The key factors in image quality assessment are resolution, color and
tone, and overall appearance. For further discussion of image quality
metrics, see an RLG DigiNewstechnical
feature by Don Williams.
Resolution is the key factor in determining image quality for textual
materials and other distinct, edge-based representations. For graphical
material, especially continuous tone images, bit-depth, color representation,
and dynamic range combine with resolution to determine the quality. Resolution
attributes to inspect are legibility, completeness, darkness, contrast,
sharpness, and uniformity. Measuring and evaluating stroke and detail
are useful in assessing image quality. See the RLG
Model RFP for examples of defining quality expectations and suggested
QC procedures associated with these requirements.
Follow emerging quality metrics for assessing resolution (see additional
Effects of Resolution on
Image Quality: Compare the quality of these
three bitonal images of a wood engraving captured at different resolutions.
For color, grayscale, and some monochrome images, color and tone reproduction
are significant indicators of quality, complementing the "detail" provided
by resolution. The goal behind assessing color and tone appearance is
to determine the extent to which a digital image conveys the same appearance
as the color and tone ranges of the original document (or intermediate
used). Tone and color assessment may be highly subjective and changeable
according to the viewing environment and the characteristics of monitors
are illustrations that demonstrate the effects of color and tone on image
the left image to the right image, which has limited dynamic range; note
the detail in shadows and hightlights.
Bit Depth: When a 24-bit
image (left) is reduced to an 8-bit one (right), the color reduction may
result in quantization artifacts.
Brightness and Contrast:
Compare the left image to the right one with high brightness and contrast
Image quality is cumulative, affected by a range of individual factors--capture
system performance, resolution, dynamic range, and color accuracy. The
final evaluation should be made on the overall image, appreciating all
the individual factors that contribute to quality.