The Hamilton Kerr Institute are working with local company SmartDrive to develop our range of digital imaging techniques. SmartDrive are a company that specialise in motion control systems, they have developed a system called SatScan which enable geometrically accurate images to be captured and accurately mosaiced together using both positional and image data.
Chris Titmus
The Hamilton Kerr Institute are working with local company SmartDrive to develop our range of digital imaging techniques. SmartDrive are a company that specialise in motion control systems, they have developed a system called SatScan which enable geometrically accurate images to be captured and accurately mosaiced together using both positional and image data.
Brief History of Infrared Imaging
Infrared images of paintings have long been used in conservation and by art historians to gain useful information about what lays beneath the surface of a picture. This technique can be useful to detect later retouching and changes to the paint layers, as well as fascinating disclosures of the original drawings used to sketch the painting. These can be used to help identify the artist by their style and characteristics. They were first captured using film, a difficult and painstaking process, which yielded no hint of an outcome until the finished photograph was examined. Frequently many hours of work would result in no new information gained.
Latterly this technique was replaced by using a Vidicon television camera. The Vidicon is an analogue camera which can be produce images from the Infra red part of the spectrum (approx 1700 – 2000 nm) when filtered to block out the visible light. This has the great advantage of live image viewing to see what information of under-drawing and retouching can be found without wasting hours of effort developing film only to find that the point of interest does not show any information in infrared. The need to store images from the Vidicon system makes the limitations of the system become more apparent. The image area can only be shot in small sections due to the limited picture size of each capture. Each image is converted to a digital file on capture and then has to be stitched together to produce an image of a usable size for examination and use. Each capture and the entire stitching process has to be carried out manually one image at a time, which needless to say, is very time consuming. There are a number of other limitations within the system which yield plenty of opportunities for errors and poor quality results. This can all be very frustrating and wasteful of valuable resources.
Modern digital cameras can be adapted to take infrared images as they have a degree of sensitivity in this area. However, although the pictures are a far higher quality that those of the Vidicon camera, they are in a slightly different part of the infra red spectrum (approx 1000 – 1300 nm) which limits the amount of information visible. What is needed is a digital system with high quality optics and working in the range of the Vidicon.
Current Project
The SatScan system developed by SmartDrive is scalable from microscope stage level to the 3 metre by 4 metre rig at the Hamilton Kerr. With suitable cameras attached the large rig allows us to capture images at up to approximately 400dpi. The painting is positioned at a suitable distance from the rig and the computer control instructs the system of the size of painting and level of detail required. The camera is then moved around capturing images at regular intervals. The position of the camera is always known to a sub millimetre level of accuracy and this positional data is used in addition to image matching techniques to stitch the many captured images together and assemble a single, large image. The image below took about 1 hour to capture at a resolution of approximately 150dpi resulting in a 32 Mega Pixel image. If you have Flash enabled you can zoom and and move around the image below by clicking on the controls on the navigation bar directly below the image.
Titian’s Nymph and Faun, from The Lord Egremont Collection at Petworth House, Courtesy of the National Trust
© Hamilton Kerr Institute
The rig is currently equipped with a Basler A631fc visible light digital camera and a VDS NIR-300 InGaAs infrared camera, although other imaging heads can be used, for example for laser measurement or multi spectral imaging. The VDS NIR-300 camera is sensitive in the infrared to 1700nm. The visible light camera allows us to capture images in visible light and ultra violet fluorescence. Below is a quick comparison of the 4 different infrared imaging system we currently have:
SatScan
VDS NIR-300 InGaAs IR camera
- theoretical 3 Giga Pixel maximum capture
- sensitive up to 1700 nm
- digital image capture time depends on the resolution of capture, the image above took about 1 hour to capture
- automated stitching with manual override
Hammamatsu Vidicon
analogue image capture from an infrared sensitive television tube.
- 400 × 300 pixel image capture
- sensitive up to 1500 – 2000 nm sensitivity depending on the tube.
- manually moving the camera for and manual image capture for each image is slow, image capture is typically reduced to cover specific areas of interest rather than the whole painting.
- manually stitching the images together using either Photoshop can easily take a day or more for one area of detail
Canon 30D
This is a digital SLR modified for us by MaxMax who removed the infrared filter.
- 8.2 Mega Pixel image
- sensitive up to 1200 nm
- exposure time: a few seconds, longer exposures create interference
- not suitable for image stitching
PhaseOne Scanning Back
A digital scanning back for mounting onto a 5×4 camera
- 7 – 8 Mega Pixel image
- sensitive up to 1300 nm
- approximately a 5 minute exposure
- not suitable for image stitching
The SatScan system clearly offers a promising alternative in terms of time, resolution and infrared sensitivity. The system is operational and in use while software development is ongoing to make the stitching sensitive to both visible and infrared light sources.