Image Quality: DPI vs Tone Reproduction
Most discussions about what makes a high quality image focus on “DPI” to the exclusion of the other elements that contribute to what makes an image good. This post is an attempt to explain why other factors should also be considered, and what they are. Steve Puglia, Erin Rhodes, and Jeffrey Reed provided an very in depth guide how to create high quality images in their 2004 Technical Guidelines that have become a standard adopted by The Federal Agencies Digitization Guideline Initiative. Very few people have seen a very useful document titled: “What went Wrong?” created by Puglia et al and hosted at the University of Maryland’s Digital Repository UM website. In this document are listed many of the common imaging problems that contribute to poor digital image quality, such as resolution (DPI, PPI), tone reproduction, clipping, noise, etc. In this document is a great graphic that illustrates the visual effect of the defects:
The use of a greyscale or other tone reproduction target (Color Bars are useless) is essential to balancing, calculating, and measuring proper exposure and tone reproduction.
In my capacity as an Imaging Specialist for the Digital Public Access Branch I am asked to inspect a lot of different digital images created by NARA staff, partners, and citizen archivists and one of the most common problems that I encounter are improper tone reproduction due to under and over exposure. The two most common causes are improper lighting and use of autoexposure on cameras and scanners. The graphic below illustrates the relationship between exposure and tone reproduction and the examination of greyscale targets and digital histograms. Highlights in the image are related to the white or lighter patches in the greyscale target and the darker tones are related to the black patches in the target. In the histogram too much exposure results in all the image data is shifted to the right and there is no data for the darker tones on the left. A properly exposed scene has a equal distribution of highlight, mid, and dark tones seen in the middle image.
In an over exposed image the highlights are washed out and there is no detail captured. In the grayscale target below the white patches are washed out and you can barely perceive a separation between the A, 1, and 2 patches. Under exposure is when there is little or no detail in the black 19, 18, 17, and B patch.
Over and under exposure in an image results in “Clipping” meaning either the highlight or Shadow detail has been “Clipped Off”. In the image below compare the image on the left with the one on the right. The one on the right has highlight clipping in the girl’s dress and shadow clipping in the detail on the grill of the car.
The image below is a scan of a document that although it has plenty of resolution, it is over exposed because all the detail is washed out.
All the tones are on the right and shoot off the scale at the top indicating over exposure.
Additionally, when creating images, it is important to follow Section 508 guidelines for images. Here are some resources to ensure 508 Compliance and accessibility:
- Creating Accessible Images: http://www.section508.va.gov/docs/Accessible_Images.pdf
- Testing Documents and Images for 508 Compliance: http://www.hhs.gov/web/508/training/testdocuments.html#mod2sec5