Image Science Associates – Golden Thread image quality control system

Digitising a collection is an enormous task. It requires the coordinated work of a great many specialists and serious institutional resources. That is why it is especially important that the quality of the finished material is adequate and that the most accurate digital copy of the collection’s objects is produced. The aim of cultural heritage digitisation is to produce, of the objects, a preservation-grade digital object (Preservation-grade Digital Object) that faithfully records the current state of the object, can be used as a reference and, for certain research tasks, can even replace the original object.

For the digital object (image and data together!) to be of reliable quality that can also be verified later, objective standards must be applied. In international museum-technology practice, digitisation that complies with the FADGI (Federal Agencies Digital Guidelines Initiative), the European Metamorfoze and the international museum-technology standards currently under development, under the code name JWG 26, which combine several ISO standards, is expected. These institutional requirements define image quality with objective, measurable, numerical target values. In Hungary the “White Book” (Fehér Könyv) provides guidance for the digitisation of cultural heritage in public collections, but this document currently does not contain data that numerically define the expected image quality.

The FADGI and Metamorfoze standards both describe the image-quality requirements with several parameters, of which the most important are defining the tolerances for deviations in colour, tone and detail reproduction, and setting the maximum acceptable level of noise and sharpening.

Both requirement systems define several compliance levels; the highest of these is the Preservation level. FADGI 4* and the Metamorfoze Strict grade both require a very high overall image quality to achieve this.

In practice, verification works as follows: after the shooting conditions have been set up, a photograph taken of a suitable test chart (target) is measured and evaluated by software, compared with the ideal reference chart, and the deviations, or the compliance, are reported numerically.

The Image Science Associates Golden Thread method

Many kinds of test chart exist; the Golden Thread target developed by Image Science Associates is one of the most complex. It is used to set and check the following parameters: Exposure, Sharpness, Resolution, Tone scale, Colour, Detail reproduction, Colour registration error, Uniformity (colour and tone), Noise, Sharpening.

Golden Thread Device Level Target ©ISa

It exists in three different versions: the Device Level Target (“device check chart”), a test chart that checks all the conditions of the camera, lens, lighting and capture; the Object Level Target (“object check chart”), which must appear next to every object on the reproductions; and the Standard Format Film Targets (monochrome), used for setting up the digitisation of transparent materials. The Golden Thread Analysis software is also part of the system, but evaluation and rating can also be carried out with other programs (Open DICE, Delt.ae).

A photograph of the Device Level Target must be taken before and after each production stage, or at the beginning and end of every day. This digitally records the state of the camera and of all conditions at the time the images are taken. It also makes later traceability possible.

The Object Level Target must appear in the shot next to every object. This tool makes it possible to record, and to numerically verify, the archival quality of the object in the image.

For example, the L*a*b values of field no. 15 on this chart are L62.15, a-1.07, b0.19. So even in 100 years it will be possible to read off numerically what tone appeared in the image.

Golden Thread Object Level Target #15

The standards also define the expected level of sharpness and detail reproduction.

Although a high theoretical resolution can be set for conventional scanners too, the richness of detail of the reproduction does not actually depend on this.

The detail reproduction of a reproduction can be quantified by the so-called sampling efficiency (Sampling Efficiency), calculated from the MTF (Modulation Transfer Function) or SFR (Spatial Frequency Response) value. This essentially means how faithfully the reproduction renders an ideal grid, taking into account every element of the reproduction – that is, all the conditions of the capture, the camera, the lens and the software settings alike. The FADGI 4* requirement is strict in this respect too, demanding an SFR value of ≥90%.

DeltaE explained ©BYK-Gardner

Colour deviations are measured by measuring the difference between the values given on the target and the digitally recorded colours. The Delta E 2000 method, developed by the CIE (Commission Internationale d’Éclairage, the International Commission on Illumination), the international organisation concerned with the scientific description of colours, uses a complex algorithm to calculate and quantify the colour deviation.

The FADGI 4* requirement allows a very small colour deviation (ΔE2000 <3), thereby guaranteeing objective colour accuracy.

Summary

Checking image quality is an indispensable element of object digitisation. The ISa Golden Thread method examines every element of the whole workflow, so quality control carried out with this tool truly guarantees the compliance of the digitisation.

The theoretical background of the method is very complex and scientifically founded, yet in practice the check is simple and can be learned with a short training course.

The Golden Thread system can be ordered here: https://www.tripont.hu/gyartok/golden_thread

If you would like detailed information about any of our products or services, or would like to take part in an (online) demonstration, contact our colleagues at the e-mail address chs@tripont.hu.

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