Digitization Basics for Archives and Special Collections – Part 1: Select and Scan
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This document discusses digital imaging concepts such as pixels, resolution, bit depth, and file size. It explains how these concepts relate to scanning quality and options for selecting a scanner. Flatbed and film scanners are described and compared based on their specifications. Guidelines for scanning, such as achieving certain levels on a gray scale, are provided. The document concludes with recommendations for image editing software and sources for further reading on digital imaging best practices.
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Digitization Basics for Archives and Special Collections – Part 1: Select and Scan
- 1. Digital Images and Scanning Josh Hickman WiLSWorld, 22 July 2015
- 2. Digital Images: Basic Concepts • Pixels = smallest units of information that make up an image; arranged in an array • Resolution = expressed in pixels per inch or as the pixel dimensions of an image • Bits = smallest units of info that can be understood by a computer; 2 values (ON or OFF, 1 or 0, BLACK or WHITE) • Bit Depth = number of bits assigned per pixel; determines the number of colors available to represent the original – 2 bits = 4 shades (2^2) – 8 bits = 256 colors (2^8) grayscale – 24 bits = millions of colors (2^24) color images – In RGB images, bits are divided among the 3 channels… 8 bits for RED 8 bits for GREEN 8 bits for BLUE
- 3. Digital Images: Basic Concepts • File Size – Height (pixels) x Width (pixels) x [Bit Depth/8] = file size (bytes) • Color Space – System for describing colors numerically – AdobeRGB v. sRGB Source: Colorspace by Cpesacreta [Image]. CC BY 2.5 <http://creativecommons.org/licenses/by/2.5/>. Retrieved from <http://en.wikipedia.org/wiki/Color_space#mediaviewer/File:Colorspace.png>
- 4. Resolution • Expressed as points per inch (ppi) or as pixel dimensions • Higher resolution allows more detail, to an extent Source: Voight, R. (1930). "Setting up camp in Sahara" [Photograph]. Voight, Robert. Courtesy of Beloit College Archives. Retrieved from http://dcms.beloit.edu/cdm/ref/collection/african/id/171
- 5. 72 ppi 300 ppi 800 ppi Source: Voight, R. (1930). "Setting up camp in Sahara" [Photograph]. Voight, Robert. Courtesy of Beloit College Archives. Retrieved from http://dcms.beloit.edu/cdm/ref/collection/african/id/171
- 6. 1200 ppi 2400 ppi The original photograph is about 2.75 x 4.5 in. Source: Voight, R. (1930). "Setting up camp in Sahara" [Photograph]. Voight, Robert. Courtesy of Beloit College Archives. Retrieved from http://dcms.beloit.edu/cdm/ref/collection/african/id/171
- 7. Selecting a scan resolution 4000 pixels on the long edge of the digital image • Select scan resolution to achieve this result • For a printed image, 300 ppi is optimal A digital image with 4000 pixels on the long edge will be… • 13 inches long at 300 ppi • 27 inches long at 150 ppi • 5 inches long at 800 ppi Digital images do not have an absolute size!
- 8. Selecting a scan resolution
- 9. Selecting a scan resolution
- 10. The Histogram
- 11. Master and Derivative Images Derivative Images Service master images with further editing for display (print, online, etc.) Lossy compression (i.e. JPG) Service Master Images Edited version of archival master Edits can be stored as Photoshop layers Archival Master Images Highest quality affordable Lossless compression, non- proprietary formats (i.e.,TIFF) Unedited
- 12. Scanning Photo courtesy of Beloit College Archives.
- 13. Scanning Goal is to achieve these levels on the Kodak Q-13 Gray Scale across each color channel: A: 239-247 M: 100-108 19: 8-16 Photo courtesy of Beloit College Archives.
- 14. An accurate image Photo courtesy of Beloit College Archives.
- 15. A clipped image Photo courtesy of Beloit College Archives.
- 16. Another inaccurate image Photo courtesy of Beloit College Archives.
- 17. Histogram gaps Photo courtesy of Beloit College Archives.
- 20. Fun with histograms Source: Welcoming the Touareg chiefs -- Insalah by W. Bradley Tyrrell (1925). Courtesy of Beloit College Archives <http://dcms.beloit.edu/cdm/ref/collection/african/id/409>
- 21. Flatbed Scanners Beyond the basics, consider…. • Document/transparency size • Optical resolution (vs. interpolated) • Dynamic range (dMax) • Supported bit depths • Added features and software No one scanner will be perfect for every situation. Keep in mind the material you expect to scan and thoroughly evaluate the specifications of each scanner to make sure it can handle your items!
- 22. Flatbed Scanners Source: Taipei IT Month Epson V370 Photo By Solomon203 (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons <http://upload.wikimedia.org/wikipedia/commons/5/56/Taip ei_IT_Month_Epson_V370_Photo_20131202.jpg> • Epson Perfection V370 (~$120) • 4480 optical resolution • 3.2 dMax • Epson Perfection V550 (~$200) • 6400 optical resolution • 3.4 dMAx • These scanners have limited transparency scanning capability. If you’ll be working with negatives, slides, glass plates, etc., be sure that these scanners can handle the format and size of your transparencies!
- 23. Flatbed Scanners • Epson Perfection v850 Pro (~$850) • 6400 optical resolution • 4.0 dMax • Includes SilverFast SE Plus and X-Rite i1 calibration • 8 x 10 in. transparency area • Epson Expression 11000XL (~$2600) • 2400 optical resolution • 3.8 dMax • 12 x 17 in. document size • Includes SilverFast Ai and IT8 calibration charts
- 24. Other Scanners • Plustek OpticFilm 8200i (~$480) • 7200 resolution • Includes SilverFast Ai • 35 mm negatives and slides • Planetary scanners • Konica Minolta PS7000C MK II • DIY Book Scanners • Caution: purchasing components can be expensive • http://www.diybookscanner.org Source: Slide scanner By HandigeHarry (Own work) [Public domain], via Wikimedia Commons <http://upload.wikimedia.org/wikipedia/commons /f/f5/Slide_scanner.JPG>
- 25. Image Editing Software • Adobe Photoshop Elements • Adobe Photoshop Creative Cloud (CC) • Adobe Creative Suite (CS) – Photoshop – Bridge, Lightroom, Illustrator, etc. • Corel Paint Shop Pro • GIMP
- 26. Further reading Bibliographic Center for Research and CDP Digital Imaging Best Practices Working Group. (2008). BCR’s CDP digital imaging best practices, version 2.0. Aurora, CO.: Bibliographic Center for Research]. Retrieved July 8, 2015, from http://mwdl.org/docs/digital-imaging-bp_2.0.pdf Kennedy, J. (2012). Preserving history: how to digitally archive and share historical photographs, documents, and audio recordings. Retrieved June 29, 2014, from http://archivehistory.jeksite.org/ Puglia, Steven T, Jeffrey Reed, and Erin Rhodes, United States and National Archives and Records Administration. Technical Guidelines for Digitizing Archival Materials for Electronic Access: Creation of Production Master Files--Raster Images. Washington, D.C.: Digital Library Federation, 2005. Retrieved July 8, 2015, from http://www.digitizationguidelines.gov/guidelines/digitize-technical.html
Editor's Notes
- Pixel = picture element Resolution = how many pixels are used to describe the image Bits = 1s and 0s; 8 bits make up a standard byte Bit Depth = more bits per pixel allows for more combinations of on/off and, therefore, more colors/shades 2 to the power of [bit depth] 24 bits = 8 bits to each color channel (R, G, B)
- Color space is a system for describing colors Note how there is a great deal in the green – human vision is most attuned to green AdobeRGB is known as a wide gamut color space, but isn’t used by the web or by printers sRGB is the color space used by the web and by most printers Most recommendations say to use AdobeRGB If you send to a printer, be sure to specify the color space used – otherwise, colors may look different Embed the color space in the file when you save it… this is an option in the “save as” window
- Many guides offer suggestions for resolutions based on size and nature of original Most suggest 4000 pixels on the long edge of the resulting image (except for very large items like maps, then maybe larger) When printing 300dpi is usually optimal (dropping below risks loss of quality, going too far above won’t yield higher quality prints) Important to understand the difference between scanning resolution and output resolution (e.g. for a print) These size/resolution combinations are based on the ppi/dpi formula (pixels / inches = pixels or dots per inch)
- Because digital images have no absolute size, we can change the size of the image to create the prints that we want. As long as we don’t drop below 300ppi, we should get good quality prints. Here, we’re resizing a very large image. Since the number of pixels don’t change, they get “stretched out” or “squeezed together” (ppi) depending on the size we select. Notice how the width/height in inches is tied to the resolution, but pixels will not change.
- We can, though, override this by selecting RESAMPLE IMAGE in Photoshop. Here, note how resolution is decoupled from the dimensions of the output image, but pixels are now unlocked. If we change the dimensions now, resolution will stay the same, but the pixels increase. To create those pixels, Photoshop guesses what they should be based on neighboring pixels. This is not good! We can, though, decrease the size without worrying, but Photoshop will “throw out” some pixels to do so. We’ll never be able to get those pixels back exactly.
- Histogram = map of the pixel distribution from dark to light (0-255) Input = what the pixels were; Output = what they will become (many advise output set to 8 and 247 to match printer capabilities) Can view all color channels at once, or individually
- Lossless compression means no pixel information is discarded Correcting for fading vs. representing original characteristics determines how many files you’ll need and the characteristics of those files (in turn influencing amount of resources and work needed to create and store the images) The scanning handout offers a few different workflows. If you want to represent the original image condition, you’ll have to do more work. You’ll need to make a file with the Kodak Q-13 reference patches and carefully set tone values. Then, you’ll need to make another file that’s optimized for display and does not include the patches. You can do that either by making another scan OR by making edits to a copy of the archival master in photoshop. Either way, you’ll want to make edits to the image at 48 bits… we’ll see why shortly. If you want to correct for fading and don’t care about preserving the original image characteristics, then you can skip the Q-13 patches. There are several different workflows that are possible The handouts give details about a few of them. The following slides present the basics of creating an archival master where preserving original condition is important.
- Here’s a sample workflow. I’ve skipped a couple of important steps that are listed in the DIGITIZATION CHEAT SHEET … calibrating your monitor and scanner …. For the sake of time. Some of the resources that I’ll show in my last slide have more information about this color management process. Histogram adjustment in scan software only for master file Across all 3 color channels Input represents the image as is, whereas output represents what it will become. Output = 8/247 (tones above and below can’t be rendered or printed)
- Use the densitometer to guide your histogram edits. The Kodak Q-13 scale has patches whose values can be measured. This chart presents accepted ranges for 3 of these patches. Use the densitometer to check edits to the histogram until you achieve these levels.
- PS histogram is for image portion only (not scale)
- Clipping occurs when some tones get dropped – here, some dark tones are lost The resulting histogram shows spiking – many pixels that were clipped off are now mapped as pure black (0) or pure white (255)
- Here, we see a histogram that’s been “squished” – all of the tones have been pushed toward the middle, making the photo look washed out
- Gaps in histograms occur when edits are made in Photoshop to a 24-bit image. 48-bit images can resist this more easily. Often, no problems will be visible, but banding is occurring. This is why it’s important to make any edits to your image at 48 bit depth. You can do this either by outputting a 48-bit image when you’re scanning or by using the 2 scan method outlined in the Scanning Methods handout if you’re preserving characteristics of the original image.