Mind the gap? Open Access spectral data for documentary heritage digitisation

by Hendrik Hameeuw & Bruno Vandermeulen

This post presents the publication of an open access spectral dataset which, beside other areas of application, provides insights and opportunities for better fine-tuned digitisation results for documentary heritage.

The Imaging Lab of KU Leuven Libraries has a strong focus on digitisation and imaging of documentary heritage. A crucial step in the digitisation process is the image creation; the transition of the physical object into a bitmap or array of pixels. During this process the colors of the object to be photographed, have to be translated into a particular colorimetric value at pixel base. The better that translation is fine-tuned, the more accurate the final result will be and the more the viewer can be assured the digital presentation matches the original object as it would appears in similar lighting conditions (=reliable data). To finetune the translation a reference color target is used in combination with specific software to build a bespoke ICC profile (=translator). This reference color target consists of an array of colored patches from which the colorimetric values are known.


… the color patches on the applied color targets do not always represent all the tints, tones, shades and hues typical for documentary heritage. So what is the problem … ?

… the digitisation of historical documentary heritage is not the same as wedding photography, product photography or even digitising the colorful art work of Kandinsky. Most of these materials, especially the historical ones, have a limited gamut (range of colors), and hold in particular less pronounced colors.

… and thus, the colors selected for most reference color targets are quite the opposite: they have a wide gamut (well spread across the theoretical color space) and they include the more pronounced colors.

Consequently, when these reference color targets are used to calibrate colors, they do not necessarily calibrate the colors photographers encounter when digitising documentary heritage. And that is unfortunate as the whole effort of profiling the colors for specific lighting conditions to obtain faithful digital representation documenting the original might be in jeopardy! Or less dramatically, we are aware this is a challenge and this job can be done better!

HOW TO ProfilE Colors

To further explore and understand the issue, let’s first take a step backwards: how is color profiled? Cameras (ranging from professional to smartphone) have embedded software or algorithms which interpret the incoming reflected light and for each pixel ‘translates’ it into a colorimetric values. These algorithms are predetermined, to assure the images look good. In fact, the standard algorithms will accentuate and shift some colors focusing more on pleasing color as opposed to accurate color. Color scientists know that, the sale managers of camera production companies know that, and such processed images are appreciated by the costumers buying those cameras. Thus, a perfect world!

When the digitisation process aims at creating a digital surrogate resembling as close as possible the original (including color), this strategy falls short. A solution is to capture the object in a raw format and process the data through specific software for processing of raw data (Capture One, Lightroom, Phocus, RawTherapee, …). In such software the color profile that comes with the camera can be disabled and replaced by a custom, tailormade profile.

Why custom made? Well, the appearance of the surface and thus the color, through the reflection of the light, changes when the lighting condition changes, such as a change in position of the light, color temperature, … In a digitisation studio all those parameters can be controlled and need to be kept the same throughout a digitisation process. In such a controlled environment, it is possible to obtain close to perfect digital representations if the correct procedures are followed and adequate hard- and software is used.

To create an in situ color profile for the specific and standardized (lighting) conditions a reference color target is used. The most popular and widely used of these targets is the ColorChecker Digital SG. The basic idea of such manual color calibration is straightforward:

  1. a standardized illumination set-up for imaging documentary heritage is established (=photo-studio)
  2. an image is taken from a reference color target for which the colorimetric values are known (=calibration target)
  3. based on the obtained result from 2. with the illumination conditions from 1. software can estimate how the colors in the data of the image should be interpreted in order to obtain a correct representation, without taking into account the color translation by the profile camera. As such, an in situ color profile is calculated. (=color profiling)
  4. the color profile calculated in 3. is applied on all images taken with the same illumination set-up as 1. (=color calibrated digitisation).

Central in this process is the reference color target from which an image is taken. This target consists of a number of different neutral and colored patches. For each patch the colorimetric values are known or measured and stored in a text file. Using a target representing all the colors in the visual spectrum is impossible. To overcome this, manufacturers of reference color targets try to include a selection of colors more or less evenly spread across the visual spectrum. At the same time they will try to include a sufficient number of tones that frequently appear in photographs, such as skin tones, sky, green vegetation, …

When the photographer has set up his equipment (camera and lighting) (1.) the target is photographed and the reflection or response in relation to the specific conditions is registered by the camera  (2.). At this moment, the response (measured energy) is uninterpreted! Color profiling software measures the response of the patches in the image and creates a table (LUT) with these values and the reference values of the target (the text file). An ICC color profile translates the measured value into the reference value. (3.). The ICC color profile is stored and the photographer applies it to all the images made within the same conditions (light position, …) (4.). Have these conditions changed (btw, that includes the position of the camera or the specific lens), a new color profile needs to be made.

Thus, with the help of a reference color targets software (in a camera or computer) can calculate how the registered (observed) energy captured by the sensor in the camera should be translated to generate the colors of a surface as they are in real.

Profiling the colors that matter

One has to define the discrepancy between the colors on the reference color target and the colors of the documentary heritage we digitize on a day to day basis. This should provide the intel to understand whether the existence of this discrepancy – the gap – is a pure theoretical problem or a real life issue. At the Imaging Lab of KU Leuven Libraries we decided to make that effort. In collaboration with our colleagues of Special Collections we defined which original historical materials are representative for library heritage and archive collections and started measuring their spectral responses with a standard reflective spectrometer (Eye-One (i1) Pro Photo) (see below: ‘The Open Access Spectral Data’). As such, an insight has been obtained by providing the spectral responses and corresponding colors with their attested tints, tones, shades and hues typical for documentary heritage.

When de spectral data is inspected it can immediately be observed the attested colors are in the yellow, brown and slightly red regions of the color space (below A & B). And secondly, when this cluster of measured colors is compared with the spread of patches on one of the most popular color calibration targets (ColorChecker Digital SG) very interesting insights are revealed (below C & D).

Visualizing and comparing the spectral data shows that the measured historical materials fall in a zone represented by very little color patches on the ColorChecker Digital SG (see also the video below for a more in depth discussion by Don Williams). That means that this zone, which represents in particular colors that are common on historical documentary heritage, will remain poorly profiled. The spectral dataset accentuates this very well.

A gap between the colors that are commonly profiled, and which should be profiled, is identified.

The profiled colors on ‘standard’ calibration targets are no perfect match with the colors which should be profiled as a gap can be observed. Consequently, even when the current digitisation standards for documentary heritage, such as metamorfoze and FADGI, are followed; it is unclear how accurate a number of specific, frequently occurring heritage colors which matter most are registered. As such, subtle variations and changes in their materiality (for ex. due to time, light exposure, conservation interactions, …), which in theory should be observable based on the colorimetric values, can remain undetected or will be poorly represented.

What NOW?

  • Further study and actions can be made. The KU Leuven Libraries spectral data shows there is still room for improvement. This needs to be explored further. The spectral dataset used for the above made conclusions counts 433 measure points, selected on typical historical documents in a Belgian Special Collections library. In the broader international context, this exercise should be repeated to establish extended spectral insights in historical documentary heritage across cultural and material traditions.
  • Based on the above, it seems wise to populate color calibration targets with other/extra patches more closely related to the type of imaged materials. This is not new, for the DT Next Generation Target (v2) a similar exercise has already been done, leading to a calibration target with extra ‘heritage colors’. The KU Leuven data has also already been matched for the selection of color patches on the new FADGI ISO 19264 (for a comparison see the video above). These new targets will need to be tested, not only in their ability to calibrate standardised colors in general, but more importantly in checking out their added value in color profiling documentary heritage materials.
  • To facilitate and support future activities and research with spectral data of historical (documentary) heritage, this data should be made available for the broad community of heritage scientists.


The entire KU Leuven Libraries spectral data has been published online as open data. Together with the needed documentation this give the opportunity to use this data for any future work for which spectral characterisation of documentary heritage materials is wanted.
The dataset is published on zenodo.org as Hameeuw Hendrik, Vandermeulen Bruno, Van Cutsem Frédéric, Smets An & Snijders Tjamke (2021): KU Leuven Libraries Open access Spectral data of historical paper, parchment/vellum, leather, inks and pigments (Version 1.0) [Data set]. http://doi.org/10.5281/zenodo.3965419.
Feel free to work with the dataset. When you do, do not hesitate to reach out to us if you have any questions. If you have feedback and/or are interested to collaborate on the topic: digitalisering@kuleuven.be.

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