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Beam Characterization of a Microfading Tester: Evaluation of Several Methods

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Beam Characterization of a Microfading Tester: Evaluation of Several Methods Title: Beam characterization of a microfading tester: evaluation of several methods Author: Paweł Świt, Marco Gargano, Julio M. del Hoyo-Meléndez Citation style: Świt Paweł, Gargano Marco, del Hoyo-Meléndez Julio M. (2021). Beam characterization of a microfading tester: evaluation of several methods. "Heritage Science" (Vol. 9 (2021), art. no. 78), doi 10.1186/ s40494-021-00556-7 Świt et al. Herit Sci (2021) 9:78 https://doi.org/10.1186/s40494-021-00556-7 RESEARCH ARTICLE Open Access Beam characterization of a microfading tester: evaluation of several methods Paweł Świt1* , Marco Gargano2 and Julio M. del Hoyo‑Meléndez3 Abstract Microfading testing allows to evaluate the sensitivity to light of a specifc artwork. Characterization of the illumination spot is important to determine its shape, dimensions, light distribution, and intensity in order to limit and account for possible damage. In this research the advantages and disadvantages of several methods used to determine the beam shape and intensity profles are described with the aim of providing various options to microfading researchers inter‑ ested in characterizing their irradiation spots. Conventional and imaging methods were employed and are compared in terms of their accuracy, cost, reliability, and technical features. Conventional methods consisted of an aperture tech‑ nique using aluminium foil and four diferent materials namely stainless steel, silicon, muscovite, and Tefon used as sharp edges. The imaging methods consisted of digital photography of illumination spot, direct beam measurement using a CMOS camera, and direct beam measurement using a laser beam profler. The results show that both conven‑ tional and imaging methods provide beam width measurements, which are in satisfactory agreement within experi‑ mental error. The two best methods were direct measurement of the beam using a CMOS camera and sharp-edge procedure. MFT illumination beam with a CMOS camera followed by a determination of the beam diameter using a direct method, more specifcally one involving a sharp-edge technique. Keywords: Beam profle, Illumination intensity, Sharp-edge method, Microfading, Spot shape Introduction objects due to the use of a high intensity spot during test- Microfading testing (MFT) has become widely accepted ing. Nevertheless microfadeometry is widely considered by the conservation science community as an adequate a non-destructive technique providing the evaluation of tool for establishing and recommending appropriate gal- the light sensitivity of objects over the last two decades lery lighting conditions that minimize damage to collec- [1–8]. Tis is due to the small spot size employed, which tions. Tese devices ofer the opportunity of measuring permits the evaluation of the durability of materials, the photostability of objects due to their optical setup, while causing no harm to the object. Te process is less which allows to conduct and quantify accelerated photo- time consuming than traditional accelerated light aging aging over a spot of approximately 0.5 mm. Also, by using tests and the results can be followed in real time. Te a high sensitivity photodetector it is possible to measure sensitivity of objects to visible light can be determined spectrocolorimetric change before it is perceived by the by short increments in exposure time and simultaneous human eye. Although a considerable amount of testing direct lightfastness measurements. Tis equipment ofers is currently performed with these instruments, there are the opportunity of direct testing the sensitivity to light still safety concerns in terms of possible damage to the of objects due to the microscopic size of the area under investigation. Prior to this equipment’s development it took weeks to acquire lightfastness data and it was not *Correspondence: [email protected] possible to obtain information directly from the collec- 1 Institute of Chemistry, Faculty of Science and Technology, University tions of unique objects. of Silesia, 40006 Katowice, Poland Full list of author information is available at the end of the article © The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://​creat​iveco​ mmons.​org/​licen​ses/​by/4.​0/. The Creative Commons Public Domain Dedication waiver (http://​creat​iveco​mmons.​org/​publi​cdoma​in/​ zero/1.​0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Świt et al. Herit Sci (2021) 9:78 Page 2 of 14 MFT is a well-established technique and many institu- temperature increase at the surface of an object caused tions are aware of the multiple advantages of microfad- by exposure to light from a microfading device has been ing such as being a unique tool for exhibition planning described by Ford [3], Lerwill [19] and Whitmore [1]. [9, 10]. Besides this, it is important to note that there are Tese three studies revealed that the temperature of the many institutions, which are not equipped with MFT investigated surface may experience an increase up to a instruments and have to just rely on illuminance meas- maximum of 5 °C. Ford has noted that these changes pre- urements. Additionally, some conservators still consider clude thermal damage to objects or having an infuence MFT harmful due to possible photo-oxidation reactions, on fading rates and mechanisms [3]. Te concern about heating of the sample, and possible interactions of indi- initiating two-photon processes has also been expressed vidual chemical components present in the sample area. in relation to fash photography [20]. However, the Other researchers have raised concerns about the difer- author concluded that modern fash units are not power- ences between the spectral power distribution and inten- ful enough to initiate two-photon degradation processes, sity of gallery lighting used to illuminate the objects and as had been suggested in the past. Te illuminance deliv- those used for MFT tests. Te authors believe that all ered to a white standard can be used to contextualize the these considerations could limit the further development discussion on potential photodegradation. For example, and spread of the MFT technique. white refectance standards, typically used in calibration Fortunately, in most cases, thanks also to the relatively of MFT instruments, have a surface with high fat spec- short testing time the techniques allows conservators tral response. It is known that factors such as movement and conservation scientists to make surveys of collec- of the light source, fber optics, or measuring probes may tions prior to their display in order to determine if there increase the uncertainty of measurements, especially are critical works having unusually high sensitivity to when working at high intensity levels. However, spectral light exposure [2, 4, 11]. Te instrument also ofers the measurements performed on a similar instrument using possibility of performing light-fastness tests on a single a barium sulphate white tile did not alter for about 40 µm material (e.g. powder pigment) or mock-ups prepared in through focus [21]. Te authors also indicated that spec- the laboratory consisting of combinations of two or more tral measurement is more sensitive than the variation in materials (e.g. pigment/binder system) [12, 13]. Recently size of the illuminated spot with probe position since the there has been considerable interest in the use of MFT alignment of illumination and measurement probes is the and the number of cultural institutions currently using key to correct interpretation of the results. or considering construction or purchase of a MFT device A well characterized spot is essential in microfading increases steadily [14]. Although some people have research since the technique makes use of a high inten- expressed concern regarding the safety of objects during sity spot that acts on the object’s surface to determine and after testing, the technique continues to be widely its stability to light. Te shape, dimensions, light distri- employed in institutions around the world. In contrast bution and intensity of the spot are important param- to research-based applications where it is important eters to assess and verify non-invasiveness. Terefore, to accurately determine the power density of the beam, characterizing the irradiation spot is essential to ensure the safety of the object under investigation is of less con- that the tests are performed in a non-invasive and non- cern in MFT as the degree of change is monitored in real destructive way. Safety of the irradiated surface is also a time and the test can be stopped once a threshold level is priority for other research applications. For example, a reached. well-defned focused laser spot is especially important Several authors have pointed out the importance of for materials processing and medical applications [22, working with a safe and stable irradiation spot when per- 23]. Tere are several methods currently used for the forming spectroscopic measurements on objects. Due to determination of the beam profle and diameter. One of concerns associated with two-photon efect at high irra- the most popular is the knife-edge method, where the diance, local heating leading to local relative humidity power of the laser beam is measured using a photo diode change, which could cause signifcant movement in the while moving a sharp edge through the laser spot using Z direction of the area being microfaded, post-exposure small distance increments [24].
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