You can learn a lot about a painting by simply taking photographs with different types of light.
Visible light sits within the electromagnetic spectrum.
This is a continuous range of wavelengths that contains different types of radiation, from very high energy, short wavelength gamma radiation down
to low energy, long wavelength radio waves.
The image below shows where visible light sits on the electromagnetic spectrum; different colours
of light have different wavelengths, red being the longest and violet the shortest.
Rainbows are created when white light is split in a process known as refraction.
Due to the varying wavelengths, low energy red light is always on the top and high energy blue/violet, is always on the bottom.
How can we use the electromagnetic spectrum to analyse paintings?
Multispectral imaging involves taking a variety of pictures of the same object using the different types of frequencies across the electromagnetic spectrum. The wavelengths can be separated by filters that are sensitive to particular wavelengths. Some paint and pigments react differently in different types of light. By comparing a set of images taken with light of different energies we can make some interesting assessments in regards to the history and makeup of the painting.Try it yourself!
See the changes in the painting across the different multispectral images. Click on a thumbnail below to drag the red button across the painting to compare the images against one taken with normal visible light.
click on a thumbnail to the left to compare
Infra red (830nm)
Infra red has a lower frequency than visible light and can penetrate deeper into the painting. This means that IR images can reveal the under drawing and underlayers of paintings. In this case, it has allowed us to see the detail underneath the cracked as discoloured varnish on the girls dress and hair.
Infra red (1000nm)
Infra red has a lower frequency than visible light and it can penetrate deeper into the painting. This means that IR images can reveal the under drawing and underlayers of paintings. The longer wavelength of 1000nm, compared to 830nm, allows it to penetrate further into the painting, which is why there is slightly more detail in this image than the previous one.
Find out more about infra-red.
Infra red false colour
False colour images are created by splitting a picture taken in visible light into its three different red, green and blue channels. The red channel is then deleted and replaced by an infra red image. When combined back together this creates an interesting visual image which will highlight details that can only be observed with infra red. The lips and tips of the girls fingers appear orange in this image and pink under UV, this indicates the artist used a pigment called carmine lake.
Raking light
Here the painting is illuminated from one side, with the light source at an extreme angle to the surface. This reveals the texture of the paint layer, as raised areas are highglighted and create shadows. This image shows clearly the cracking above the girls hair as well as the thick paint layer that was used to create her sleeve. At the top of her shoulder a section of white paint appears to have flaked away, a fact not immediately obvious in any of the other images
UV Fluorescence
UV images are paticularly useful for analysing the varnish layers of paintings. On exposure to high energy ultra violet light, some materials will fluoresce. Traditional and popular varnishes like mastic and dammar give off a greenish colour. New varnishes will not fluoresce as much, and so will appear darker as they prevent. Dyes, especially red ones, flouresce quite strongly in UV light. Can you spot any areas where red dyes may have been used?
UV Reflectance
UV reflectance records the radiation that is reflected back from the painting when it is illuminated with an ultraviolet light. New varnish layers will reflect darker, which means touch ups to the painting are much more visible in this light. In this image you can clearly see a darker spot on the girls nose, which indicates that it has been restored.
UV False colour
This is similar to Infra red false colour, except instead of replacing the red channel with IR, the blue channel is replaced with UV reflectance.
Transmitted light
This effect is created when the light source is placed behind the canvas. This shows where the paint layer is weakest and, in this case, where any significant losses or holes are in the canvas. In this case, there are several areas across the painting that need to be patched up!