Barcode symbols provide a fast and accurate means of representing information about an object. Decoding or reading a barcode is accomplished by translating the pattern of barcode features, such as bars and spaces in linear (1D) barcodes or blocks or other features in a two-dimensional (2D) barcode, into the corresponding numbers or characters. Barcodes are widely used for encoding information and tracking purposes in retail, shipping and industrial settings.
Barcodes are typically read by a handheld barcode reading device that includes a camera or imager (these terms being used interchangeably herein) for capturing an image of the barcode. Since the reading device is handheld, the quality of the captured image can suffer due to hand movement of the reading device relative to the barcode during the image capture period. For example, movement in the direction perpendicular to the bars and spaces with a linear barcode can result in a blurring that causes misdetection of the edges between the bars and spaces in decoding the barcode. For 2D barcodes, motion in any direction can induce image blur that results in misdetection of the barcode feature. The image blurring problem can be exasperated when using low cost image sensors that require longer exposure periods to capture images with suitable signal to noise ratios or when attempting to capture an image of a barcode under less than ideal lighting condition which also requires a relatively longer exposure period.
Optical stabilization techniques are often used to reduce blurring associated with the motion of a camera during exposure to compensate for movement of the imaging device. Optical stabilization varies the optical path to the image sensor of the camera in response to movement. This technology can be implemented in the lens itself, or by moving the sensor as the final element in the optical path. The key element of all optical stabilization systems is that they stabilize the image projected on the sensor before the sensor converts the image into digital information. Optical stabilization techniques typically require additional sensors, such as gyroscopes and accelerometers, and devices to move lenses or sensors in the optical path. These additional sensors and devices create a number of potential disadvantages in a handheld device, such as increasing the expense of the handheld device, increasing the number of moving parts in the handheld device, possibly reducing the overall reliability, and increasing the power consumption within the handheld device.
Another known technique used to minimize motion blur relies on using an accelerometer to detect pauses in hand motion. This technique only allows the camera to attempt to capture an image during a detected pause in motion. This technique requires knowledge of the expected motion pattern of hand jitter in order to correctly detect the pauses. This technique is used by some cell phone cameras, one example being the Glogger VS2 software available for Nokia phones running on Symbian OS. Published U.S. Patent Application 20050236488 to Kricorissian, and assigned to the assignee of the present invention, discusses another implementation of a delay-based system. These approaches have proven not to be practicable for barcode readers because they often create an unacceptable delay when capturing the image of the barcode. Furthermore, these approaches may still result in blurring of the captured image in lower light environments or when using a low-cost image sensor where a longer exposure time is required since these approaches do not prevent blurring or compensate for blur causing movement, but instead merely attempt to avoid capturing the image of the barcode when movement is occurring.
An alternative approach to reduce image blurring is to reduce exposure times. However, imagers used in barcode reading devices are typically lower cost image sensors that require relatively longer exposure times. In the scanning environments where barcode reading devices are typically used, the light levels are often not sufficient to use reduced exposure times to eliminate image blurring. Providing supplementary lighting, such as a flash or other light source, can reduce the exposure time, but would result in increased power consumption by the handheld device with a commensurate reduction in the useful operating time of the battery powered handheld device.