Technical Photography of Historical Pigments

I’m sharing in this post the Technical Photography 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.

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American institute of Conservation Photo Documentation target upgraded with a stripe of zinc white and cadmium red for calibration of Ultraviolet fluorescence and infrared Fluorescence.

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.

AIC Photo documentation Target. The color swatches in the UVR are dark indicating that no infrared is reaching the camera. The cadmium red stripe is used to check exposure for infrared fluorescence.

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 a 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 a 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.

Historical Pigments swatches. Visible (normal ) photography. Historical Pigments swatches. Ultraviolet Fluorescence photography. Zinc white on the bottom exhibits its characteristic yellow UV fluorescence. Historical Pigments swatches. Ultraviolet Reflected photography. Titanium white and zinc white – modern whites – are immediately distinguished from the historical lead white. Historical Pigments swatches. Infrared photography. Most of the pigments become transparent. Historical Pigments swatches. Infrared Reflectography with InGaAs camera (900-1600 nm). More pigments become transparent at longer wavelengths.

 

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.

Historical Pigments swatches. Infrared Reflectography with Vidicon tube. Historical Pigments swatches. Infrared false color photography. It helps with the identification of pigments. Historical Pigments swatches. Infrared Fluorescence. It helps with the identification of cadmium pigments. The brighter spot is Egyptian blue.