Multispectral imaging has been applied to the field of art conservation and art history since the early 1990s. It is attractive as a non-invasive imaging technique because it is fast and hence capable of imaging large areas of an object giving both spatial and spectral information. This paper gives an overview of the different instrumental designs, image processing techniques and various applications of multispectral and hyperspectral imaging to art conservation, art history and archaeology. Recent advances in the development of remote and versatile multispectral and hyperspectral imaging as well as techniques in pigment identification will be presented. Future prospects including combination of spectral imaging with other non-invasive imaging and analytical techniques will be discussed.

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Advances in multispectral and hyperspectral imaging for archaeology and art conservation

ConspectusDiffuse reflectance hyperspectral imaging, or reflectance imaging spectroscopy, is a sophisticated technique that enables the capture of hundreds of images in contiguous narrow spectral bands (bandwidth < 10 nm), typically in the visible (Vis, 400–750 nm) and the near-infrared (NIR, 750–2500 nm) regions. This sequence of images provides a data set that is called an image-cube or file-cube. Two dimensions of the image-cube are the spatial dimensions of the scene, and the third dimension is the wavelength. In this way, each spatial pixel in the image has an associated reflectance spectrum. This “big data” image-cube allows for the mining of artists’ materials and mapping their distribution across the surface of a work of art.Reflectance hyperspectral imaging, introduced in the 1980s by Goetz and co-workers, led to a revolution in the field of remote sensing of the earth and near planets (Goetz, F. H.; Vane, G.; Solomon, B. N.; Rock, N. Imaging Spectrometry for Earth Remote Sensing. Science, 1985, ...

Reflectance Hyperspectral Imaging for Investigation of Works of Art: Old Master Paintings and Illuminated Manuscripts

In the last two decades, several new concepts for improving the performance of infrared detectors have been proposed. These new concepts particularly address the drive towards the so-called high operating temperature focal plane arrays (FPAs), aiming to increase detector operating temperatures, and as a consequence reduce the cost of infrared systems. In imaging systems with the above megapixel formats, pixel dimension plays a crucial role in determining critical system attributes such as system size, weight and power consumption (SWaP). The advent of smaller pixels has also resulted in the superior spatial and temperature resolution of these systems. Optimum pixel dimensions are limited by diffraction effects from the aperture, and are in turn wavelength-dependent. In this paper, the key challenges in realizing optimum pixel dimensions in FPA design including dark current, pixel hybridization, pixel delineation, and unit cell readout capacity are outlined to achieve a sufficiently adequate modulation transfer function for the ultra-small pitches involved. Both photon and thermal detectors have been considered. Concerning infrared photon detectors, the trade-offs between two types of competing technology-HgCdTe material systems and III-V materials (mainly barrier detectors)-have been investigated.

http://iopscience.iop.org/article/10.1088/0034-4885/79/4/046501/pdf

Challenges of small-pixel infrared detectors: a review

For decades detection and monitoring of forest and other wildland fires has relied heavily on aircraft (and satellites). Technical advances and improved affordability of both sensors and sensor platforms promise to revolutionize the way aircraft detect, monitor and help suppress wildfires. Sensor systems like hyperspectral cameras, image intensifiers and thermal cameras that have previously been limited in use due to cost or technology considerations are now becoming widely available and affordable. Similarly, new airborne sensor platforms, particularly small, unmanned aircraft or drones, are enabling new applications for airborne fire sensing. In this review we outline the state of the art in direct, semi-automated and automated fire detection from both manned and unmanned aerial platforms. We discuss the operational constraints and opportunities provided by these sensor systems including a discussion of the objective evaluation of these systems in a realistic context.

https://www.mdpi.com/1424-8220/16/8/1310/pdf

Airborne Optical and Thermal Remote Sensing for Wildfire Detection and Monitoring.

A paradigm using multispectral visible and near-infrared imaging spectroscopy is presented to semi-automatically create unbiased spectral maps that guide the site selection for in situ analytical methods (e.g. fiber optic reflectance spectroscopy and X-ray fluorescence) in order to identify and map pigments in illuminated manuscripts. This approach uses low spectral resolution imaging spectroscopy to create maps of areas having the same spectral characteristics. This paradigm is demonstrated by analysis of the illuminated manuscript leaf Christ in Majesty with Twelve Apostles (workshop of Pacino di Buonaguida, c. 1320). Using this approach the primary pigments are mapped and identified as azurite, lead-tin yellow, red lead, a red lake (likely insect-derived), a copper-containing green, brown iron oxide, and lead white. Moreover, small amounts of natural ultramarine were found to be used to enhance the blue fields around Christ, and a red lake was used to highlight different colors. These results s...

https://www.tandfonline.com/doi/pdf/10.1179/2047058412Y.0000000078

Use of imaging spectroscopy, fiber optic reflectance spectroscopy, and X-ray fluorescence to map and identify pigments in illuminated manuscripts

Reflection imaging spectroscopy is a useful technique to remotely identify and map minerals and vegetation. Here we report on the mapping and identification of artists' materials in paintings using this method. Visible and infrared image cubes of Picasso's Harlequin Musician are collected using two hyperspectral cameras and combined into a single cube having 260 bands (441 to 1680 nm) and processed using convex geometry algorithms. The resulting 18 spectral end members are identified by comparison with library spectra, fitting by nonlinear mixing, and using results from luminescence imaging spectroscopy. The results are compared with those from X-ray fluorescence spectrometry, polarized light microscopy, and scanning electron microscopy–energy dispersive spectrometry (SEM/EDS). This work shows the potential of reflection imaging spectroscopy, in particular if the shortwave infrared region is included along with information from luminescence imaging spectroscopy.

Visible and Infrared Imaging Spectroscopy of Picasso's Harlequin Musician : Mapping and Identification of Artist Materials in Situ

Reflectance imaging spectroscopy, the collection of images in narrow spectral bands, has been developed for remote
sensing of the Earth. In this paper we present findings on the use of imaging spectroscopy to identify and map artist
pigments as well as to improve the visualization of preparatory sketches. Two novel hyperspectral cameras, one
operating from the visible to near-infrared (VNIR) and the other in the shortwave infrared (SWIR), have been used to
collect diffuse reflectance spectral image cubes on a variety of paintings. The resulting image cubes (VNIR 417 to 973
nm, 240 bands, and SWIR 970 to 1650 nm, 85 bands) were calibrated to reflectance and the resulting spectra compared
with results from a fiber optics reflectance spectrometer (350 to 2500 nm). The results show good agreement between
the spectra acquired with the hyperspectral cameras and those from the fiber reflectance spectrometer. For example, the
primary blue pigments and their distribution in Picasso's Harlequin Musician (1924) are identified from the reflectance
spectra and agree with results from X-ray fluorescence data and dispersed sample analysis. False color infrared
reflectograms, obtained from the SWIR hyperspectral images, of extensively reworked paintings such as Picasso's The
Tragedy (1903) are found to give improved visualization of changes made by the artist. These results show that
including the NIR and SWIR spectral regions along with the visible provides for a more robust identification and
mapping of artist pigments than using visible imaging spectroscopy alone.

Visible and Infrared Reflectance Imaging Spectroscopy of Paintings: Pigment Mapping and Improved Infrared Reflectography

There is a strong motivation for smaller pixels based on end-user demand for lower-cost, higher-resolution camera systems both for military and commercial applications. Uncooled detector technology fits the need for a low size, weight, and power system. We explore the tradeoffs and challenges to achieving pixel designs smaller than the current 17-μm state-of-the-art detectors without loss in sensitivity or resolution. For illustration we consider a 12-μm design. We also address modulation transfer function issues as the pixel size shrinks, and examine the difference between the performance of present devices and the theoretical performance limit for uncooled detectors.

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-52/issue-6/061305/Uncooled-long-wave-infrared-small-pixel-focal-plane-array-and/10.1117/1.OE.52.6.061305.pdf

Uncooled long-wave infrared small pixel focal plane array and system challenges

An apparatus for emulating various known night sky illumination conditions. The apparatus comprises a plurality of electrically-powerable LEDs which are disposed in an array and have respective spectral curves centered at different wavelengths in the visible to the short wave infrared wavebands, and means for fixing the temperatures of the LEDs to avoid temperature-induced changes in their spectral curves. Additionally, the apparatus includes means for varying the light intensities of the individual LEDs so that the combination of their spectral curves matches the spectrum of the known night sky illumination condition to be emulated, and means for regulating the total amount of light collected from the array so that the cumulative spectrum has the same intensity as the known night sky illumination condition to be emulated.

Programmable LED spectral light source

This paper discusses the design and development of a dual field of view, all-refractive infrared optical system
that images the mid-wave infrared (MWIR) light in one field of view and the short wave infrared (SWIR) light
in the narrower second field of view onto the same detector. The narrow field of view images the SWIR
radiation at a slow f/number of 10.0, while the wide field of view images the MWIR radiation at f/1.9. The
field of view is changed via a single lens that changes its axial position within the lens, resulting in an axial
zoom and an overall magnification change of 6X. The change in focal length and f/number at the same time
enables an increased focal length without having to increase the aperture size by the ratio of the focal length
change. The large field of view change is achieved by both the motion of the lens, and the use of the spectral
properties of the materials that change with wavelength. The change in spectral bands is accomplished via the
use of a spectral filter wheel.

Roy Littleton

Papers

Visible and Infrared Imaging Spectroscopy of Picasso's Harlequin Musician : Mapping and Identification of Artist Materials in Situ

Visible and Infrared Reflectance Imaging Spectroscopy of Paintings: Pigment Mapping and Improved Infrared Reflectography

Uncooled long-wave infrared small pixel focal plane array and system challenges

Programmable LED spectral light source

Compact dual field of view SWIR/MWIR optical system