The present invention relates generally to the generation of an electronic image and, more particularly, concerns the generation of an image of a moving object, making use of a rolling shutter.
Two well-known types of electronic image sensors are CMOS sensors and charge-coupled devices (CCDs). CMOS sensors, generally, represent the lower cost option and have a number of other advantages. For example, CMOS fabrication technologies lend themselves to the inclusion of timing circuits, analog-to-digital converters, and other functional blocks within an area of the image sensor. CCD sensors, on the other hand, must be accompanied by separate circuits that provide those functions.
CCD sensors have the advantage that they incorporate an electronic shutter mechanism known as a global shutter. In imagers of this type, the entire device is reset before integration (collection of light) to remove any residual signal in the sensor wells (pixel defining elements) of an imager. The pixels then accumulate charge for a predefined integration period, with light collection starting and ending simultaneously for all pixels. At the end of the integration period, all charges are transferred simultaneously to light shielded areas of the sensor to prevent further accumulation of charge during the readout process. The charges are then shifted out of the light shielded areas of the sensor and read out. As a result, with a global shutter sensor, an image of a moving object is “frozen” in time, provided that the integration time is short enough to avoid motion blur. It is known that, with a global shutter, motion blur can be reduced or avoided by using a short duration flash of illumination, as when a flash photograph is taken.
CMOS image sensors use a rolling shutter, which operates differently, in that the photodiodes (pixels) do not collect light at the same time. Al pixels in one row of the imager collect tight during exactly the same period of time, but the time light collection starts and ends is slightly different for each row, the top row of the imager being the first one to start collecting the light and the first one to finish collecting. The start and end of the light collection for following rows is slightly, although increasingly, delayed. The total light collection time for each row may be exactly the same; and the delay between rows may be constant. The time delay between a row being reset and a row being read is the integration time. By varying the amount of time between when the reset sweeps past a row and when the readout of the row takes place, the integration time can be controlled, Since the integration process moves through the image over some length of time, some motion blur may be apparent, even with a relatively slow moving object. Some reduction in motion blur may be achieved with flashed illumination, but a very bright flash is necessary to permit sufficient light to be collected by the last row of the imager, but the increased expense of a bright flasher and the wasteful energy consumption are undesirable.
One type of imaging apparatus which often utilizes a CMOS imager is a barcode reader, Scanning barcodes on objects on a moving conveyor belt can represent a particularly challenging task. Not only are the barcodes relatively small, but the code elements are very closely spaced. The barcode must be imaged accurately for proper decoding, and image blur can impede that process. Even a relatively slowly moving barcode may therefore be the equivalent of a rapidly moving object, from the point of view of the quality of the image to be formed. It will be appreciated that the difficulty with barcodes is substantially increased when a two-dimensional barcode is utilized. It would be desirable to be able to image a moving barcode without resorting to the use of intense flashing light.
In accordance with the present invention, a relatively rapidly moving object may be imaged with a sensor incorporating a rolling electronic shutter, while using flash illumination which is comparable to or dimmer than ambient light. This is achieved by utilizing a physical shutter between the object and sensor which is synchronized to the flash. Preferably, the physical shutter is also operated so as to be open for a time interval which is coextensive with the presence of the flash.
In a preferred embodiment, an optical mechanism may is the provided between the object and sensor which causes the image to be focused when the light is flashed and blurred otherwise.
In a preferred embodiment, an optical filter is positioned between the object and sensor, and this filter is constructed to transmit to the CMOS sensor light at the wavelength of the flashing light source but to attenuate ambient light.
During the time that the shutter is open, the ambient light and flashing light source combine to provide sufficient power for imaging. However, because the ambient light is being used in combination with the flashing source, the exposure time can be minimized. Hence, distortion and blur are minimized by having a small exposure time, but ambient light and a flashing light source combine to make the incident power enough to image the desired object.
The foregoing brief description and further objects, features, and advantages of the present invention will be understood more completely from the following detailed description of presently preferred, but nonetheless illustrative, embodiments in accordance with the present invention, with reference being had to the accompanying drawings in which:
Turning now to the drawings,
In contrast, in the CMOS imager of
That is, an upright rectangular object takes on a trapezoidal shape tilt shape, making it seem to tilt away from the direction of movement, because the object has moved a little in between each line being imaged. This makes reading a two-dimensional barcode problematic. Also, the image is blurred, because the object is moving during the imaging time. The blurring of the image is even more difficult to correct for.
Referring to
One exemplary manner in which to implement the physical shutter is to use a rotating disk or similar member with apertures so that there is periodically an opening for the light to impinge upon the sensor. In such an arrangement, the flash illumination and the image sensor must be synchronized to capture the light reflected during the time that each opening of the physical shutter permits the passage of light.
It is also notable that when the ambient light is bright enough, the flash illumination may not be needed. The effective integration time can thus be set by changing the pulse width of the incident light, when the flash is used, or by changing the electronic shutter period of the sensor array. The system designer may choose either, or the system may include a light sensor that chooses which manner in which to control the effective integration time of the CMOS or other sensor, depending upon whether the ambient light is beyond a predetermined threshold or not.
Although preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that many additions, modifications, and substitutions are possible without departing from the scope and spirit of the invention as defined by the accompanying claims.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US07/09421 | 4/18/2007 | WO | 00 | 7/14/2010 |