B-2,083,968. In a further application of this device, freeze frame image analysis can be achieved at twice the rate normally afforded by a TV system without sacrificing image area. By purging image charge from the integration region prior to an exposure period, allowing charge to accumulate for a short integration period (generally less than 0.5 ms), and then rapidly transferring the image charge into one of the storage regions - a movement freezing snapshot may be taken. The second "snapshot" would normally be taken half a TV field later (10 ms) and stored in the second storage region. Two conventional video tape recorders would record the two output signals for subsequent freeze frame analysis. The two events recorded in each TV field may also be independently triggered from an external source related to the event. This would allow a critical point in a process to be precisely monitored such as a high speed automated manufacturing plant. A third application for the CCD device would be to observe phenomena in two spectral bands without the collimation errors that will occur if two imaging means are utilised. In this case a filter wheel containing two spectral filters is rotated in synchronism with the camera at the TV field rate (i.e. 3,000 RPM for the CCIR 50Hz field rate). The exposure periods will be constrained to occur only when each spectral filter is correctly positioned in front of the focal plane. This method may also be utilised to observe phenomena in two planes of polarisation by replacing the spectral filters with polarising filters.

Claims: Solid state imaging apparatus comprising a charge-coupled frame transfer image sensor including an image-sensitive integration region (13) for forming a charge pattern representative of an incident image, first and second storage regions (11,12) associated with respective opposed ends of said integration region, and charge transfer means for selectively shifting the charge pattern formed in said integration region into said storage regions, characterised in that said apparatus further comprises control means for defining a continuing series of field time periods each including a long portion and a short portion, said control means being operable for causing said charge transfer means to shift a charge pattern formed during the long portions of said field time periods into one of said storage regions, and to shift the charge pattern formed during the short portions of said field time periods into the other storage region. Solid state imaging apparatus according to Claim 1, wherein said integration region (13) and said storage regions (11,12) are formed on a common substrate. Solid state imaging apparatus as claimed in Claim 1 or 2 wherein said control means is operable to cause the charge-pattern formed during a further variable portion of said field time period to become lost. Solid state imaging apparatus as claimed in any preceding claim, comprising output processor means (16 to 20) for receiving data representative of the charge-patterns in each of said storage regions and for combining said data to provide for each field period image-representative data of extended dynamic range. Solid state imaging apparatus as claimed in Claim 4, wherein said output processor means includes means (16,17) for digitising data representative of the charge-patterns and frame store means (20) for storing said image-representative data of extended dynamic range, and wherein the data representative of the charge pattern formed during said short portion defines the most significant digits of the combined data and the data representative of the charge pattern formed during said long portion defines the least significant digits. Solid state imaging apparatus as claimed in any preceding claim, wherein filter means are provided to filter the image incident on the integration region during at least one of said long and said short portions.

Inventors: BUSBY, PETER WILLIAM C/O BRITISH AEROSPACE
Applicants: BRITISH AEROSPACE PUBLIC LIMITED COMPANY
Application year: 1987
Main classification: H04N3
IPC codes: H01L27, H04N3, H04N5