Digitisation glossary

Bit depth

The level of detail of the colour information of a pixel. The usual level is 8 bits per channel (24-bit colour), while 16 bits per channel (48-bit colour) is the highest-quality archival level, allowing conservation and colour analysis.

Calibration target (colour target)

A colour and grey scale of known values placed in the shot (e.g. a FADGI/UTT target), against which the colour accuracy and tones of the digitisation can be objectively measured and corrected.

Checksum (fixity)

A unique fingerprint calculated from a file (e.g. MD5, SHA-256) which, when recalculated regularly in the archive, shows if a file has been damaged or changed, even by a single bit.

Colour space

The mathematically described range of colours that can be captured and displayed (e.g. sRGB, Adobe RGB, ProPhoto RGB). A correctly managed colour space, together with the embedded ICC profile, ensures faithful, device-independent colour reproduction.

Derivative (access copy)

A smaller, compressed version derived from the master copy (JPEG, PDF, web image) for everyday browsing, downloading and online display. It can be replaced or regenerated without compromising the original archival quality.

Digital preservation

The process and strategy that ensure the long-term readability and integrity of digital files in the face of the obsolescence of media, formats and technologies – through regular migration, redundant storage and integrity checks.

Digitisation

Converting information held on a physical carrier (book, manuscript, photograph, film, object) into a digital form that can be stored and searched on a computer, through imaging and processing, to archival quality.

DPI (dots per inch)

The unit of digitisation resolution: the number of pixels captured per inch. In cultural heritage digitisation the standard is typically 300–600 DPI for text-based material and a higher resolution for small objects.

Dublin Core

An internationally widespread, simple descriptive metadata standard with fifteen core elements (title, creator, date, subject, rights, etc.), which enables data exchange and aggregation between institutions.

Dynamic range

The range between the brightest and the darkest pixels that still carry detail, measured in exposure values (EV, stops). A wide dynamic range is needed so that the tonal nuances of a negative or of a high-contrast object are not lost.

Europeana / aggregator

Europeana is the common digital aggregation portal of European cultural heritage; an aggregator (in Hungary, e.g. Forum Hungaricum) passes institutional material on to it with uniform metadata, making the material more visible and easier to research.

FADGI

The Federal Agencies Digital Guidelines Initiative is a US system of quality guidelines for digitisation. Its star rating (1★–4★) grades the colour accuracy, sharpness and noise level of the digitisation; 4★ is the highest, archival level.

Georeferencing

Fitting historical maps or aerial photographs to real geographic coordinates, so that they can be overlaid on present-day maps and analysed in a geographic information system (GIS).

Glass plate negative

A light-sensitive glass carrier used in the late 19th and early 20th centuries. It has an extremely high resolution but is fragile; it is digitised with calibrated transmitted light and gentle handling.

glTF

An open, compact standard format for displaying three-dimensional models on the web (“the JPEG of 3D”). It is used to publish models that can be rotated interactively in the browser, without a plug-in.

HTR (handwritten text recognition)

Handwritten Text Recognition – the machine-learning-based version of OCR extended to handwriting, which makes the text of historical manuscripts searchable. Its model is based on training samples of the period and handwriting style in question.

ICC profile / colour management

A standardised colour description that ensures the colours of the digitised image are displayed faithfully, independently of the device. A calibrated, profiled workflow is the basic prerequisite for faithful reproduction.

IIIF

International Image Interoperability Framework – an open standard for publishing and displaying high-resolution images and their metadata in a uniform way. It allows material from different institutions to be accessed in the same viewer, with deep zoom, and compared side by side.

Master copy (master)

The primary, lossless, highest-quality file of a digitisation (typically TIFF), which is archived for the long term. Every access and display copy is derived from it; the master copy itself is not edited.

Metadata

Descriptive, technical and preservation information assigned to a digital object (title, origin, rights, resolution, format), which makes the content searchable and identifiable and keeps it manageable in the long term.

Metamorfoze

A digitisation quality standard of Dutch origin, widely used in Europe, for cultural heritage material; like FADGI, it lays down measurable, objective requirements for image quality.

METS / ALTO

Two XML formats that are standard in library and archive digitisation: METS describes the structure and metadata of the digital object, while ALTO stores a description of the text and layout of the page (with coordinates).

Microfilm and microfiche

Analogue, reduced-size photographic media for the mass preservation of documents. The purpose of digitising them is to make the content searchable and available online, and to spare the original, ageing film.

Moiré

A distracting interference pattern that can arise when digitising halftone prints or textile patterns, caused by a clash between the resolution and the pattern. It can be avoided with a suitable resolution, angle or filtering.

Multispectral imaging

A series of images, also taken in ranges beyond visible light (UV, infrared), that reveals details the naked eye cannot perceive: faded writing, corrections, underdrawings – for conservation and research.

OCR (optical character recognition)

Machine recognition of the text in a scanned image, which produces searchable, copyable text. For historical material it is often supplemented with manual post-processing or with HTR (handwritten text recognition) technology.

PDF/A

The version of PDF standardised for long-term archiving (ISO 19005), which preserves the document as a self-contained file, without external dependencies and with unchanged rendering – which is why it is an established format for archival preservation.

Persistent identifier

A durable, unchanging reference (e.g. DOI, ARK, Handle) to a digital object that keeps working even if the storage location or web address changes in the meantime – the basis of scholarly citability.

Photogrammetric point cloud

The intermediate result of 3D digitisation: a large set of points positioned in space that describe the surface of the object, from which the final, textured three-dimensional model (mesh) is built.

Photogrammetry

A true-to-scale three-dimensional (3D) model calculated from several photographs taken from different viewpoints. An established method for the digital documentation of objects, archaeological finds and built heritage.

RAW

The unprocessed raw data of the camera sensor, which retains the most scope for later correction (white balance, tone, colour space). In reproduction photography, the calibrated, archival master copy is made from the RAW file.

Reproduction photography

High-resolution, calibrated photography of flat or three-dimensional objects on a fixed copy stand, with even, gentle lighting – a method for documenting manuscripts, paintings and objects.

Structured-light scanning

A high-precision 3D technology: light with a known pattern is projected onto the object, and the geometry of the surface is calculated from the distortion of the pattern. Suitable for micron-level documentation of small and medium-sized objects.

TIFF

An image format for storing images uncompressed or with lossless compression, and the standard storage format for the primary, archival master copy of a digitisation; the access copies (JPEG, PDF) are derived from it.

V-shaped book cradle

A gentle scanner support that does not open the volume wider than 100–120°, so that the binding and the spine are not damaged. An established tool for digitising rare and fragile volumes (e.g. ScanRobot, BookTEK).

Vinegar syndrome

The decay of acetate-based film (microfilm, negatives), signalled by a characteristic smell of vinegar. The process is irreversible and accelerating – which is why digitising the affected analogue material is a time-sensitive rescue.

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