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An enhanced optical micro-fading device
Abstract
This paper introduces a new set-up for the determination of colour change on cultural heritage objects, referred to herein as a stereo-microfading tester. The system uses high quality optics through the implementation of a stereo-microscope as its central element. This technology enables new developments such as incorporation of high quality imaging systems, and separation of fading and colour measurement processes. This paper describes this new micro-fading set-up and evaluates its performance against traditional devices based on the measurement of blue wool standards. The results show a correlation between the fading performance obtained on different devices while highlighting a significant variability inherent to the blue wool samples
... In view of the above characteristics, it was capable of conducting accelerated fading measurements in a very short time, for example less than 30 min. With the advantages of less time consuming than traditional accelerated aging methods and in situ testing in a microscopic area, it was broadly acknowledged as a nondestructive technique for lightfastness assessment for the last two decades [6][7][8] . Color changes during the fading process were generally evaluated by color differences based on CIE76 standard ( E 76 ) which is more suitable for archaeological application [ 9 , 10 ]. ...
... Light is cast over an area so small, that the fading effect is almost unperceivable by the naked eye, making this technique appropriate for the measurement of real artworks. Since the pioneering work of Whitmore et al. [13], who was the first to design a microfading setup, several other systems with increased spectral accuracy and higher portability for in-situ handling have been proposed [14][15][16]. As a consequence, microfading has enabled the color degradation analysis of a series of artworks from museum collections [17,18] and even outdoor rock art specimens [19]. ...
In a previous article, we modelled the spectral and temporal dimensions of the photodegradation behaviour of pigments in the painting “A Japanese Lantern” by Oda Krohg. In particular, we extracted the endmembers and spectral fading rate of pigments by applying tensor decomposition on a time-series of spectroscopic point measurements. Now, we capture the same painting with a hyperspectral imaging setup and propose an approach to render the fading effects as 2D images. More precisely, from the hyperspectral image, we compute the concentration maps of each previously identified endmember with a least-squares unmixing method. Subsequently, by using tensor algebra, we multiply the concentration maps with the endmembers and their corresponding fading rate and obtain a 4D tensor where each pixel in the image is described by a spectrum and a fading function. This way, we generate past and future spatio-temporal simulations of the painting’s appearance by reversing and elevating light exposure, respectively.
... The MFT consists of a probe containing several lenses to focus a high energy light source into a small spot (≈0.3 mm) to induce accelerated light ageing. Advances are still being made to the set-up, indicating that this method is of interest to many [40,41]. The use of an MFT takes little time and the result can be followed in real-time. ...
It has been known for many years that dyes and pigments are subject to light-induced degradation, or photodegradation, when exposed to light. It is the very reason why some beverages or medicines are wrapped in light-tight packaging materials, and why museums cover their windows with UV-blocking filters. The exact chemistry of light-induced degradation can be quite complex. Why and how are these dyes and colorants affected by light? How fast do these processes occur? Are there ways to prevent this from happening in a straight-forward and durable way? These were and still are questions that are relevant to the many fields where colorants are applied. In order to support these questions, we have tried to provide a broad overview of the research that has already been conducted on photodegradation of dyes and pigments and the analysis of photodegradation products. In those papers, the most important parameters that were discussed are the influence of the irradiation source, intensity and time, the presence or absence of oxygen, temperature, the effects that catalysts have, as well as the dye or pigment concentration. Additionally, we have investigated the differences found for photodegradation in solution and on substrates and specific parameters that may affect the processes in these media.
Array programming provides a powerful, compact and expressive syntax for accessing, manipulating and operating on data in vectors, matrices and higher-dimensional arrays. NumPy is the primary array programming library for the Python language. It has an essential role in research analysis pipelines in fields as diverse as physics, chemistry, astronomy, geoscience, biology, psychology, materials science, engineering, finance and economics. For example, in astronomy, NumPy was an important part of the software stack used in the discovery of gravitational waves1 and in the first imaging of a black hole2. Here we review how a few fundamental array concepts lead to a simple and powerful programming paradigm for organizing, exploring and analysing scientific data. NumPy is the foundation upon which the scientific Python ecosystem is constructed. It is so pervasive that several projects, targeting audiences with specialized needs, have developed their own NumPy-like interfaces and array objects. Owing to its central position in the ecosystem, NumPy increasingly acts as an interoperability layer between such array computation libraries and, together with its application programming interface (API), provides a flexible framework to support the next decade of scientific and industrial analysis.
Drawing with fineliners and rollerball pens has become one of the most popular artistic activities for many reasons. They are
comfortable to use, are available at relatively low cost and in a wide range of colors.
Hundreds of artworks created with that type of pens are now being exhibited in galleries and museums worldwide. Unfortunately,
several of the colorants used in these pens were not intended to be permanent, and light can seriously damage them.
We examined the light sensitivity of some ink based pens commonly used by contemporary artists and graphic designers
through microfading testing. Modern pen drawings by Sol LeWitt and Jorinde Voigt from the Kupferstichkabinett Berlin were
also analyzed.
The microfade tester is used to assess fading rates of fugitive colors of collection items. The paper presents the research considerations to design a simple, less expensive and portable contact microfade tester that could serve as a screening tool for conservators. Hardware design for such an instrument is presented that includes variations in light source (xenon or LED) and measuring head (ball lens or angled holder fiber), and does not need refocusing between measurements. Performance of the portable microfade tester versions was tested on lab samples and a paper based collection item and based on the ability to rank light sensitivity relative to that of ISO Blue Wool Standards 1, 2 or 3. The results are compared to the ranking obtained with the bench instrument when testing sensitivity of the same items for the same duration. All versions of the portable hardware were found suitable for use as a screening tool to discriminate light-sensitive collection items, with performance of the portable microfade testers using an LED optimized for samples more fugitive than Blue Wool Standard 3. These portable microfade testers need not replace the bench microfade tester as the presented portable microfade tester versions are only suitable to test collection items tolerating surface contact with the instrument.
A technique for determining the lightfastness of artifacts is presented. The method developed gives reliable results with a well-defined degree of precision and allows confident prediction of the time it will take for light-induced colour changes to become visible. It uses an integrating head both to focus intense illumination onto selected sampling areas and to allow reflectance spectra to be recorded at desired intervals using a dual-beam spectrophotometer, thereby overcoming the chief sources of uncertainty associated with the alternative techniques considered (relocation, exposure, precision of data). Data are converted to CIELAB coordinates. Changes in the principal components of colour (lightness, hue and chroma) are monitored as a function of exposure, as well as the overall CIE94 colour change. All uncertainties are derived directly from the (normally distributed and random) uncertainty in reflectance spectra and not simply from data clusters. The technique is being used to determine the lightfastness of the Bullerswood carpet, designed by William Morris, which will feature in the Victoria & Albert Museum's new galleries dedicated to British art and design.
A colour-difference equation based on CIELAB is developed. It includes not only lightness, chroma, and hue weighting functions, but also an interactive term between chroma and hue differences for improving the performance for blue colours and a scaling factor for CIELAB a* scale for improving the performance for gray colours. Four reliable colour discrimination datasets based upon object colours were accumulated and combined. The equation was tested together with the other advanced CIELAB based equations using the combined dataset and each individual dataset. It outperformed CMC and CIE94 by a large margin, and predicted better than BFD and LCD. The equation has been officially adopted as the new CIE colour-difference equation. © 2001 John Wiley & Sons, Inc. Col Res Appl, 26, 340–350, 2001
The design and experimental method for the use of a novel instrument for lightfastness measurements on an artwork is presented. The new micro-fading spectrometer design offers increased structural stability (which enables portability) and increased versatility over the previous, published design, broadening the scope of locations at which data can be acquired. This reduces the need for art handling or transportation in order to gain evidence-based risk assessments for the display of light-sensitive artworks. The instrument focuses a stabilized high powered xenon lamp to a spot 0.25 mm diameter (FWHM) while simultaneously monitoring colour and spectral change. This makes it possible to identify pigments and determine the lightfastness of materials effectively and non-destructively. With 2.59 mW or 0.82 lumen (1.7·107 lux for a 0.25 mm focused spot) the instrument is capable of fading Blue Wool 1 to a measured 11 ΔEab value (using CIE standard illuminant D65) in 15 min. The temperature increase created by focused radiation was measured to be 3 to 4 °C above room temperature. The system was stable within 0.12 ΔEab over 1 h and 0.31 ΔEab over 7 h. A safety evaluation of the technique is discussed whichconcludes that some caution should be employed when fading smooth, uniform areas of artworks. The instrument can also incorporate a linear variable filter. This enables the researcher to identify the active wavebands that cause certain degradation reactions and determine the degree of wavelength dependence of fading. Some preliminary results of fading experiments on Prussian blue samples from the studio materials of J. M. W Turner (1755-1851) are presented.
Since 2008, the Canadian Conservation Institute (CCI) has provided microfade testing (MFT) as a service to more than 13 Canadian museums, archives, and galleries. In addition to obtaining lightfastness data for objects and collections planned for exhibition, MFT is also used for research and for training. This article summarizes the experiences and practices that arose from the variety of objects tested, the demand for the service, and the lessons learned. These include protocols to ensure reliable and reproducible results with multiple users, ways to report large number of results, and how MFT data are used with the CCI Light Damage Calculator for effective communication of the results with other museum staff for exhibit planning.
It is increasingly necessary for researchers in all fields to write computer code, and in order to reproduce research results, it is important that this code is published. We present Jupyter notebooks, a document format for publishing code, results and explanations in a form that is both readable and executable. We discuss various tools and use cases for notebook documents.
The effect of visible radiation of different wavelengths on the deterioration of several fugitive artists' pigments and dyed textiles has been assessed. For red and yellow pigments, damage decreases as the wavelength increases. The relationship between wavelength and colour change for blue pigments and for ISO blue wool standards nos. 1-3 is less straightforward: Radiation in the blue region is not necessarily the most damaging for these colorants. The deterioration caused by particular wavelengths depends on the energy of the radiation and the spectral absorbance properties of the colorant. The implications of the results on the selection of light sources and the use of blue wool standards to assess damage to museum collections are discussed.
In our laboratory we have developed a micro-fading tester, a device capable of performing non-destructive accelerated fading tests on submillimetre areas of coloured materials. The original purpose of these micro-fading tests was to provide rapid ‘screening’ of the colourants present on artifacts and to identify those having extreme sensitivity to visible region light exposure. Recent field tests have made it clear that another important application of these test results is the prediction of colour changes that would result from light exposure in particular exhibition conditions and schedules. In this paper we explore the accuracy of the micro-fading tests, and the factors that affect that accuracy. An overall judgment of the use of micro-fading test results and the prospects for future refinement of the tests are discussed.
A modified microfading spectrometer incorporating a linear variable filter is used to investigate the wavelength dependence of fading of traditional watercolour pigments, dosimeters and fading standards at a higher spectral resolution and/or sampling than had previously been attempted. While the wavelength dependence of photochemical damage was largely found to correlate well with the absorption spectra of each material, exceptions were found in the case of Prussian blue and Prussian green pigments (the latter includes Prussian blue), for which an anti-correlation between the spectral colour change and the absorption spectrum was found.
It is frequently assumed that sensitive museum materials follow the reciprocity principle of light exposures. Thus, equivalent exposure doses obtained by using either high-illuminance levels for short periods of time or lower illumination for longer exhibition periods are believed to cause similar degrees of damage to an object. Microfading spectrometry permitted the investigation of this phenomenon by evaluation of light-induced changes in a series of samples. The effects of equivalent exposure doses on materials such as Blue Wool 1, LightCheck Ultra, and various dyed silks from a reference collection were compared. The results indicate that reciprocity is obeyed by the most stable colorants, while materials with lower stability to light may experience deviations that are proportional to the intensity of illumination. This study confirms that reciprocity failure is associated with the use of high-intensity lamps during accelerated-aging trials. Therefore only those tests conducted at low-illumination intensity ranges where reciprocity holds should be employed when one estimates the extent of damage occurring in a museum environment.
Matplotlib is a 2D graphics package used for Python for application development, interactive scripting, and publication-quality image generation across user interfaces and operating systems. The latest release of matplotlib runs on all major operating systems, with binaries for Macintosh's OS X, Microsoft Windows, and the major Linux distributions. Matplotlib has a Matlab emulation environment called PyLab, which is a simple wrapper of the matplotlib API. Matplotlib provides access to basic GUI events such as button_press_event, mouse_motion_event and can also be registered with those events to receive callbacks. Event handling code written in matplotlib works across many different GUIs. It supports toolkits for domain specific plotting functionality that is either too big or too narrow in purpose for the main distribution. Matplotlib has three basic API classes, including, FigureCanvasBase, RendererBase and Artist.
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