About MSI
Multispectral/multiband imaging (MSI) is a nondestructive photographic technique that can be used to identify pigments, fibers, and dyes and to map their locations on an object.
Imaging techniques
Visible Light (VIS)
Visible light is what most people use in day-to-day photography. VIS images capture the reflectance and absorption of visible light off the surface of an object, a phenomenon our eyes perceive as color. In MSI, the VIS image is used as a reference to compare with other images and for processing false color images (more on this below).
Ultraviolet-Induced Visible Luminescence (UVL)
This technique’s light source is a longwave UV–generating black light, and although we can see some purple light that emerges with it, we can’t actually see UV light with our eyes. The energy in longwave UV can “excite” electrons in the atoms of certain materials, causing them to emit a characteristic colorful glow that we can see (a phenomenon known as fluorescence or luminescence) in materials such as shellac and madder lake. Other materials (such as modern restoration paints and ancient copper mineral pigments) can appear very dark under UVL. These characteristics make UVL useful for pigment and binder identification, pigment mapping, and—in rare cases—revealing ancient painted inscriptions on artifacts.
Infrared Reflectance (IRR)
Infrared light is used in many imaging applications, from night vision to astrophysics. As with UV light, IR is invisible to our eyes, but materials still absorb and reflect it in characteristic ways. Infrared imaging is often used to visualize underdrawings on paintings and can sometimes reveal hard-to-see inscriptions on artifacts. The resulting image is in grayscale.
Infrared Reflected False Color (IRRFC)
IRR and VIS images can be combined through RGB channel substitution in Photoshop to create an infrared “false color” image that indicates reflectance/absorption characteristics in a colorant. For example, indigo appears red in IRRFC images, while red iron oxides appear brownish yellow. More on this in the “Image Processing” section below.
Visible-Induced IR Luminescence (VIL)
Like UVL, this technique involves excitation—but in this case, the light source is in the visible range and luminescence is captured in the infrared range. This is because Egyptian blue pigment (and a few others, including Han and Maya blue and some cadmium-based pigments) luminesces at 960 nanometers. The resulting image is in grayscale, and if Egyptian blue is on the object, it will appear as a white glow.
Indigo Subtraction (MBR Indigo)
Indigo dye molecules absorb light at a high level (660 nanometers, the end of the visible light range) and reflect light at a high level (735 nanometers, the start of the infrared light range). These contrasting behaviors can be captured by using lens filters that allow the camera to record these specific wavelengths. Images captured at 660 and 735 nm can then be combined using the “difference” blend mode in Photoshop. The resulting image is in grayscale, and areas that contain indigo (which absorbs and reflects at those specific wavelengths) appear white.
Our Setup Includes:
- Nikon D80 DSLR camera with a “full spectrum” modification (LifePixel, Adorama)
- Longwave UV, visible, and infrared light sources (B&H Photo, AnalytikJena)
- A set of bandpass lens filters, which allow the camera to capture characteristic reflectance or luminescence from dyes and fibers, while blocking unwanted radiation (B&H Photo, PECA, MidOpt)
- Magnetic ring adaptors to making changing filters quick and easy—the filter is held in place with a magnet, eliminating the need to screw it on and off the lens
- Calibration standards, including a color checker and 99% diffuse reflectance standard (B&H Photo, Labsphere)
- Adobe Photoshop for image processing
A Note on Image Calibration and Additional Equipment
- The workflows presented here are optimized for the equipment and processing techniques used at the Kelsey Museum. They produce useful and consistent, but not calibrated, images
- Calibrated images are good if you wish to publish the images or compare them between institutions; we recommend following the MSI capture and processing guidelines in Dyer et al. (Dyer, J., G. Verri, and J. Cupitt. 2013. Multispectral Imaging in Reflectance and Photo-Induced Luminescence Modes: A User Manual. European CHARISMA Project, British Museum) to produce calibrated images
- To aid in future calibration, we include color and reflectance standards in each of our images. Those are:
Additional Considerations
MSI results can tell us which colorants are likely present on an artifact rather than what is definitively there—mixtures can be difficult to identify with this technique alone, for example. Binding media produce their own luminescence colors and can therefore impact imaging results.
Helpful Hints
- Do not move the camera or lens while shooting
- Be careful not to disturb the camera or lens when changing lens filters (a remote shutter release and magnetic ring adapters make this process easier)
- Make sure battery-powered light sources (if applicable) are charged up and ready to go