I’m sharing in this post the Multispectral Images of my Historical Pigments swatches collection painted with gum Arabic. I used a Nikon D800 36 MP modified for UV-VIS-IR and the set of filters for multispectral imaging described here. All the images are shot RAW and then edited in Camera RAW.
VIS (Visible). Be sure to shoot RAW, add a color checker target and follow calibration of your camera and monitor as described in my post. I’m using a modified AIC Photo Documentation Target. I added on the right two stripes — one of zinc white and one of cadmium red — which I use respectively for calibration of Ultraviolet Fluorescence and Infrared Fluorescence. Camera RAW editing: I adjust the exposure in order to have color checker’s black swatch at RGB 60 and white swatch at RGB 230. This is an arbitrary set of values which I implemented to have standardized IR False Color images.
UVF (Ultraviolet Fluorescence). Camera RAW editing. Camera Calibration: adobe standard. I noticed the custom calibration of the camera, see this post, doesn’t provide a good rendition of the UV fluorescence image. Temperature =15000, Tint =+70, in order to render the characteristic yellow ultraviolet fluorescence of zinc white which I use as a reference.
UVR (Ultraviolet Reflected). Camera RAW editing: Saturation = –100 and exposure adjusted to have the black and white swatches at RGB 30 and 50 respectively. If no infrared light is passing into the camera, as should be, the color swatches in the AIC color checker are all dark. Compare the same color swatches with the infrared image as well as the zinc white and cadmium red stripes. I’m saying this because the Wood’s glass in the UV lamp allows through UV and IR, so you must be sure your on camera filters stop both UV and IR.
IR (Infrared). Camera RAW editing: Saturation = –100 and exposure adjusted to have AIC PhD Target black swatch at RGB 50 and white swatch at RGB 200. This is an arbitrary set of values which I implemented to have standardized IR False Color.
IRR (Infrared Reflectography). I use an InGaAs Camera Merlin Indigo System. Images are collected with a frame grabber Imaging Source DFG/USB2Pro. Stitching is done with PTGui Pro. As you can tell comparing IR (done with digital camera) and IRR (InGaAs camera) more pigments become transparent, such as Prussian blue, azurite, verdigris and yellow ochre. It’s interesting to note that smalt, cobalt blue and cobalt green become more opaque. Indeed, just few pigments, such as smalt, have a particular transmittance curve. They are more transparent in the closest digital IR than in the IR Reflectography range. This is a good point to consider the two techniques complementary.
IRF (Infrared Fluorescence). Camera RAW editing: Saturation = –100 and exposure to make cadmium red swatch in the AIC PhD Target with RGB 30.
I didn’t notice any loss in transparency using InGaAs camera compared with the Vidicon I previously owned. I got below a Vidicon image of the same collection of pigments.