Seeing Red

Seeing the invisible:
What is infra-red reflectography?

Infra-red light lies just outside the visible spectrum. It overlaps with the red area of the spectrum and the microwave region. Conservators use wavelengths of radiation from the near infra-red part of the spectrum – usually in the range of 750–2000 nanometres. One nanometre is equal to one-billionth of a metre. This relatively long, low frequency wavelength is able to penetrate through the upper layers of a painting or work on paper, such as a watercolour, to the drawing underneath. An infra-red reflectogram is created by capturing an image of the infra-red wavelengths that are reflected into a camera lens.

Infra-red examination is commonly used to look at an artist’s working technique. It provides clues as to how a work of art has been constructed, and often gives an indication of the materials that the artist has used. As some paints and varnishes can appear transparent in infra-red light, details hidden beneath the surface may be revealed. Graphite pencil, charcoal lines and other carbon-based drawing media used during the early stages of developing a work of art are enhanced using infra-red reflectography.

Seeing

The limitations The ability to see underdrawing on paintings and watercolours by infra-red examination varies with a number of factors. These include the equipment being used to collect the information, and the actual materials used to create the work of art. Typically in paintings the underdrawing is a preparatory stage not intended to be seen, and is therefore painted over as the artist completes the work. The materials used for the underdrawing, the pigments, and the thickness of the paint layers and drawing layers can be crucial to the success of this method of examination.

Even if a drawing material is usually visible by infra-red examination, for example graphite pencil, it can be hidden by pigment layers that absorb infra-red light strongly, or have been applied in thick layers. The ability of a pigment to transmit, reflect or block infra-red depends not on the colour of the pigment, but on the size, shape and density of the particles in the paint layer.

Capturing the invisible
The reflectogram

An infra-red reflectogram is created by capturing the invisible infra-red energy reflected from a work of art when it is exposed to infra-red wavelengths. This is converted to a black and white image and stored in a computer. The image may then be manipulated to increase contrast or size, and a direct comparison can made between the work of art and the infra-red reflectogram.

To achieve this, special equipment is required:

• An infra-red energy source (infra-red emitting lamps)
• An infra-red sensitive vidicon consisting of an infra-red TV camera with video monitor
• A computer and software
• A motorised easel for scanning the surface of the work

Precision is required to set up the lamps, camera and work of art to ensure an accurate match of the many images is created during the scanning process. The work of art is moved in front of the camera at close range and infra-red images are collected in a grid pattern across the surface. Each image is labelled and stored in a computer until the entire area is scanned. The many small images are then assembled like a mosaic using computer software, to produce an overall image of the underdrawing.

Glossary

Electromagnetic radiation – waves of energy associated with electric and magnetic fields, resulting from the acceleration of an electric charge.

Electromagnetic spectrum – the range of frequencies over which electromagnetic radiations spread. The lowest frequencies (longest wavelengths) are radio waves; increasing frequency produces infra-red radiation, visible light, ultraviolet radiation, X-rays and gamma rays.

Infra-red reflectography – a technique that relies on capturing reflected infra-red energy which is converted to a visible image on film or as a digital image. It is a technique that allows the recording of underdrawing in works of art.

Infra-red vidicon unit – a self-contained unit that translates invisible infra-red radiation into a visible black and white image to produce an infra-red reflectogram.

Mosaic – an image formed as a composite of many small images. An infra-red reflectogram may be one small detail, or a composite of many details carefully re-positioned to reproduce a larger image.

Underdrawing – preparatory sketches made by the artist during the conception and development of a composition. Preparatory sketches are generally concealed under layers of paint and are seldom fully visible to the naked eye.

Visible spectrum – the range of electromagnetic radiations that is visible to humans – approximately 400–700 nanometres, and is described as visible light. In order of shortest to longest wavelength the colours of the spectrum are: violet, indigo, blue, green, yellow, orange, and red.

Useful Links

Infrared satellite photos help track coral reef health

MolArt: Infrared reflectography and the spectroscopic analysis of paintings

Harvard University Art Museums: Infrared reflectography

Getty: Watch a paintings conservator explain how infrared reflectography reveals the artist’s process