INTERLINE CCD SENSOR ARRANGEMENT

Abstract: In an interline image sensor arrangement, comprising an interline image CCD sensor (S) having a charge integration period, a shutter (LCS) is provided in front of the interline image CCD sensor (S). The shutter (LCS) is controlled to be transparent substantially during the charge integration period, and to be non-transparent otherwise. Preferably, a liquid crystal shutter (LCS) is used which is scattering during the non-transparent periods.

Description: Interline CCD sensor arrangement. The invention relates to an interline CCD sensor arrangement. US-A-4, 805,026 discloses an interline transfer type CCD area image sensor which includes a plurality of photo sensors arranged in a plurality of vertical lines. A plurality of vertical shift registers are disposed along respective ones of the lines of photo sensors. A horizontal shift register is formed to receive charges from the vertical shift registers. An output circuit converts the charges transferred through the horizontal shift register into electrical signals. With interline image sensors, smear is caused by a leakage from the image cells to the neighboring storage cells during the full field duration of 20 ms. If the charge integration duration equals 20 ms, the smear suppression is, for example, about 86 dB for a given sensor. However, if the charge integration duration is, for example, 200 times shorter as a consequence of the operation of an electronic iris, the smear suppression is reduced by the same factor resulting in a just acceptably low smear suppression of only 40 dB. Consequently, an electronic iris control with a range exceeding 200, such as required in outdoor situations, becomes impossible. It is, inter alia, an object of the invention to provide an interline CCD sensor arrangement in which a good smear reduction is obtained also when short exposure times are used. To this end, a first aspect of the invention provides an interline image sensor arrangement as defined in claim 1. A second aspect of the invention provides a camera as defined in claim 3. Advantageous embodiments are defined in the dependent claims. In accordance with the invention, a shutter is used which only becomes transparent substantially during the integration period. The advantages of the invention can be illustrated by means of the following example. The dynamical range of a camera in an outdoor situation runs from 100,000 lux down to 0.1 lux, i.e. the brightest scene may be 1,000,000 times brighter than the darkest scene. A factor 50 of this 1,000,000 can be dealt with by automatic gain control and compression, so that the electronic iris has to have a dynamical range of 20,000. A reduction of the original integration duration by a factor 20,000 implies that from the original smear suppression of 86 dB, nothing is left as 1/20,000 equals 86 dB and 86 dB - 86 dB = 0 dB. If a shutter yields 40 dB (which is the contrast ratio between light transmission in transparent state and light transmission in non-transparent state), at 100,000 lux a just acceptable smear suppression of 40 dB can be obtained. It is to be noted that Research Disclosure, May 1993, 327, discloses a scattering liquid crystal shutter for use in frame transfer CCD cameras. To avoid smear as far as possible, d e scene is shut-off from the CCD image sensor during vertical transport of all the pixel information to the storage section of the frame transfer CCD image sensor. When scattering liquid crystal materials are used for the shutter, these shutters offer high speed switching combined with good on/off optical contrast without needing polarizers. These scattering liquid crystal shutters can be used in series because these shutters do not give a loss in light sensitivity. This gives, for example, a 1000: 1 (open.close) contrast ratio by using three shutters in series. A smear suppression of more than 120 dB can be achieved by such an arrangement. However, with frame transfer CCD sensors, the smear phenomenon is obtained in a completely different manner dian with interline CCD sensors. With frame transfer CCD sensors, the smear phenomenon is the direct result of charge transport via the uncovered image portion of the sensor, and the smear phenomenon is thus only obtained during the frame transport from the image portion to the storage portion. Consequently, the frame transfer CCD sensor needs to be covered during that frame transport only. On the contrary, wid interline CCD sensors, the smear is caused by a secondary effect, viz. by leakage from die image cells to the neighboring covered storage cells during the full frame duration of 20 ms (in a 50 Hz system). At die maximum integration duration of 20 ms, no furdier smear reduction can be effected by a shutter. Such a further smear reduction is not needed anyway, because die smear reduction in that situation is already quite considerable (about 86 dB). So, widi interline CCD sensors, the shutter needs to be scattering during the full field period minus the integration period, while widi frame transfer sensors, the shutter needs to be scattering during me frame transport period. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows an embodiment of a camera comprising an interline image sensor arrangement in accordance with the present invention; and Fig. 2 shows a timing diagram for use with the embodiment of Fig. 1. In the embodiment of Fig. 1, light L is passed tiiru an objective O and a liquid crystal shutter LCS to a CCD sensor S. The liquid crystal shutter LCS is controlled by a control circuit CC. The output signals of me sensor S are applied to an image signal processing circuit IPC to form an image signal IS. The image signal processing circuit IPC and me control circuit CC receive timing signals from a timing generator TG which also provides horizontal and vertical synchronizing signals for use with any further processing of the image signal IS. The liquid crystal shutter LCS comprises two glass plates between which liquid crystal material is arranged. The boundaries of the glass plates are provided wim glue such diat the liquid crystal material is captured in between. Each glass plate has a transparent electrically conducting layer with a contact such that a strong electrical field can be put across the very thin liquid crystal layer (about 15 μm). The active area of the liquid crystal shutter LCS exceeds that of me CCD sensor S, so mat the sensor S is fully covered during the periods in which die liquid crystal shutter LCS is non-transparent. In principle, two types of liquid crystal shutters can be used: a conventional liquid crystal shutter which is fully impermeable during the non-transparent periods, and a scattering liquid crystal shutter which is scattering during me non-transparent periods. The advantage of a scattering liquid crystal shutter over a conventional liquid crystal shutter which becomes fully impermeable, is mat me transmission during the transparent state exceeds 80% as a result of die fact mat a scattering liquid crystal shutter does not need a polarizing filter. With a conventional liquid crystal shutter, the transmission is theoretically at most 50% in view of the need for a polarizing filter, but in practice the transmission may be 40% or 30%. Thus, with a scattering liquid crystal shutter, the advantage is obtained that during me transparent state, a very high transmission of over 80% can be obtained, while during the scattering state, die shutter is diffuse which results in a uniform additional charge. This uniform additional charge results in a DC component which can easily be compensated for in the image signal processing circuit IPC. The following liquid crystal materials can be used for a scattering liquid crystal shutter: polymeric dispersed liquid crystal (PDLC) materials, and an anisotropic gel comprising a polymerized liquid crystalline material (LC gel). An LC gel shutter is transparent when no voltage is applied, and it becomes scattering when an alternating voltage is applied. In general, a PDLC shutter is scattering when no voltage is applied, and it becomes transparent when an alternating voltage is applied. In one embodiment, the alternating voltage is 200 Vpp at 5 kHz. PDLC materials are known as such from EP-B- 0,238,626, and from B. Bahadur (ed.), Liquid Crystals, appl. and uses, World Scientific '90, Chap. 14, by J.W. Doane. LC gels are known as such from US-A-5, 188,760, and from R.A.M. Hikmet, Adv. Mater. 4, 679 ('92). In the timing diagram of Fig. 2, at a a field period of the image signal IS is shown. The field duration from instant t2 to instant t2' lasts 20 ms for PAL signals and 16.6 ms for NTSC signals. The field blanking period is indicated between the instants t2 and t4. At b the instant t3 of the charge transport is indicated. As shown, it falls within the field blanking period t2-t4. At c a plurality of sub-pulses is shown. At a sub-pulse, undesired charge collected at the pixels is discharged to d e substrate. This is because during the full active field period between me instants t3 and t3\ charge is collected whereas only during the integration period (shown at d) a charge integration is required: more charge collection is not necessary. Preferably, the sub-pulses are within the line blanking periods to reduce disturbances. At d the integration period is shown. As can be seen from Fig. 2, me integration period lasts from the "last" sub-pulse in a field at instant tl, and the charge transport at instant t3. The number of sub-pulses, and thereby the instant tl of the "last" sub- pulse in a field, can be varied to establish a shorter or longer time slot between the "last" sub-pulse in a field at instant tl, and d e charge transport at instant t3. So, the variation of the duration of die integration period is a means to control sensitivity of the sensor: the longer the integration period, d e longer the period during which charge is collected and die more sensitive the sensor is. However, at bright scenes, a long integration period only results in an overexposed picture. At e shutter control is illustrated. Between die instants tO and t3 the shutter is transparent, while between t3 and tO' the shutter is scattering. The instant tO is somewhat before tl (the start of me integration period as shown at d) to accommodate for a slow switching witiiin the liquid crystal shutter. The instant t3 is the end of me integration period as shown at d. When the integration period is made variable to allow for a sensitivity adjustment of the sensor, d e period t0-t3 during which me shutter is transparent must vary accordingly such that the instant tO remains a little bit earlier than the instant tl whatever the latter may be. The sensor sensitivity control which is obtained by varying d e number of sub- pulses at which undesired charge is discarded, is known as "electronic iris". As set out above, in interline sensors, smear is obtained during the full field period, this in contrast with frame transfer sensors, in which smear is only obtained upon transport from the image section to the storage section during me field blanking period. Consequently, with interline sensors, a shutter is only useful if the effective integration period tl-t3 is shorter than the field duration t2-t2'. The liquid crystal shutter must be transparent during me integration period tl-t3. In accordance widi a preferred embodiment, die invention thus proposes a scattering liquid crystal shutter in combination with an interline CCD sensor, to improve smear suppression when short exposure times are used. An advantage of a scattering liquid crystal shutter is die high transmission in the transparent mode. An important advantage of the use of a shutter in color cameras is the fact that color filters on the CCD sensor IC gradually turn pale during die course of time, in dependence upon the amount of light received by the CCD sensor. A shutter causes the CCD sensor to receive a smaller amount of light at e short integration periods used when mere is an abundance of light, so that the life of the on-chip color filters is extended when a shutter is present in accordance with the present invention. It should be noted that the above-mentioned embodiments illustrate rauier than limit die invention, and diat those skilled in me art will be able to design many alternative embodiments without departing from the scope of die appended claims. Instead of a liquid crystal shutter, any other electrically controllable light scattering device can be used. In the claims, any reference signs placed between parend eses shall not be construed as limiting e claim. The driving of the shutter can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer.

Claims: Claims: 1. An interline image sensor arrangement, comprising: an interline image CCD sensor (S) having a charge integration period (tl- t3); and a shutter (LCS) controlled to be transparent substantially during (t0-t3) said charge integration period (tl-t3), and to be non-transparent otherwise (t3-t0'). 2. An interline image sensor arrangement as defined in claim 1 , wherein said shutter (LCS) is a liquid crystal shutter which is scattering during ie non-transparent periods. 3. A camera comprising an objective (O) positioned in front of an interline image CCD sensor (S) having a charge integration period (tl-t3); and a shutter (LCS) controlled to be transparent substantially during (t0-t3) said charge integration period (tl-t3), and to be non-transparent otherwise (t3-t0'). 4. A camera as claimed in claim 3, wherein said shutter (LCS) is a liquid crystal shutter which is scattering during die non-transparent periods. 5. A camera as claimed in claim 4, further comprising image signal processing means (IPC) coupled to said interline image CCD sensor (S) for removing a DC- component obtained during a period (t3-t0') in which the liquid crystal shutter (LCS) was controlled to be scattering.

Inventors: KLIJN JAN, SEPPEN CONSTANCE JEANNE ELIZAB, KLIJN, JAN, SEPPEN, CONSTANCE, JEANNE, ELIZABETH
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1996
Main classification: H04N3
IPC codes: H04N5