Visual assessment of images guide
From UA Libraries Digital Services Planning and Documentation
The practice of optimizing images is mainly one of observing the materials and the on screen scan, and making a comparison. The goal of the comparison is to retain as much visual information about the materials being scanned as possible. things to look to when making the visual assessment.
Evaluate the digital image!
distinguish between attributes of the digital image and those present in the physical item. For instance the difference between the focus of a digital capture and the "focus" of the physical item Is this an accurate representation of the physical item? This includes any "flaws" the physical item may have.
Exposure - correct
The white point reference values found on a white balance target reads evenly across all three channels (R, G, and B) in the range of 215 - 235. A surrogate white point reference can be used for exposure evaluation if you are sure the material is "white" Dense black areas will read between 10 - 20, the "black" background paper of the document table will read between 30 - 60, with the "B" value slightly higher.
some questions to ask of the image when evaluating
- are the light areas and the dark areas retaining their details in the on screen scan. (if not there may be a problem with brightness)
- are there grey tones that act as a "bridge" between light and dark tones. (if not there may be a problem with contrast)
- are the lightest areas a "flat" white tone or is there some hint of texture, detail, or gradient within that light area. (bringing down the level of contrast, or bringing down the brightness can reveal these details.
Exposure - even
in Adobe camera raw turning on the highlight and or shadow limit warning while moving the master exposure slider back and forth. if the image is evenly lit and exposed the out of gamut warning mask color will appear in a consistent manner all across the item in the image. if the lighting and exposure is uneven you will see a corner or a side of the item get masked out first.
Focus - correct
- Make this evaluation at 100% viewing resolution
- fibers in the paper
- edges of paper
- sharpness of text, printed graphic elements
Focus - even
- At a 100% resolution preview; compare each of the four corners of the material in the image for a consistent focus level
- 600 dpi
- 300 dpi (only for 100% typescript items)
Color - correct
- neutral reference values read evenly across all three channels
Alignment - correct
- meeting guidelines for best practice defined here (link to alignment guide needed)
- A high level of right-side-upedness
Alignment - even
- all border areas within an image match
- the border spacing is also reflected in the images for the rest of the item and for collections in general
border areas are kept to a minimum that avoids a "full bleed" crop.
The image below illustrates the key point to take into consideration when assessing proper focus.
Out of gamut and banding problems can occur when the level of contrast has been lowered to much
The image below illustrates several states an image can be in with the center image being the best.