A vertical field-transfer type of CCD imager has a photo-responsive image A register exposed to radiant energy from a scene and has a field storage B register and an output line C register, both of the latter being masked from illumination. Radiant energy descriptive of the scene is focused onto the A register during an image integration interval corresponding substantially to a vertical trace period to generate a field of charge packets representative of the scene. These charge packets are shifted into the B register during a field transfer interval (commonly also called a "pull down" or A/B transfer interval) occuring within the next vertical retrace period. During the next vertical trace period the charge packets of each line of the field stored in the B register are shifted in parallel into the C register during respective horizontal retrace periods and then read out serially during the following horizontal trace periods until the entire field of charge packets in the B register is read out.
Solid-state imagers undesirably produce temperature-dependent dark currents which accumulate as charge in all of the registers of the imager. These dark currents appear as signal even in the absence of light. The accumulation of dark current in the A register during the field integration interval results in a dark current DC pedestal, which accompanies the image representative charge packets. Video signal field shading, a variation in black level across the picture field in the direction of line advance, is caused by successive lines of charge packets representative of image samples spending progressively longer periods of time in the B register of the imager. The increased length of time each line of charge packets spends in the B register allows a longer time for the accumulation of dark current thereby shifting the black level for each successive line further towards white. Although dark current does accumulate in the C register, its level is very small.
Therefore, it is desirable to obtain an indication signal responsive to dark currents which can be used for compensating the video signal against dark current response. Since the rate of dark current accumulation increases with increased temperature of the semiconductive substrate on which the imager is formed, it is desirable to derive the indication signal as a direct measure of an accumulation of dark current in an area of the same substrate as that which the imager itself is constructed upon.
Charge left in the B register after the field-trace period can, as described in U.S. Pat. application Ser. No. 382,422 filed May 27, 1982 by P. A. Levine, entitled "COMPENSATION AGAINST FIELD SHADING IN VIDEO FROM FIELD-TRANSFER CCD IMAGERS" be removed during field retrace and integrated over time to develop a signal that is a measure of dark current accumulation during imager field-trace. The area of the substrate over which remnant charge is accumulated is that of the B register, so the sensitivity of the measure of dark current accumulation is good since the indicating signal tends to have a large enough amplitude due to the above-described line-progressive build up to be accurately measured. The practical problem that one runs into in practicing this method is that the opaque mask over the B and C registers used to shield them from illumination does not, at least in imagers presently commercially available, adequately preclude photo-responsive components from being produced in the lines of the masked B register which are close to the unmasked A register. This photo-responsive component is clocked forward during the line-by-line advance of the charge packets through the B register in the field trace period and therefore appears in every line of the remnant charge clocked out of the B register during the field retrace period. This photoresponse results in an inaccurate measurement of dark current response.
In an alternative dark current response measurement method, this Levine application describes the use of a B register which is larger than the A register, in that it has an "extra" vertical charge transfer channel which does not receive charge packets from the A register and, therefore, solely accumulates dark current response. Time-division-multiplexing of the CCD output signal allows the charge packets of this extra B register charge transfer channel to be integrated for generating an indication signal which is proportional to imager dark current. In some imagers, such as those having relatively low dark current response, it may be difficult to generate a large enough indication signal for accurate measurement from a single extra B register charge transfer channel. Increasing the number of extra B register charge transfer channels may improve the accuracy of the dark current measurement, but at the undesirable expense of increasing the semiconductor area of the imager.
U.S. Pat. application Ser. No. 659,460 filed Oct. 10, 1984 in the name of P. A. Levine and entitled "IMPROVED DARK CURRENT SENSING WITH A SOLID-STATE IMAGER HAVING A CCD LINE REGISTER OUTPUT", describes a dark current response measurement method for field transfer CCD imagers wherein dark current accumulated in the C register is sensed for deriving the indication signal. In this method, read out of the C register is continued for an extra time interval after it has already been emptied of charge packets supplied from the B register and before the next line of charge packets from the B register is transferred into the C register. An integration over time of the signals read out of the C register during this extra time interval is used as a measure of dark current response. Although this method of measuring dark current response may be used quite succesfully for some imagers, recently fabricated imagers have a reduced dark current response which may be too small to be accurately measured by this technique, since only a small amount of dark current can accumulate in the one line C register.
In color television cameras, where gamma-correction is used, small video variations close to black level are stretched by about four times with respect to video signal variations in brighter portions of the televised scene. Therefore, even slight black level shifts caused by inaccurate dark current compensation can cause disturbing color shifts when the televised scene is reproduced.
Consequently, it is desirable to provide an accurate indication signal responsive substantially solely to integrated dark currents for use by the camera processing circuitry for purposes such as dark current correction.