“Making the Invisible Visible” – Creation of a multiband image dataset based on an established multispectral imaging methodology*, using the FUJIFILM GFX100 II IR.
Multispectral imaging – A key to new insights
Multispectral imaging is gaining increasing importance across numerous disciplines. It enables the acquisition of information that remains invisible to the human eye and to conventional photography, and supports the detailed characterisation of materials, structures, and surface properties. The resulting increase in knowledge is achieved through a contactless and non-invasive approach, making it particularly suitable for the examination of sensitive or unique objects. This opens up a wide range of applications in both research and applied science.
Typical fields of application include art analysis, where underdrawings can be revealed, retouchings identified, and pigments distinguished. In forensic science, multispectral imaging enables the visualisation of latent fingerprints, the detection of document manipulation, and the analysis of trace evidence. Materials testing benefits from the ability to precisely detect fine cracks, coating defects, or inhomogeneous structures. In archaeology, the method allows the examination of sensitive artefacts and manuscripts without physical contact. All of these applications are based on the distinct absorption, reflectance, and luminescence behaviour of materials in the ultraviolet and infrared spectral ranges, which enables more precise differentiation and analysis than is possible with conventional photography.
The Expert
Annette T. Keller, an expert in multispectral imaging and analytical methods, has over 25 years of experience in documentation and art diagnostics, as well as more than seven years of experience in the field of forensic analysis. Since 2000, she has maintained close collaborations with institutions and museums worldwide and has worked on internationally significant works of art. This extensive expertise enabled a well-founded assessment of the performance of the FUJIFILM GFX100 II IR.
FUJIFILM GFX100 II IR – Imaging with extended spectral sensitivity
The FUJIFILM GFX100 II IR provides a powerful tool for this purpose: a large-format camera with 102 megapixels that delivers exceptional image quality not only in the visible range but also in the near-ultraviolet and infrared spectral regions. It offers a dynamic range of more than 14 f-stops and a 16-bit colour depth, enabling the precise capture of even the finest gradations in brightness and colour. The full functionality of the standard GFX100 II version is available, including precise autofocus, a wide range of shooting modes, and direct control via computer tethering. In addition, the camera supports industrial integration via a software development kit (SDK), allowing flexible integration into analysis and automation systems. For imaging applications, selected FUJINON GF lenses can be used, ensuring optimised imaging performance across a wide focal length range, including the near-ultraviolet and infrared spectral regions.
Background and Objectives
As part of a practical test, the performance of the FUJIFILM GFX100 II IR was evaluated under realistic conditions. Individual images were acquired in various spectral ranges, assessed, and analysed with regard to their relevance for the respective applications. The test was conducted under the expert supervision of Annette T. Keller to ensure a qualified assessment of the camera and its suitability for multispectral documentation and analysis in scientific and cultural contexts.
Test Setup and Image Acquisition Process
The practical test using the FUJIFILM GFX100 II IR was based on a clearly structured setup:
• A defined workflow for the creation of a multiband stack according to the CHARISMA Manual (a systematic image series consisting of recordings from different spectral ranges, enabling comparative and comprehensive analysis)*
• Use of different light sources and optical filters for various wavelength ranges and imaging modalities
• Systematic testing of the extended spectral sensitivity of the camera and lenses
Results and Findings
The FUJIFILM GFX100 II IR demonstrated its suitability for multispectral multiband imaging during the test. The GFX system offers a range of lenses appropriate for these requirements, covering a broad focal length spectrum, including macro applications.
Various imaging modalities in reflectance and luminescence were employed using different illumination setups and spectral filtering, producing meaningful results
across a wide wavelength range. The following suitable lenses were used:
GF45mmF2.8 R WR, GF63mmF2.8 R WR, GF80mmF1.7 R WR, and GF120mmF4 R LM OIS WR Macro.
• Visible reflectance (VIS-R): Documentation in the visible spectral range serving as a reference basis.
• Ultraviolet reflectance (UV-R): Observation of surface-related material properties and conditions that are not visible to the naked eye.
• Infrared reflectance (IR-R): Visualisation of, for example, underdrawings and material differentiations that appear similar in the visible range.
• Visible luminescence (UV-excited): Observation of materials such as varnishes and binding media based on their fluorescent properties.
• Infrared luminescence (VIS- or UV-excited): Identification and differentiation of pigments and layers that emit in the infrared range when excited by visible or ultraviolet light.
The information contained in each image is highly multidimensional and provides significantly more relevant insight into materials, condition, painting technique, and the overall “readability” of the scene than conventional imaging alone.
Individual images of the completed stack using the different imaging modalities (© artIMAGING, Annette T. Keller and FUJIFILM, Renate Lange):
Visible reflectance (VIS-R_vis):
The visible image serves as a reference for locating and interpreting phenomena observed using other imaging modalities in the non-visible spectral ranges.
Ultraviolet reflectance (UV-R_uv):
Observation of reflectance in the ultraviolet spectral range allows different white pigments to be distinguished. Titanium white and zinc white can be identified due to their strong absorption in the UV range, whereas lead white exhibits higher reflectance and can therefore be excluded.
UV false-colour image (UVFC):
The UV false-colour image is generated by combining the visible image with the UV reflectance channel and displays the results using artificial colours. Modern white pigments such as titanium white and zinc white appear in yellow tones, allowing lead white (rendered as white) to be reliably excluded.
Infrared reflectance (IR-R_ir):
Infrared reflectance imaging reveals underdrawings in the case of IR-transparent pigments, as well as later modifications to the composition (pentimenti), for example in the hand area, and the signature in the lower left. In this example, the deep blue Prussian blue pigment in the upper right blocks infrared radiation. Due to its chemical composition, infrared radiation does not penetrate through to the canvas in these areas, preventing underlying layers or drawings from becoming visible.
Infrared false-colour image (IRFC):
The infrared false-colour image is generated by combining the visible image with the infrared reflectance channel. Differences in the reflectance and absorption properties of blue pigments are translated into distinct artificial colour tones, enabling blue pigment differentiation. For example, Prussian blue (rendered as dark violet in the upper right) can be clearly distinguished from ultramarine, which appears in strong red or pink tones.
Visible luminescence under UV excitation (VIS-L_uv365):
This UV-excited luminescence is often referred to as UV fluorescence. Many red pigments absorb nearly all ultraviolet radiation and therefore do not exhibit fluorescence. In this case, the presence of luminescence is observed, indicating the presence of madder lake.
Infrared luminescence under UV excitation (IR-L_uv365):
When the image is excited with ultraviolet light, certain pigments such as cadmium-based pigments emit infrared luminescence. This makes it possible to visualise the distribution of modern cadmium pigments.
Infrared luminescence under visible light excitation (IR-L_vis):
Infrared luminescence can also be used under visible light excitation to visualise pigment distribution. In this example, the distribution of cadmium pigments (red and yellow) becomes apparent.
This method is also effective for the clear identification of very ancient pigments such as Egyptian blue and Han blue.
Annette T. Keller explicitly emphasises that the multiband approach must provide sufficiently robust and reliable image-based evidence to support material identification and interpretation before further quantitative, measurement-based analytical methods are applied on this basis. The method is particularly well suited for visualising the distribution of materials, retouchings, and underdrawings in the case of infrared-transparent pigments. It should be regarded as a qualitative approach, as insufficient data are available for direct quantitative evaluation. Nevertheless, the information obtained is highly relevant for improving the understanding of an object and represents the recommended first step in any comprehensive investigation campaign.
Assessment by Annette T. Keller
Annette T. Keller evaluated the results as a demonstration of the performance of the FUJIFILM GFX100 II IR. She summarises her assessment as follows:
“The tested camera–lens combinations are very well suited for multiband photography in combination with the appropriate filters, light sources, and reference materials. They expand the possibilities of digitisation and documentation by providing relevant information about the object. Even if not all image information contained in a multiband stack may appear relevant to every user, I recommend making use of the complete information and selecting the relevant methods only after reviewing the results on an object-specific basis. Features that are invisible can only be fully assessed once a complete stack has been created. And indeed, based on experience, this process often reveals not only a clear increase in readability or material classification, but also new questions that require further clarification and investigation.”
The practical test demonstrated that the FUJIFILM GFX100 II IR is fully suitable for multispectral imaging applications. Its combination of 102 megapixels extended ultraviolet and infrared sensitivity, and flexible handling makes it a valuable tool for contactless analysis in research, cultural heritage, and applied sciences.
* The combined multiband stack according to the CHARISMA approach (Cultural Heritage Analysis for Regional and Integrated Spectral Multimodal Assessment, developed within European research projects under the leadership of the British Museum – Joanne Dyer and Giovanni Verri) enables a precise comparison of individual spectral recordings and thus provides a comprehensive extension of information beyond the visible range. In addition, the use of electronically generated infrared false-colour (IRFC) and ultraviolet false-colour (UVFC) images is recommended. These add a visually expressive layer that supports material classification by clearly differentiating differences between individual pigments or surface properties that cannot be distinguished with the naked eye. As such, they provide relevant information that is valuable for both art-historical and scientific interpretation.
Link : https://artimaging.de/
