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Quality Control-definitionQuality Control-assessing quality

4. Quality Control

Key Concepts

developing a program
assessing quality

additional reading



The following steps outline the main points of a quality control program. A fully developed strategy for establishing such a program is presented in Moving Theory into Practice: Digital Imaging for Libraries and Archives.

1. Identify Your Products
The first step is to clearly identify the products to be evaluated. These might include master and derivative images, printouts, image databases, and accompanying metadata, including converted text and marked-up files.

2. Develop a Consistent Approach
To measure quality and judge whether the products are satisfactory, clearly define baseline characteristics for "acceptable" and "unacceptable" digital products.

Example: Defining Image Quality Parameters for Different Project Goals

If the goal is faithful representation, quality assessment will be based on how well the image conveys the appearance of the original document (detail, color, tone, paper texture, etc.).

Faithful Representation: The color image (left) represents the essence of the original more fully than the grayscale image (right).

If the goal is removing a color cast introduced during the photographic process, quality will be judged against the original scene or document (rendering intent), rather than the photograph at hand.

Removing Color Cast: The color shift caused by photography (left) was detected and removed during quality inspection (right).

3. Determine a Reference Point
What are you judging the images against? Answering this question is not always straightforward. For example, if conversion is based on an intermediate, the digital image is two "generations" away from the original. It has been copied to film (first generation), which is then scanned (second generation). What should be the reference point in assessing such an image, the original document or the transparency? Will the master or derivative (or both) be the focus of image quality inspection?

4. Define the Scope and Methods
Determine the scope of your quality review. Will you inspect all the images, or only on a sampled subset (e.g., 20%)?

Describe your methodology and define how quality judgements will be made. For example, will you visually evaluate the images at 100% (1:1) magnification onscreen and compare them to the original documents? Or, will your evaluation be based only on a subjective assessment of images onscreen, without reference to the originals?

Example: Because Cornell University Library replaces brittle volumes with digital reprints, image quality evaluation is based on the printouts created from the digital images. A 100% inspection is conducted, comparing each printout to the corresponding original page.

5. Control the QC Environment
The impact of image-display conditions on perceived quality is often underestimated. Given an improper environment, even a high-quality image may come across as unsatisfactory. For example, a 24-bit color image might look heavily "posterized" when viewed using an improperly configured computer that cannot provide a full palette of colors. More information on controlling the viewing environment is provided in Using Kodak Photo CD Technology for Preservation and Access.

QC Environment: Image quality evaluation conducted in a controlled environment. Courtesy of William Blake Archive.

Factors affecting on-screen image quality

Hardware Configuration
It is difficult to prescribe the ideal hardware configuration. The rule of thumb is to assemble a system that supports your requirements for speed, memory, storage, and display quality. What kinds of images are being created? How many? To serve what purposes? What level of on-screen review is needed? You will need a fast and reliable computer with ample processing power and memory to be able to retrieve and manipulate the large files you are creating, especially when creating color images. See also: Technical Infrastructure: Image Creation.

Image Retrieval Software
Use retrieval software appropriate to your images. For example, if you are evaluating images created and stored in Kodak ImagePac format, retrieve them using one of the viewing freeware and shareware products available on the Web that support the format and color space. Cornell used Adobe Photoshop with the Kodak Photo CD Acquire Module plug-in to ensure correct mapping of Photo CD colors. More information is provided in Using Kodak Photo CD Technology for Preservation and Access.

Viewing Conditions
Control your viewing environment. Understand that the monitor and the source document require distinct viewing conditions. The original is best viewed in a bright surrounding, and the monitor works best in a low-light environment. However, a low-light environment does not equate to a dark room. Viewed in the dark, an on-screen image would appear deficient in contrast.

Human Characteristics
Image quality assessment requires visual sophistication, especially for subjective evaluations. Ideally, the same person should evaluate all images, using the same equipment and settings. In particular, staff need training to communicate color appearance information effectively.

Some color vision deficiencies are linked to a defective, recessive gene on the X chromosome. Since females have two X chromosomes and males have one, the chance of color-deficient vision is 1 in 250 females, but 1 in 12 for males. Even among expert viewers, differences in judgments due to normal variation in the human eye are not uncommon. A color vision test can be used to evaluate a viewer's vision.

Monitor Calibration
Images may appear different on different monitors. Calibration is the process of adjusting monitor color-conversion settings to a standard, so that the image displays the same on a variety of monitors. The ideal method is to use monitor-calibration hardware and accompanying software. However, if you do not have access to these resources, use your application program's calibration tools. For example, Adobe Photoshop includes a basic monitor-calibration tool, which can be used to eliminate color cast and standardize the display of images.

Color Management
One of the main challenges in digitizing color documents is to maintain color appearance and consistency across the digitization chain, including scanning, displaying, and printing. Accurately reproducing colors is difficult because input and output devices treat colors differently. The goal of color management system (CMS) software is to ensure that the colors of the original match as precisely as possible the digital reproduction on-screen or printed out.

6. Evaluate System Performance
Whether conversion takes place in-house or is outsourced, system performance should be evaluated to ensure consistency throughout the conversion process. Among characteristics to evaluate are resolution, linearity, flare, scanner noise, color reproduction, and various artifacts. Several publications noted at the end of this section provide further information on system calibration.

7. Codify Your Inspection Procedures
Quality control data has long-term value, from supporting different stages of quality inspection to facilitating future manipulation and migration. For in-house components of QC, we recommend detailing the inspection procedures in a short manual (or in a series of workforms) to be used in training and to facilitate workflow. Issues that need to be addressed include: QC procedures; staff involved and skills needed; instruments, hardware, and software needs; rejecting and replacing unacceptable products. An example of this approach is demonstrated by the Library of Congress in its Internal Training Guide.

Reality Check

You retrieve an image from a CD that just arrived from the image production unit. The image is not crisp and looks darker than what you had expected. What is the first thing to do to determine the cause of the problem?

(A) Confirm that your viewing environment is correct.
(B) Use the adjustment tools (sharpening, color correction) available in your image viewing software to bring the image in line with expectations.
(C) Call the image production unit to find out what happened.

© 2000-2003 Cornell University Library/Research Department

Quality Control - definitionQuality Control - assessing quality

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