Abstract:
A novel method and apparatus is disclosed that is able to extend the intra-scene dynamic range of Time Delay and Integrate Charge-Coupled Device imagers. In accordance with the principles of the invention, the charge collected and accumulated over a plurality of photosensitive devices is limited by adjusting the barrier levels at which collected accumulated charge is removed from the photosensitive devices. By limiting the amount of accumulated charge that is collected, images of high intensity are prevented from overflowing or saturating the photosensitive devices. Thus, information that is included in the brighter levels of the image is not lost because of clipping the photosensitive device to prevent saturation. In one embodiment of the invention, the blooming barrier levels are adjusted in a step-wise linear manner to allow a known amount of charge to be retained during the initial collection phase while allowing progressively greater amounts of charge to be retained as the collection phase proceeds. This stepped increase in barrier level causes the imager to compress highly intense images to prevent saturation during the initial collection phase, while not influencing the response to images of lower intensity.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application relates to Provisional Application Serial No. 60/125,670, entitled, “TIME DELAY AND INTEGRATE CCD WITH EXTENDED DYNAMIC RANGE” filed on Mar. 22, 1999, which is assigned to the same assignee and is incorporated by reference herein. Applicant claims the benefit of the priority filing date of Mar. 22, 1999 pursuant 35 U.S.C. §119(e)(1). 
    
    
     FIELD OF THE INVENTION 
     The present invention is related to the field of imaging devices. More specifically, to extending the dynamic range of Time Delay and Integrate (TDI) imagers, 
     BACKGROUND OF THE INVENTION 
     Typical single line imagers, for example, Charge Coupled Device (CCD) imagers, incorporate a single row of photosensitive electrical devices, such as photo-detectors, to collect photonic charge. A photo-detector converts the collected photonic charge into an electrical charge that is related to the amount of photonic charge collected. In moving target imaging the detector integration time is limited by the allowable time the target image moves past the imaging device or scanner. Consequently, as the target image velocity across the imager increases, the allowable time to collect photonic charge decreases. A Time Delay &amp; Integration (TDI) method of operating CCD imagers incorporates multiple single row CCD scanners in an imaging array. Each row in the array collects image data for a finite period of time as the target image and the imager move relative to each other. Typically, the collected charge moves from one row to the next at the same speed as the target image moves across the imager to prevent blurring. TDI-CCD scanners offer an advantage over single line scanners in that TDI-CCD devices provide for longer times to collect and integrate photonic charge from the target image. The longer collection time also increases the sensitivity of the imager. 
     However, when TDI-CCD devices are used to collect highly intense images, the longer integration time may cause the accumulated charge to exceed the capability of the photosensitive devices. To prevent the excess charge from influencing adjacent photosensitive devices, the excess charge is generally drained away. This draining causes details of an image to be lost. One method of avoiding this loss of detail is to increase the level that is deemed excessive charge—i.e., increase the threshold beyond which charge drainage begins. This method, however, has the disadvantage that the maximum threshold drainage level is set by the saturation level of the photosensitive device. In this case, information is lost not because of draining of the excessive charge, but rather from the inability of the cells to collect any additional charge. Another method to avoid excess charge-accumulation problems is to reduce the amount of time that is used in collecting an image. This method, however, has the disadvantage of reducing sensitivity of the imaging device. Thus, dark images remain relatively dark. Hence, there is a need to extend the dynamic range of TDI-CCD devices that allows for the collection of sufficient image data to view dark images while not losing data from relatively bright images. 
     SUMMARY OF THE INVENTION 
     In accordance with the principles of the invention, a method is disclosed that increases the intra-scene dynamic range of a TDI-CCD device by selectively limiting the amount of charge collected in each stage by selectively setting blooming barrier levels. By selectively setting barrier levels, the maximum accumulated charge collected in each cell is incrementally increased as the charge is passed from one cell to a subsequent cell. In one embodiment, the level of the barrier is set in a step-wise increasing manner that limits the amount of charge collected in each stage while allowing for the accumulation of progressively greater amounts of collected charge in subsequent cells. In this embodiment, a step-wise increase in the barrier levels changes the reference level at which charge is accumulated in a cell. These changes in the reference level shift the imaging device response characteristic to cause a compression of the accumulated photonic charge for highly intense images while the response to lower intensity images is not substantially affected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments to be described in detail in connection with the accompanying drawings. In the drawings: 
     FIG. 1 illustrates a schematic diagram of an exemplary TDI-CCD device; 
     FIG. 2 depicts a cross-sectional view of a pixel cell and barrier of a CCD structure; 
     FIG. 3 illustrates typical response characteristic of a TDI-CCD device; 
     FIG. 4 illustrates a schematic diagram of a TDI-CCD device in accordance with the principles of the invention; 
     FIG. 5 illustrates a response characteristic of a TDI-CCD device illustrated in FIG. 4 in accordance with the principles of the invention; 
     FIG. 6 a  illustrates one exemplary embodiment of limiting charge capacity in accordance with the principles of the invention; 
     FIG. 6 b  illustrates a second exemplary embodiment of limiting charge capacity in accordance with the principles of the invention; 
     FIG. 7 a  illustrates a third exemplary embodiment of limiting charge capacity in accordance with the principles of the invention; 
     FIG. 7 b  illustrates a fourth exemplary embodiment of limiting charge capacity in accordance with the principles of the invention; 
     FIG. 8 illustrates an exemplary response characteristic of a TDI-CCD device in accordance with a second embodiment of the invention; 
     FIG. 9 illustrates an exemplary response characteristic of a TDI-CCD device in accordance with another embodiment of the invention; 
     FIG. 10 a  illustrates a cross-sectional view of an exemplary embodiment of a back-illuminated TDI-CCD device having a response characteristic as illustrated in FIG. 8; 
     FIG. 10 b  illustrates a cross-sectional view of a second exemplary embodiment of a back-illuminated TDI-CCD device having a response characteristic as illustrated in FIG. 8; and 
     FIG. 10 c  illustrates a cross-sectional view of an exemplary embodiment of a top-illuminated TDI-CCD device having a response characteristic as illustrated in FIG.  8 . 
     It is to be understood that these drawings are for purposes of illustrating the inventive concepts of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As would be understood the principles of the invention disclosed are related to TDI-CCD array imagers. However, the principles of the invention may also be applied to other solid-state TDI imaging devices, for example, CMOS TDI-APS devices. 
     FIG. 1 illustrates an exemplary schematic diagram of an 8-stage TDI-CCD device  100 . Only four photosensitive devices (pixel cells)  115 , among a plurality of cells in each stage are shown to describe the operation of the TDI-CCD device. In operation, the photonic charge from a target image is collected beginning in pixel cells  115  associated with the first stage—i.e., stage  101 . As the target image or imager moves, the image moves past the pixel cells in a next stage ( 102 ). The cells in this stage then collect photonic charge from the target image. In addition, the charge collected in previous cells is added to the charge collected in the currently-imaging cell. Thus, the charge collected in pixel cell  115  associated with stage  101 , for example, is added or transferred to pixel cell  115  associated with stage  102 , as the image moves past stage  102 . Similarly, the accumulated charge in pixel cell  115  associated with stage  102  is added to the charge collected in pixel cell  115  associated with stage  103 , as the image moves past stage  103 . This transferring of accumulated photonic charge from a cell in one stage to a cell in a subsequent stage continues, one stage at a time, until an end stage is reached—in this case, stage  108 . The photonic charge collected for the image is accumulated and integrated over the depth of the TDI-CCD device,—i.e., stages  101 - 108 . At stage  110  the accumulated charge is read out from the TDI-CCD device. 
     In transferring the collected charge for each pixel cell  115  to a subsequent pixel cell  115 , the charge is transferred along a channel of pixel cells  115 , known as a transfer channel, illustrated for one channel of pixel cells as  120 . When the energy collected in one cell  115  exceeds the maximum capability of the cell, the excess energy would normally overflow the cell and “spread” into an adjacent cell. To prevent the accumulated charge in the cells of one transfer channel from influencing the accumulated charge in cells in an adjacent channel, such as transfer channel  121 , channels are isolated from one another by blooming drains  125  and barriers  130 . The level at which accumulated charge is deemed excessive is determined by barrier  130  that exists between, for example, transfer channel  120  and drain  125 . The greater the level, or potential, of barrier  130 , the greater the amount of accumulated charge that may be collected in each cell  115 . Generally, the levels of barrier  130  are set to achieve the maximum accumulation of charge—i.e., saturation level—in a pixel cell. As illustrated barrier  130  is positioned horizontally adjacent to channel  120  and  121 . As would be known in the art, barrier  130  may also be positioned vertically adjacent to channels  120  and  121  to limit the charge capacity of pixel cell  115 . 
     FIG. 2 illustrates the operation of barrier  130  in controlling excessive charge. As shown, pixel cell  115  fills to a value that is set by barrier  130 . When the accumulated charge exceeds the level of barrier  130 , the excess, as represented by charge  140 , is directed into drain  125 . In this illustrative example, when the maximum charge is achieved, any further charge that may be collected is simply discarded. The maximum level of barrier  130  thus limits the accumulated charge. 
     FIG. 3 illustrates an exemplary performance characteristic of TDI-CCD device of FIG. 1 for three images of different brightness levels. In the first example, the charge collected by the imager is represented by response characteristic  400 . In this example, each pixel cell  115  in a stage adds a substantially similar amount of charge to pixel cell  115  in the next subsequent stage as the imager and target image move past each other. The accumulated charge collected increases proportionally to the number of stages in the imager. For this first exemplary image, the total amount of charge collected after the eight stages remains below the barrier level  465  set by barrier  130 . In a second example, wherein the image is brighter than the first image, the charge collected is represented by response characteristic  420 . In this example, more charge is collected in each pixel cell  115  (relative to that collected for response characteristic  400 ) as the target image and imager move past one another. The accumulated energy collected would normally continue along dashed line portion  425  of response characteristic  420  until the accumulated charge is read out at stage  110 . However, when the collected charge exceeds barrier level  465  (as happens in this example near the end of the sixth CCD stage), the excess charge is drained into drain  125 . The accumulated charge thus remains limited, or clipped, at the barrier level  465 , as represented by response characteristic  430 . Any image detail that may be captured in the dashed line portion of response characteristic  420  is not processed and is therefore lost. 
     As a third example, the charge of an even brighter image is collected. The response to this brighter image is represented by response characteristic  440 . In this example the brightness of the image causes the accumulated charge to exceed the maximum level set by barrier  130  (at level  465 ) after scanning of the image by only about two of the CCD stages. Were the accumulated charge permitted to accumulate through continuing CCD stages, that charge would be represented by the dashed line portion  445  of response characteristic  440 , but the charge is limited by barrier level  465 . Barrier level  465  thus limits the response characteristic of the imager. Charge above barrier level  465  is simply discarded into drain  125  and the accumulated charge for this third exemplary image thus remains limited at level  465  as represented by response characteristic  460 . 
     FIG. 4 illustrates an 8-stage TDI-CCD in accordance with the principles of the invention. As is illustrated, barrier  130  is selectively set in accordance with the capacity charge curve  510 . Accordingly, the amount of charge accumulated and retained in each stage is incremental increased. In the exemplary embodiment, barrier  130  is set to limit the accumulated charge to twenty-five per cent (0.25) of maximum barrier level  465  in stages  101 - 105  of the imager, and fifty percent of that level for stages  106  and  107 . At stage  108 , the level of barrier  130  is set to the maximum barrier level  465 . The maximum barrier level may be set independently of the maximum capacity of pixel cell  115 , but is typically set substantially equal to the maximum capacity of the pixel cell. 
     In this illustrated example, barrier  130  is selectively set in an step-wise pattern that limits the amount of data collected in the early stages of collection while allowing for incrementally greater amounts of charge in latter stages. Thus, stages  101 - 105  accumulate 25 percent of the charge that could be collected. Stages  106  and  107  accumulate 50 percent of the charge that could be collected, and stage  108  allows for the accumulation of a maximum amount of charge. With the illustrated configuration, the maximum amount of charge may be collected without causing a saturation of pixel cell  115 . 
     FIG. 5 illustrates the response characteristics of a TDI-CCD device having barrier characteristics selectively set in accordance with the exemplary characteristics illustrated in FIG. 4 for the three exemplary images discussed in regard to FIG.  3 . With regard to the first exemplary image (least bright), the charge collected is represented by response characteristic  600 . Response characteristic  600  is similar to the response characteristic  400  of FIG. 3, as the accumulated charge does not exceed the selectively set barrier levels of stages  101 - 105 ,  106 - 107  or  108 . Accordingly, the response characteristic is not altered and the accumulated charge is not limited. 
     With regard to the second exemplary image, the accumulated charge, as represented by response characteristic  610 , is shown to exceed the first barrier level while charge is collected in stage  102 . That is, the accumulation of the charge collected in stage  101  and the charge being collected in stage  102  exceeds the barrier level selectively set for stages  101  through  105 . Any additional charge collected in stages  103  through  105  would, accordingly, cause the accumulated charge to exceed the level of barrier  130  and is thus drained off. The accumulated charge in stages  103  through  105  remains limited to 25 percent of the charge that may be collected, as represented by response characteristic  620 . At stage  106 , the level of barrier  130  is increased to 50 percent of the maximum barrier level to allow an incremental increase in the charge collected. As stage  106  moves past the target image, charge is accumulated in pixel cells  115  associated with stage  106 . The increase in accumulated charge is represented by response characteristic  630 . As shown, the accumulated charge exceeds the level of barrier  130  during the charge collection of stage  107  and the excess charge is drained off. The accumulated charge remains limited to the 50 percent level, as represented by response characteristic  640 . At stage  108 , the level of barrier  130  is increased to allow an incremental increase in the charge collected. As stage  108  moves past the target image, charge is accumulated, as represented by response characteristic  650 . The charge level continues to accumulate until the collected charge is read out in stage  110 . 
     In this exemplary case, the accumulated charge level is substantially less than the maximum level; hence, the brightest image data is not lost because the image data is compressed. As is illustrated, an imager incorporating a step-wise increase in the level of barrier  130  causes alterations or shifts in reference points that are used by subsequent cells to accumulate charge. These alterations in the reference points allow for the collection of more data from brighter images. 
     In a third exemplary image, similar to the brightest image discussed in regard to FIG. 4, the brightness of the image causes the charge collected to exceed the first level of barrier  130  during the collection phase of stage  101 . Any charge collected as the target image moves past stages  101  through  105  is thus drained off. The level of the accumulated charge remains fixed at the barrier level as represented by response characteristic  620 . Beginning at stage  106 , the level of barrier  130  is increased. As stage  106  moves past the target image, charge is again accumulated in pixel cell  115  associated with stage  106 , as represented by response characteristic  670 . As shown, the brightness of the image causes the accumulated charge to quickly exceed the increased barrier level. Thus, any charge collected after this increased barrier level is exceeded is drained off. Accordingly, the accumulated charge level remains limited during stage  106  and continuing through stage  107 , as represented by response characteristic  640 . At stage  108 , the level of barrier  130  is incrementally increased and charge is again accumulated, as represented by the response characteristic  680 . In this embodiment, the brightest portions of the image are collected and the brightest portions of the image are not lost. The altered reference points compress the image data to allow pixel cells to accumulate charge that would otherwise be greater than their maximum capability. These alterations in the reference points allow for the collection of image data from brighter images that would otherwise be lost without affecting low light level performance. 
     In accordance with the principles of the invention, the dynamic range of a TDI-CCD device is extended by selectively setting levels of barriers that limit the charge collection capability. The selectively set barrier levels shift the reference point of accumulated charge. The altered, or shifted, reference point causes the imager to compress brighter portions of an image while not influencing the lower intensity portions of the same image. 
     As should be understood, the scope of the invention is not limited to the step-wise increase in charge capacity in the exemplary  8- stage TDI-CCD device illustrated. Rather the method of setting progressive barrier levels, and thereby progressively accumulating charge in a TDI-CCD device, may also be determined using, for example, exponential, logarithmic, inverse logarithmic, geometric or polynomial functions. Furtherstill, the number of barrier levels also may be increased commensurate with the number of stages in a TDI-CCD device. Thus, the number of different barrier levels may be set substantially equal to the number of stages. 
     FIG. 6 a  illustrates one embodiment for fabricating a TDI-CCD imager having a charge capacity illustrated in FIG.  4 . In this illustrative embodiment, the barrier level  130  is set to limit the amount of charge collected. For example, barrier levels are set to limit pixel cell  115  associated with stages  101  through  105  to collect only twenty-five (25) percent of full capacity of pixel cell  115 —i.e., maximum level  465 . For stages  106  and  107 , barrier levels are set to limit pixel cell  115  associated with stages  106  and  107  to collect and retain an accumulated charge that corresponds to fifty (50) percent of full capacity. At stage  108 , the barrier levels are set to the full width of pixel cell  115  to collect and retain an accumulated charge that is substantially equal to full capacity. FIG. 6 b  illustrates a second embodiment for fabricating a TDI-CCD imager having a charge capacity illustrated in FIG.  4 . In this embodiment, the barrier levels, or potentials, are constant but are positioned to adjust the width of pixel cells  115  to limit the charge collected. In this case, the amount of charge collected is limited by the size of the pixel cells. Further, the width of barrier level  130  remains substantially constant. 
     FIG. 7 a  illustrates a second embodiment of fabricating a TDI-CCD device to achieve the exemplary step-wise accumulated charge characteristics illustrated in FIG.  4 . In this embodiment, the pixel cell charge capacity is set in designated stages to limit the accumulated charge. As illustrated, the width of pixel  115  in stages  101  through  104  is set at a maximum level  465 —i.e., full collection capacity. At stage  105 , the width of pixel  115  is reduced to twenty-five (25) percent of maximum level  465 . At stage  106 , the width of pixel  115  is again increased to maximum level  465 , and at stage  107 , the width of pixel  115  is reduced to fifty (50) percent of maximum level  465 . Finally, at stage  108 , the width of pixel  115  is again increased to maximum level  465 . The narrow channel regions at stages  105  and  107  act as charge clippers. FIG. 7 b  illustrates another embodiment, similar to that described in regard to FIG. 7 a,  wherein the width of barrier  130  is remains substantially constant. This embodiment is advantageous as the width of drain  125  is correspondingly increased when the size of pixel  115  is decreased. This increase in drain  125  allows for the draining of excessive accumulated charge. 
     In another embodiment, which is not illustrated, the depth of the pixel cell  115 , as illustrated in FIG. 2, may also be used to control pixel charge capacity. For 8-stage TDI-CCD device discussed herein, the depth of pixel cells  115  associated with stages  101 - 105  may be set to collect twenty-five percent of full charge capacity. Similarly, the depth of pixel cells  115  associated with stages  106 - 107  may be set to collect fifty percent of full charge capacity and the depth of pixel cell  115  associated with stage  108  may be set to accumulate a full charge. 
     FIG. 8 illustrates the response characteristics of this embodiment of the invention to the three exemplary images discussed in regard to FIG.  3  and FIG.  5 . The response characteristic to the first exemplary image is represented by the response characteristic  800 . As discussed previously, the accumulated charge in stages  101  through  108  remains lower than the associated barrier levels. Thus, the selectively set levels do not influence the response characteristic of the TDI-CCD device to this first exemplary image. With regard to the second exemplary image as discussed in regard to FIG. 3, the charge accumulated in stages  101  through  104 , as represented by response  810 , rises to maximum level  465 . At stage  105 , however, the charge capacity of pixel cell  115  is set to twenty-five percent of maximum level  465 . Accordingly, as the target image passes stage  105 , the accumulated charge above this twenty-five percent level is drained off, as represented by response  820 . Furthermore, any additional charge collected during stage  105  is also drained off. Thus, the accumulated charge remains limited, as represented by response characteristic  830 . At stage  106 , the charge capacity of pixel cell  115  is again increased to maximum level  465  and charge is again accumulated, as represented by response characteristic  840 . At stage  107 , the charge capacity of the associated pixel cell  115  is reduced to fifty percent. Any charge collected above this fifty-percent level is, accordingly, drained off and the accumulated charge remains at a fixed level, as is represented by response characteristic  850 . At the beginning of stage  108 , the charge capacity of the associated pixel cell  115  increased to maximum level  465  and charge again is accumulated, as represented by response characteristic  860 . Accordingly, the brighter portions of the imager are again captured in the imaging device similar to that illustrated in FIG.  5 . 
     With regard to the third exemplary image as discussed in regard to FIG. 3, the accumulated charge quickly rises to maximum level  465 , as illustrated by response characteristic  865 . Any additional charge that may be collected in stages  102 - 104  is drained off, as represented by response characteristic  870 . At stage  105 , the charge capacity of the associated pixel cell  115  is set at twenty five percent and any accumulated charge above this level is drained off, as represented by response characteristic  875 . Further, any additional charge collected is similarly drained off and the accumulated charge level remains constant, as represented by response characteristic  830 . At stage  106  the charge capacity of the associated pixel cell  115  is again increased to maximum level  465 . The charge is again accumulated, as represented by response characteristic  880 . At the beginning of stage  107 , the charge capacity of the associated pixel cell  115  is again reduced to fifty percent, and the accumulated charge above this reduced level is again drained off, as represented by response characteristic  885 . The accumulated charge remains fixed at this level as any additional charge collected is drained off, as represented by response characteristic  850 . At stage  108 , the charge capacity of the associated pixel cell  115  is again increased and the accumulated charge again increases, as represented by response  890 . The brightest portions of this third exemplary image are thus captured in a manner similar to that illustrated in FIG.  5 . 
     It should be understood, the representation of the transfer channel having varying size is intended merely to illustrate different methods of achieving the exemplary model of varying barrier levels. Other methods may also be used to achieve similar increases in accumulated charge. 
     A further means of increasing dynamic range is achieved by placing a filtering element over at least one stage of a TDI-CCD device. Filter elements are used to further extend dynamic range by reducing the intensity of an image as the image moves with respect to the collection stages. FIG. 4 illustrates the placement of filter  500  over stage  108 . In this embodiment, in addition to selectively setting of barrier levels, filter  500  is used to reduce the image intensity as the image passes stage  108 . Filter  500  may be, for example, a neutral-density filter, or a slotted metal filter. Filtering is well known in the art and need not be discussed in detail herein. 
     FIG. 9 illustrates the response characteristic of the TDI-CCD imager illustrated in FIG. 4 with filter  500  positioned over stage  108 . In this case, the response to the three exemplary images is similar to the responses illustrated in FIG. 5, except for the response in the filtered stage. That is, the response of this embodiment to the first exemplary image is similar to response characteristic  600  of FIG.  5 . However, the imposed filter  500  causes the image intensity to decrease during charge collection at stage  108 . The amount of charge collected is thus reduced and this reduction in the accumulated charge is represented by response characteristic  910 . Similarly, the responses of the second and third exemplary images, as illustrated in FIG. 9 are equivalent to corresponding responses illustrated in FIG. 5, except for the response during stage  108 . With the imposed filter  500  placed over stage  108  the charge collection is reduced as represented by response characteristics  960  and  990 , respectively. Thus, in accordance with the principles of the present invention, dynamic range of TDI-CCD is further extended by selectively using filters over at least one collection stage to collect highly intense images. 
     FIG. 10 a  illustrates a cross-sectional view of an exemplary embodiment of an  8 stage back-illuminated TDI-CCD imager incorporating filtering over one stage (stage  108 ). FIG. 10 b  illustrates a cross-sectional view of a second exemplary embodiment of an 8-stage back-illuminated TDI-CCD imager incorporating filtering over one stage (stage  108 ). As would be understood the principles of the invention are equally applicable to top-illuminated TDI-CCD imagers. FIG. 10 c  illustrates a cross-sectional view of an exemplary embodiment of an 8-stage top-illuminated TDI-CCD imager incorporating filtering over one stage (stage  108 ). 
     Using FIG. 10 a  as an illustrative example of TDI-CCD device constructed in accordance with the principles of the invention, light energy  1040  from a target image is sequentially collected in each of the collection stage as imager  1010  moves past the target image in the direction  1050 . As illustrated, TDI-CCD imager  1010  is composed of eight (8) stages,—i.e., stages  101 - 108 . Stages  101 - 108  are formed in wafer  1000 , which is typically made of a semi-conductor material, such as silicon or GaAs. 
     Attached to wafer  1000  is block  1020 . Block  1020  provides support and protection for wafer  1010 . Typically block  1020  is an optically transparent material. For example, for imagers used in the visible light range, block  1020  may be constructed of materials such as glass, plastic, acrylic or polymer, that allows light  1040  to pass without significant degradation or loss. For imagers operating in the IR region, block  1020  may be constructed of materials such as silicon. For imagers operating in the UV region, block  1020  may be materials such as quartz and calcium chloride. 
     Filter  1030  is positioned on surface  1070  between block  1020  and wafer  1010 . Filter  1030 , similar to filter  500  discussed previously, is positioned over stage  108  to reduce the charge collected in this stage. As should be understood, the placement of filter  1030  is not limited to surface  1070 , as illustrated. Rather, filter  1030  may be placed, for example, on surface  1060 , or may even be incorporated within block  1020  to achieve a reduction in the intensity of the image. For example, a filter may be included within block  1020 , by altering the index of reflective of the material of block  1020 . The altered index of reflective reduces the intensity of light that traverses block  1020  and is collected in stages  101 - 108  in wafer  1000 . Further still, filter  1030  may be incorporated onto wafer  1010 . However, placing filter  1030  on wafer  1010  has the disadvantage that when a change of filtering is necessary wafer  1010  is affected. Accordingly, incorporating filter  1030  onto block  1020  allows for filter changes without affecting wafer  1010 . 
     Filters may be used over a plurality of stages and each of these filters need not cause the same reduction in image intensity. For example, filters may be graduated along the depth of a TDI-CCD such that no filtering occurs in the earlier collection stages while significant reductions of intensity occurs in the latter collection stages. Graduated filtering, thus, does not affect the low intensity portions of any image but will affect the brighter portions, as illustrated in FIG.  9 . 
     The examples given herein are presented to enable those skilled in the art to more clearly understand and practice the instant invention. The examples should not be considered as limitations upon the scope of the invention, but as merely being illustrative and representative of the invention. Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention and is not intended to illustrate all possible forms thereof. It is also understood that the words used are words of description, rather than limitation, and that details of the structure may be varied substantially without departing from the spirit of the invention and the exclusive use of all modifications which come within the scope of the appended claims is reserved.