GUIDELINES FOR THE CREATION OF DIGITAL COLLECTIONSDigitization Best Practices for ImagesThis document sets forth guidelines for digitizing two-dimensional, non-textual materials for theCARLI Digital Collections. The issues described concern image quality, file formats, storage, andaccess.This document was created by the CARLI Digital Collections Users’ Group (DCUG) and ismaintained and updated by the CARLI Created Content Committee (CCC).For questions about this document, please contact CARLI at [email protected] may include such items as photographs, maps, plans, blueprints, drawings, paintings, andother two-dimensional visual media.In many instances, images will contain or have accompanying textual material. Due to this dualnature, the digitization of images is very similar to the digitization of text. For further information ontext digitization, consult the “Guidelines for the Creation of Digital Collections: Digitization BestPractices for Text” document maintained by Created Content s/digital collections/documentation/guidelines for text.pdfThe sections below provide guidance on the processes of creating digital images.
Image CollectionsAlthough no universal standards for quality image capture exist and technical standards areconstantly evolving, member institutions participating in the CARLI Digital Collections shouldadhere to the best practices adopted by recognized leading institutions when the collections' purposewould benefit from those actions.This document does not address best practices for preserving digital files. Digital preservationresources can be found at: Digital Preservation Best Practices and Guidelines, made available by theNorth Carolina Department of Natural and Cultural Resources.Digital ImagesA digital image is a two-dimensional array of small square regions known as pixels. Images typicallyfall into one of three categories: monochrome (bitonal), grayscale, and color. In the case of amonochrome image, the brightness of each pixel is represented by a numeric value. Black and whiteare the most common pairing of pixel values, but any two colors may be used. Grayscale imagestypically contain values in the range from 0 to 255, where 0 represents black, 255 represents white,and values in between represent shades or intensity of gray. A color image can be represented by atwo-dimensional array of Red, Green and Blue triples, where 0 indicates that none of that primarycolor is present in that pixel and 255 indicates a maximum amount of that primary color.Creating ImagesAt least one high-quality digital master or archival image file should be created for each objectphotographed or scanned. From that master file derivative files can be created, such as: An access image (an image used for detailed on-screen viewing) A thumbnail image (for fast access during search, browse and retrieval). Often the thumbnailis automatically created by the digital asset management software (DAMS) used to displayimages.The Federal Agencies Digital Guidelines Initiative (FADGI), defines two master image file types thatcan be created, along with access images.CARLI Created Content CommitteeLinks revised: 2/12/20202
Archival Master ImageProduction MasterImageAccess Image Represents as closely aspossible the informationcontained in the original Produced from archivalmaster Used in place ofmaster image forgeneral web access Uncompressed, orlossless compression Unedited and “useneutral” Serves as long term,sustainable resource Can serve as surrogatefor the original Highest quality that canbe produced by thedigitizing organization Large file size Uncompressed, orlossless compression May be edited fortechnical corrections to thefile, such as adjusting toneor stitching togethermultiple images of a largeitem (e.g. maps) into onefile Generally fitswithin viewing areaof average monitor Reasonable file sizefor fast downloadtime; does notrequire a fastnetwork connection Used to create derivativefiles for access Acceptable qualityfor general research Large file size Compressed forspeed of access Stored in the TIFF fileformat Stored in the TIFF fileformat Usually stored inJPEG or JPEG2000file formatFrom the Federal Agencies Digital Guidelines Initiatives (FADGI), Technical Guidelines for Digitizing Cultural HeritageMaterials: Creation of Raster Image Files. Publication Date: September 2016. May be accessed nes/digitize-technical.htmlMaster ImagesDue to the stress of digitizing unique materials, a digital master should be generated for every objectcreated. The digital master image represents as accurately as possible the visual information in theoriginal object. If possible, both an archival master file and a production master file should be kept.CARLI Created Content CommitteeLinks revised: 2/12/20203
The archival master image’s primary function is to serve as a long-term archival record forpreservation and be used as infrequently as possible. From that file, a production master file can becreated, and used to create derivative images (access and thumbnails) or for other purposes. Theproduction master may also have to serve as the archival master file if digital storage is limited, or ifthe institution is not concerned with preservation of digital images. Digital master files are measuredin ppi (pixels per inch). Master files are most often saved to a designated server or other long-termstorage device. See the NSDA link provided above for more resources on digital preservation.Master images should be scanned at the highest level of quality the digitizing institution can produceto avoid re-handling of any original materials. Archival master images should not be edited for anyspecific output or use, and should be saved as large TIFF files with lossless or no compression.Production master images may have minimal editing.File FormatResolution in PixelsPer Inch (ppi)Bit Depth percolor channelTIFF400-600 ppi. Very finedetails might requirehigher ppi.8- or 16-bit color(24- or 48-bittotal)MasterImage,Photographsand PrintsMasterImage,OversizedItems (e.g.maps,posters)600 is preferable forhighest-quality archivalmaster.TIFF orJPEG2000Large originalsmay be savedas tiles ifnecessary.300-400 ppi. Very finedetails might requirehigher ppi.400 is preferable forhighest-quality archivalmaster.8- or 16-bit coloror grayscale,depending onoriginal (24- or48-bit total)Values can vary outside of the ranges given depending on source material. Specifications taken from FADGI, TechnicalGuidelines for Digitizing Cultural Heritage Materials: Creation of Raster Image Files. September nes/FADGI Federal Agencies Digital Guidelines Initiative-2016Final rev1.pdfCARLI Created Content CommitteeLinks revised: 2/12/20204
Creating digital master files: Guidelines for file size and resolution of digital master files will vary by collection based onend user needs, sizes and types of original objects, software specifications, available filestorage space, etc. Each library should develop specific scanning guidelines based on individual collection needsand requirements. Where possible, scanning guidelines for creation of digital master files should follow thespecifications outlined in the FADGI Technical Guidelines for Digitizing Cultural HeritageMaterials: Creation of Raster Image ines/FADGI Federal Agencies DigitalGuidelines Initiative-2016 Final rev1.pdf CARLI member libraries using CONTENTdm should not upload full resolution TIFF files tothe CARLI server. Archival image file storage is the responsibility of each contributinginstitution and must be managed locally. The CONTENTdm Project Client can automaticallyconvert TIFF files into JPEG2000 or JPEG display images.Derivative ImagesFrom the master file, derivative files can be created for general use. Derivative images can be used tocreate access image used in DAMS systems and/or for creating reproductions. They may be editedand enhanced or converted to different formats. For each master image, two derivative files are oftencreated: an access image (for more detailed onscreen viewing) and a thumbnail image (for searchingand browsing). In the case of collections using CONTENTdm, the software is configured toautomatically generate access and thumbnail images from a master file.CARLI Created Content CommitteeLinks revised: 2/12/20205
RecommendationsFile FormatResolution inPixels Per Inch(ppi)Bit Depth percolor channelAccessImageAny, normallyJPEG orJPEG2000400-200 ppi1-bit bitonal, 4- or8-bit grayscale, or8-bit color (24-bittotal)ThumbnailImageAny, often JPEGor GIF100-200 ppi1-bit bitonal, 4- or8-bit grayscale, or8-bit color (24-bittotal)Some recommendations based on 1-. 2-, and 3- star specifications from FADGI, Technical Guidelines for DigitizingCultural Heritage Materials: Creation of Raster Image Files. September nes/FADGI Federal Agencies Digital Guidelines Initiative-2016Final rev1.pdfFile Naming ConventionsEach digital object in a collection should be assigned a unique identifier. Unique identifiers shouldfollow a consistent naming format to ensure ongoing identification and retrieval of digital files.Guidelines for file names will vary by collection based on local needs and specifications. Eachlibrary should develop specific file naming conventions based on individual collection needs andlocal requirements.Examples from the CARLI CONTENTdm collections include the following:ACWC0002bru004 03 nFImages W-04.jpgLor137 01.jpgCARLI Created Content CommitteeLinks revised: 2/12/20206
More information on best practices for file naming conventions may be found on the NationalInstitute of Standards and Technology website 016-03.pdf.Monitor CalibrationMonitors used for image editing and color correction should be calibrated according to the followingspecifications1: Set to 24 millions of colors Set monitor Gamma at 2.2 Color temperature at 5000 degrees K. The FADGI recommendation is to “Adjust theillumination and color temperature of the monitor to provide the best approximation of whitein the viewing environment to the digital representation of white on the monitor.”Monitor calibration software can be selected and purchased by member libraries and will varydepending on local budgets, equipment and software specifications.Technical MetadataIn the interest of preservation and reproduction, it is helpful to capture technical metadata in thecreation of the digital image file. Digital cameras and scanners can automatically capture thisinformation and embed it in the object file. NISO Standard Z39.87-2006 (R2017) (Data Dictionary Technical Metadata for Digital Still Images) is widely accepted for use in the management oftechnical metadata. Among the attributes that can be described by the technical metadata are thefollowing2:1From FADGI Technical Guidelines for Digitizing Cultural Heritage Materials: Creation of Raster Image ines/FADGI Federal Agencies Digital Guidelines Initiative-2016Final rev1.pdf2From Best Practices for Technical zationMetadata/bestpracticesCARLI Created Content CommitteeLinks revised: 2/12/20207
file formatfile resolution (pixels per inch)dimensions (image dimension or size in inches or centimeters)bit-depth (e.g., 8-bit, 16-bit, 24-bit, etc.)color space of digital image (e.g., RGB, CMYK, or grayscale)information about method of digitization: scanner or digital camera brand, name, and modelnumber information about software used to manipulate or compress the image, including the softwarename and versionCollection of technical metadata will vary according to equipment used and local policy. Inclusion oftechnical metadata is not required for items in CARLI Digital Collections, but it can be included. Formore information, see Summary of Data Dictionary Elements, NISO Standard Z39.87-2006 (R2017)pp. 94-101, which lists mandatory containers and elements for technical atadigital-still-imagesCARLI Created Content CommitteeLinks revised: 2/12/20208
Guidelines for file size and resolution of digital master files will vary by collection based on end user needs, sizes and types of original objects, software specifications, available file storage space, etc. Each library should develop specific scanning guidelines based on individual collection needs and requirements.