Efficient dark current subtraction in an image sensor

A method for dark current subtraction which enables a dark frame to be reused for dark current subtraction for multiple image frames. The dark frame is reused by scaling it according to changes in the dark current levels associated with the dark frame and the image frames.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention pertains to the field of image sensors. More particularly, this invention relates to efficient dark current subtraction for an image sensor.

2. Art Background

A typical image sensor includes a set of sensing elements which generate electrical charge in response to light. For example, one type of image sensor is a charge-coupled device (CCD) array. A typical CCD array includes an array of CCD sensing elements and circuitry for sampling the charge levels of the CCD sensing elements.

A variety of images sensors including CCD arrays have light sensing elements which accumulate an electrical charge even in the absence of light. The electrical charge that accumulates in a sensing element in the absence of light is commonly referred to as dark current. Typically, the dark current in an image sensor creates an undesirable dark image that overlays the optical image.

One prior method for removing the effects of dark current in an image sensor is to perform dark current subtraction. Typically, an image is obtained from the image sensor when it is illuminated by light from an image scene and an image is obtained from the image sensor when it is not illuminated. The image obtained when the image sensor is illuminated may be referred to as an image frame. The image obtained when the image sensor is not illuminated may be referred to as a dark frame. The dark frame is usually subtracted from the image frame to yield an image frame without the dark current component.

Prior methods for dark current subtraction usually obtain a dark frame each time an image frame is obtained because the dark current typically varies according to temperature and exposure. Unfortunately, the time consumed in obtaining a dark frame each time an image frame is obtained usually slows the rate at which useful images may be obtained with an image sensor.

SUMMARY OF THE INVENTION

A method for dark current subtraction is disclosed which enables a dark frame to be reused for dark current subtraction for multiple image frames. The dark frame is reused by scaling it according to changes in the dark current levels associated with the dark frame and the image frames. In one embodiment, the changes in dark current levels are determined by examining changes in charge samples from optically black sensing elements and dummy samples from circuitry in the image sensor.

Other features and advantages of the present invention will be apparent from the detailed description that follows.

DETAILED DESCRIPTION

FIG. 1 shows a method for dark current subtraction according to the present teachings. This method may be used, for example to perform dark current subtraction for an image sensor in a digital still camera or digital video camera.

At step 100 , a dark frame is obtained from the image sensor. Step 100 may be performed, for example, by closing a shutter on the camera that contains the image sensor and obtaining samples from the image sensor.

At step 102 , the dark frame obtained at step 100 is reused for dark current subtraction for multiple image frames obtained from the image sensor. The dark frame obtained at step 100 may be reused at step 102 by obtaining an image frame from the image sensor when it is illuminated by light from an image scene and then determining a difference between dark current levels associated with the image frame obtained at step 102 and dark current levels associated with the dark frame obtained at step 100 . The dark frame obtained at step 100 may then be scaled according to the difference and then subtracted from the image frame obtained at step 102 .

The dark frame obtained at step 100 and the image frame obtained at step 102 each include a sample from each of a set of active elements of the image sensor and a sample from each of a set of optically black elements in the image sensor. In addition, the dark frame obtained at step 100 and the image frame obtained at step 102 each include a set of dummy samples that represent a low charge level in the image sensor. In one embodiment, the dummy samples are obtained by over-clocking an output register in the image sensor.

In one embodiment, the difference between the dark current levels associated with the dark frame and the image frame is determined by determining differences between samples from the optically black elements and the dummy samples.

FIG. 2 shows a camera 10 which incorporates the present teachings. The camera 10 includes a lense mechanism 12 that conducts light from an image scene through an aperture mechanism 14 and a shutter mechanism 16 to an image sensor 18 . An image processor 22 controls the aperture mechanism 14 and the shutter mechanism 16 via a set of control signals 24 .

The image sensor 18 accumulates electrical charge in its sensing elements during exposure periods which the image processor 22 controls via a set of control signals 26 . The image sensor 18 also accumulates charge in its sensing elements during exposure periods as a result of dark currents in the image sensor 18 .

The image processor 22 generates signals via the control signals 24 and 26 to start exposure periods, stop exposure periods and to read out samples from the image sensor 18 . The samples obtained from the image sensor 18 are provided to an analog-to-digital (A/D) converter 20 via an output signal 28 . The A/D converter 20 digitizes the samples carried by the output signal 28 and provides the corresponding digitized samples to the image processor 22 .

The image processor 22 controls the aperture mechanism 14 and the shutter mechanism 16 , and the image sensor 18 to obtain digitized samples for a dark frame at step 100 and for an image frame at step 102 . For example, a dark frame may be obtained by closing the shutter mechanism 16 using the control signals 24 and then using the control signals 26 to obtain samples from the image sensor 18 . An image frame may be obtained by setting an exposure with the aperture mechanism 14 and the shutter mechanism 16 using the control signals 24 and then using the control signals 26 to obtain samples from the image sensor 18 .

In one embodiment, the camera 10 is a still image camera. In another embodiment, the camera 10 is a video camera. The image sensor 18 may be a CCD array or complementary metal-oxide semiconductor (CMOS) array. The image processor 22 including dark current subtraction functionality may be implemented in hardware and/or software or firmware. The aperture mechanism 14 and the shutter mechanism 16 may be any known mechanisms useful in cameras.

FIG. 3 shows an example arrangement of elements in the image sensor 18 . In this example, the image sensor 18 includes an array with a set of active elements 32 , two sets of optically black elements 30 and 34 , and an output register 50 . In one embodiment, the active elements 32 are CCD light sensing elements and the optically black elements 30 an 34 are CCD light sensing elements that are covered with a material that blocks light.

The output register 50 is used to read out charge samples from the active elements 32 and the optically black elements 30 and 34 . Circuitry (not shown) in the image sensor 18 selects the rows the active elements 32 and the optically black elements 30 and 34 in sequence. Charge from a selected row of the active elements 32 is provided to the output register 50 via a set of signal lines 42 and charge from a selected row of the optically black elements 30 and 34 is provided to the output register 50 via sets of signal lines 40 and 44 .

Once a row of charge samples has been collected in the output register 50 , the image processor 22 clocks out the row of samples serially onto the output signal 28 using the control signals 26 . In this example, a row of samples includes 2 optically black samples, 8 active samples and 2 optically black samples. The image processor 22 obtains these samples by generating 12 transitions or edges of clock signal portion of the control signals 26 to shift out these samples onto the output signal 28 .

The image processor 22 obtains dummy samples from the image sensor 18 by over-clocking the output register 50 . The dummy samples represent a low charge level in the image sensor 18 for purposes of scaling dark current when reusing a dark frame. The dummy samples may be treated as if the image sensor 18 included an area 36 of dummy elements which are devoid of dark. For example, the image processor 22 obtains 4 dummy samples for a row by generating 4 extra transitions or edges of the clock signal on the control signals 26 after clocking out the samples from the optically black and active elements.

FIG. 4 shows a data frame 60 obtained from the image sensor 18 via the output signal 28 . The data frame 60 may be a dark frame obtained at step 100 or an image frame obtained at step 102 . The data frame 60 includes a series of row samples (row 1 n) wherein n is the number of rows of sensing elements in the image sensor 18 .

Each row 1 n includes a set of samples (OB) from the optically black elements 30 followed by a set of samples (ACTIVE) from the active elements 32 followed by a set of samples (OB) from the optically black elements 34 followed by a set of dummy samples (DUMMY) from what is represented as the area 36 .

The image processor 22 determines an average intensity of the samples from the optically black elements in a dark frame (D OB ) and an average intensity of the dummy samples in the dark frame (D DUMMY ). The averages D OB and D DUMMY may be computed for all of the samples or a subset of the samples in the dark frame.

The image processor 22 determines an average intensity of the samples from the optically black elements in an image frame (I OB ) and an average intensity of the dummy samples in the image frame (I DUMMY ). The averages I OB and I DUMMY may be computed for all of the samples or a subset of the samples in the image frame.

The samples from the active elements 32 in a dark frame obtained at step 100 are D ACTIVE (1,1) through D ACTIVE (n,m), where n is the number of rows and m is the number of columns in the active elements 32 . The samples from the active elements 32 for an image frame obtained at step 102 are I ACTIVE (1,1) through I ACTIVE (n,m). The dark frame obtained at step 100 is scaled at step 102 in one embodiment by multiplying D ACTIVE (1,1) through D ACTIVE (n,m) by the following scale factor: I OB - D DUMMY D OB - D DUMMY

The scaled dark frame obtained using the above scale factor is subtracted from I ACTIVE (1,1) through I ACTIVE (n,m) element by element when performing dark current subtraction at step 102 .

The time taken to generate a scaled dark frame from a stored dark frame using the above technique is less than the time taken to close the shutter mechanism 16 and obtain a new dark frame. This reuse of a dark frame enables an increased rate of image frame sampling in the camera 10 . A dark frame may be stored in a memory associated with the image processor 22 and re-scaled and reused for each acquired image frame thereafter.

The foregoing detailed description of the present invention is provided for the purposes of illustration and is not intended to be exhaustive or to limit the invention to the precise embodiment disclosed. Accordingly, the scope of the present invention is defined by the appended claims.