Research

Traditionally, most attention in Bosch research focuses on the meaning of the artworks by Jheronimus Bosch. Issues of painting technique, workshop participation, and condition of the works have received much less consideration. To convey the meaning of these often enigmatic images, however, the clearest possible view of them is crucial. Therefore, the BRCP’s initial concern is for the careful, standardized documentation of the paintings.

All paintings are documented with infrared reflectography and ultrahigh-resolution digital macro photography, both in infrared and visible light. X-radiographs are digitized. This photographic documentation is processed via stitching and coregistration, and specially developed user-friendly viewers allow careful comparison of the images. Together with microscopic study of the paintings, all of this enables us to write extensive research reports describing the techniques and conditions of the works. These form the basis for further research and conclusions about the iconography and (symbolic) meaning of the pictures.

Below are some technical details for our project: a description of our imaging setup, overview of our digital image processing and online viewers, and a glossary of terms.


Imaging

Infrared reflectography
CameraQueen’s University’s OSIRIS infrared camera.
Spectrograph arrayInGaAs array, spectral response 900–1700 nm.
Standard lens6-element Rodagon f/5.6, 150mm–, specially coated for NIR.
Standard lens documentation settings 400 x 400 mm of paint surface; 4,096 x 4,096 pix (260 ppi).
Camera movement on XY device 320 mm V&H.
Working distance 900 mm front of body to painting; focusing scale: 43 mm.
Illumination EV 7.7 at 100 ISO; aperture f/9.5.
Macro lens f=75 mm, Rogonar f/4.5, 75 mm, specially coated for NIR.
Macro lens documentation settings 36 x 36 mm of paint surface; 4,096 x 4,096 pix (2884 ppi).
Working distance352 mm front of body to painting, lens at maximum extension, 270 mm.
Macro photography
Camera Hasselblad H4D-60-IR.
Lens f/4, 120 mm macro.
Filter BG-39 (VIS); Schott 093 (IRP).
Sensor 6,708 x 8,915 pixels, 40 x 54 mm.
Capture
Resolution 1250 dpi.
Height 6,708 pix/1,250 ppi=5.4 inch=135 mm.
Width 8,956 pix/1,250 ppi=7.1 inch=180 mm.
With 20% overlap 100 x 140 mm effective area=0.1 x 0.14=0.014 m² per capture.
Camera movement V 100 mm, H 140 mm.
Working distance 435 mm from lens to painting.
Camera movement/grip Specially designed XY-axis camera movement device, aligned to painting with laser measurement device.
Lighting and exposure 2 Broncolor monoblocks, Minipuls C200 1500 J with softboxes. With Hasselblad, flash is used for both visible and IR. Preferred aperture f/13. With OSIRIS, 2 tungsten 650 W modeling lights are used.
Color management Metamorfoze guidelines used. Camera calibrated for visible with BasICColor input, on Gretag ColorChecker® SG.
Output color space eci-RGB v2 16-bit, Hasselblad visible; grayscale 16-bit, Hasselblad IR; sRGB 8-bit bitmap, OSIRIS.
Microscopy
Microscope Olympus stereo microscope SZ61TR/45.
Working distance 110 mm.
Camera adaptor Olympus 0.5x C-mount camera adapter.
Oculars Olympus WHSZ 10x-H oculars.
Objective
110AL objective 0.5x working distance 200 mm.
110AL objective 1.5x working distance 61mm.
Holder Olympus STS holder with focus adjustment.
Light source Photonic PL 300 cold light source 150 W adjustable.
Illumination Fiber optic illumination system.
Camera Leica EC3 high-speed digital color camera.
Movement Linhof Microrail for accurate horizontal movement.

Image Processing and Digital Infrastructure

Stitching
When a painting is digitally imaged, we are generally left with dozens to hundreds of overlapping images, each of which only covers a small fraction of the painting. Though every effort is made to ensure that the lighting and camera angles are consistent from image to image, some variation still occurs. We developed new software by combining algorithms from computer vision (computer science) with the theory of multiscale microstructural analysis (materials science and engineering) to identify exactly how the individual images overlap and then to resample them and "stitch" them together into a seamless whole. This gives the appearance that each painting was imaged by a single super-resolution camera.
Registration
In addition to visible-light photography, we also image the paintings using infrared photography and infrared reflectography. The stitching process results in one stitched image for each wavelength. In order to obtain maximum benefit from each image, it is important to be able to identify the exact same point in a painting in images captured at different wavelengths. This is only possible if the images are "registered" (aligned) with respect to each other, so that the images from the various wavelengths can be perfectly overlaid on each other. We developed multiscale algorithms to automatically perform this step by locating certain features that appear in every wavelength, despite the fact that they may have very different appearances in the different images.
Online Viewers
The stitching step can result in huge images that are tens of thousands of pixels wide and that occupy tens of gigabytes of storage, making them extremely unwieldy, especially given the Project's international character. To solve this problem and to make maximum use of these images, we extended existing open-source technology to develop a new suite of fast online viewers. The viewers, which are featured on this site, enable art historians and conservators to explore the images in several ways: from different wavelengths individually, in a synchronized side-by-side view, or using a new technique that we refer to as a "curtain view."

Glossary

cradle
A cradle is a gridlike construction at the back of a wood panel to prevent warping. In the nineteenth century, cradles were routinely applied as a preventive treatment. However, wood panels keep responding to changes in relative humidity, and the cradle’s restraint of the panel’s movements can cause serious damage, such as the splitting of the panel and flaking of the ground and paint layers. In such cases, the cradles will need to be adjusted or removed—interventions that require highly specialized skills and knowledge.
dendrochronology
Dendrochronology is a dating method for trees and wooden artifacts such as panel supports. The growth-ring pattern on the end grain of a plank is measured and, based on statistical probabilities, compared with the master chronology for a particular species from a particular area. This way the earliest possible felling date for the tree can be determined, and thus an earliest possible date of production. The method is especially useful in the study of Netherlandish painting because virtually all these works were executed on oak that was imported from the same climatic region in the Baltic.
infrared photography
Infrared photography (IRP), which requires an adapted digital camera and a lens filter, registers patterns of absorption and reflection in the 700–1,100 nanometer (nm) range of the electromagnetic spectrum. At these wavelengths, paint can be partially penetrated to reveal components of layers below a painting’s surface. The technique is used to study the condition of paintings, changes in compositions, and underdrawings. Pinkish skin tones, whites, browns, and reds are penetrable, but in general, IRP cannot reveal any underdrawing beneath most blues and greens. The BRCP uses IRP at ultrahigh resolution, through macrophotographs that are stitched together into new, very large overall images.
infrared reflectography
Infrared reflectography (IRR) was developed to overcome the limitations of IRP in revealing underdrawing by utilizing higher wavelengths (1,500–2,000 nm). In this range, underdrawings can also be revealed below blue and green paints, but black will remain opaque under infrared. Because of the low resolution of IRR systems, it is necessary to take many close-ups, which need to be assembled into overall images. To partly overcome this, the BRCP uses an OSIRIS camera (sensitive to 1,700 nm) that automatically assembles 64 reflectograms in each scan. Such IRR studies are used in issues of attribution, in distinguishing original from copy, and to clarify workshop practices.
photomacrograph
A photomacrograph is a close-up photograph that can be reproduced at the actual size of the object or larger. The BRCP documents all paintings with macrographs in the visible and infrared ranges of the electromagnetic spectrum. These individual macrographs are then stitched together into new, very large images of the overall paintings, which allow for the study of the works in unprecedented detail. A photomicrograph, on the other hand, is a photograph taken through a microscope.
underdrawing
An underdrawing is a sketch that an artist lays out on the ground layer of a painting to prepare the composition before the actual painting starts. A variety of different materials may be used for underdrawings, both dry (such as black chalk or charcoal) and liquid (such as water-soluble pigment or diluted ink) applied with a brush. Underdrawing may be executed in contour lines only, but often shadow is also rendered, using systems of hatching or cross-hatching. The stylistic analysis of underdrawings can be useful to support or refute attributions, and changes between the initial design and the final composition can give insight into the genesis of a painting.
visible light
Visible light is a form of electromagnetic radiation, and the human eye perceives color as a function of the different wavelengths present in visible light. Only a narrow band of the electromagnetic spectrum is visible to the human eye, from violet at about 400 nm, to red at around 700 nm. Forms of radiation are distinguishable by their wavelengths, and range from long radio waves to ultrashort gamma rays. In the examination of paintings, infrared light, ultraviolet light, and X-rays are used routinely. Infrared has a slightly longer and ultraviolet a slightly shorter wavelength than the visible region of the spectrum; X-rays have much shorter wavelengths than those of visible light.
X-radiography
X-radiography utilizes X-rays—invisible radiation with much shorter wavelengths than those of visible light. Different materials absorb X-rays to varying degrees, and absorbency patterns can be registered on a special film or digital plate, the X-radiograph. This can be used to study the materials, internal structure, and condition of a painting. To make an X‑radiograph, the painting is placed between an X-ray source and the film. Paints (such as lead white or vermilion) that contain elements with relatively high atomic weights will absorb the X-rays to a greater extent than other paints do and will block the X-rays from darkening the film, thus appearing as light areas on the X-radiograph.