Sections:
Motion Blur
Out of Focus
Chromatic Aberration
Clipping
Image Skew
Object Interference
Image Orientation
Motion Blur
Motion blur is a distortion of pixels in an image caused by movement. Motion blur results in images that appear fuzzy (transitional pixels) to the end user, making the record difficult to read. Motion blur is characterized by transitional pixels exceeding more than two pixels in one direction, as opposed to “out of focus,” which indicates transitional pixels exceeding more than two pixels in all directions. Blur can occur in random places, anywhere in an image. Some possible sources of movement that can cause blur include the following:
- Camera movement
- Wind or air movement caused by air ducts
- Nearby trains or elevator shafts
- Earthquakes
- Bumping the copy table
- Vibrations
Out of Focus
An image is out of focus when the camera was not focused properly, as opposed to motion blur, when the image is moved during capture. That is, relative to the camera, the out of focus image lies outside the depth of field—the distance from the lens in which the subject appears sharp for a particular set of lighting conditions.
The out-of-focus condition is characterized by transitional pixels exceeding more than two pixels in all directions, as opposed to “motion blur,” which indicates transitional pixels exceeding more than two pixels in one direction.
Generally, when the lens is set at F-8, at least 0.5 inches (1.27 centimeters cm) is in focus. This is important when capturing a book. For a thick book during image capture, you need to occasionally reset the focus, since some of the pages will move outside of the depth of field as the pages are turned.
Put wedges under the front and back covers of a book so the entire surface of the book is in focus. When calibrating, open the book to a middle page, and put the calibration tools on top.
The F-stops on a camera affect the depth of field. Smaller numbers, such as F-2 and F-4, have less depth of field. Larger numbers, such as F-11 and F-16, have a greater depth of field. The DCam software is set to calibrate effectively when the F-stop is set to F-8. Do not change this setting unless specifically requested by your field supervisor.
Chromatic Aberration
For color images, chromatic aberration is an out-of-focus condition caused by lens dispersion. Dispersion is when different colors of light travel at different speeds when passing through a lens. Lens dispersion causes red pixels on one side of a focus character and blue pixels on the other side. While a black and white focus image can have no more than 2 transitional pixels, color images that show chromatic aberration allows 3 transitional pixels.
Clipping (Overexposed or Underexposed)
Clipping is when gray values exceed the desired tonal range, resulting in images that are overexposed (too light) or underexposed (too dark). Clipping is caused by improper calibration, lighting changes, or environmental changes. The DCam system monitors images being captured and warns you when an image contains pixels that are outside of the normal range of tonal values.
Image Skew
Image skew occurs when a document is not straight under the camera lens. A document is out of spec when it is skewed more than 3 degrees.
Object Interference
Object Interference results when unapproved extra material is captured in frame. Such extra material may include hands, fingers, or other body parts, stacks of loose papers behind or to the side of the captured record, and/or camera operator tools that are not used to keep the document flat or book open.
Image Orientation
An image should be oriented so that it is right-reading. Right reading means the image of the document can be read from left to right and top to bottom. Attachments should also be right reading, which may require the attachment to be rotated before being captured.
Note: There are exceptions to right-reading in some languages and cultures.
For more detail on problems that occur with image capture, see Image Quality Feedback Errors.