Infrared False Color photography (IRFC)

Infrared False Color photography is part of the Technical Photography documentation and allows to detect inpaints and to tentatively identify pigments. It has been used in the past as a photographic film which was first developed for totally different applications than art but soon it was used for paintings and since then IRFC has been familiar to art conservators. They used it mainly for preliminary assessment of art works, such as to distinguish azurite from ultramarine in old masters paintings. It is obvious that this method doesn’t have the accuracy that analytical methods can provide but since it is fast and easy to implement with filters and modified digital cameras, it is one of the most common imaging techniques implemented in art conservation laboratory.

Infrared False Color photography is part of the Technical Photography documentation and allows to detect inpaints and to tentatively identify pigments.

Applications in Art examination

IRFC is helpful to detect retouches and for the tentative identification of pigments. While IRFC does not provide conclusive results, it is recognized as a valid tool to select areas of inter-
est for further analytical studies. Visit Pigments Checker to examine what false colors we can expect from different pigments.

Visit Pigments Checker to examine what are the actual false color that we can expect from historical and modern conservation pigments.

Tentative identification of pigments

The identification of pigments based on IRFC is just tentative because a number of factors can affect their actual false colors.

Infrared False Color. Pigments identification. — A number of factors can affect the actual false colors of pigments. This slide shows an example showing the effect of an under layer. Indigo and phthalo blue were painted on prussian blue. As we see already in the IR image, both the pigments appear bright but this is due to very different optical properties of the pigments. Indigo became transparent in the IR, while phthalo blue reflects infrared and it is pretty opaque to this spectral range. Consequently, their false color is different depending on the under layer.Looking at the IRFC image, we see that if they are painted over a white cardboard that reflects infrared, they both look bright in the IR image and consequently they have a vivid red false color. On the other hand, when they are painted over prussian blue, which strongly absorbs infrared, they have totally different false colors. Phthalo blue maintains the same false color because it is opaque and the under layer doesn’t affect its IR image while indigo is transparent to the infrared that is then all absorbed by the under-layer of prussian blue. The result is now a black false color for indigo.

Mapping retouches on paintings

IRFC is very effective to quickly differentiate materials (paints and inks). It is fast, at least compared for example to a more accurate but much more time-consuming multispectral imaging mapping.

Infrared False Color. Mapping inpaints — *Retouches with a blue paint become evident on this oil painting using IRFC. The original blue paint shows a vivid red false color while the blue retouch has a bluish false color.*

Infrared False Color. Mapping inpaints in wall paintings — The IRFC method works well with any binder. so far we saw examples of tempera and oil paintings. This slide shows a renaissance wall painting with plenty of paint losses filled with modern pigments.

IRFC and different paint binders

The IRFC is not much influenced by the different painting techniques since IR images are not significantly changed by the different binders. In the case of wall paintings, for example, the increased brightness of the mineral support could be responsible for the difference in the resulting IRFC. The brighter infrared images can shift the IRFC images toward more intense red components. As an example, chrome green was identified through pXRF on mural painting shown in this slide and its false color is compatible with that attribution.

Experimental Setup

To create the IRFC image the VIS green (G) and red (R) channels become respectively the IRFC blue (B) and green (G) channels. The IRFC red (R) channel is represented by the IR image. Pigments of the same color feature a different IRFC color if they behave differently in the infrared. Malachite absorbs infrared, red, and blue light. Consequently, only its green component participates in the IRFC image, providing a final blue IRFC color for malachite. On the other hand, viridian additionally reflects the infrared and consequently, its IRFC is the additive result of the red and blue channels, making it appear purple.

Infrared False Color.Digital editing — *Infrared False Color photography is now realized with a digital method which uses a* photo-editing software to opportunely mix the channels of a VIS and IR image. *The Infrared False Color image is created by digitally editing the VIS and IR images of the same subject. This slide shows the editing of the VIS and IR images of a wall painting* *into the IRFC image.* *The resulting blue IRFC color of the green drapery* suggests malachite. IRFC is helpful to detect retouches.While IRFC does not provide conclusive results, *it is recognized as a valid tool to select areas of interest for further analytical studies.*

False color images with special filters

False color images have been acquired with specific filters, such as the XNite BP1 (the images are indicated with the acronym BP1) but this approach is not recommended. This filter transmits visible light in the range 350-660 nm and infrared after 800 nm and it can be used as an alternative to the IRFC method but with significant limitations. Digital color cameras feature CCD or CMOS imaging detectors whose photosensors cannot distinguish the wavelength of the incoming light and are covered with a CFA (color filter array) to select only red, green or blue
light. The CFA is transparent to the infrared transmitted by the XNite BP1 and the photosensors can detect it. The photo that is obtained with XNite BP1 would have the infrared light contributing more to the red channel since the far red has been cut out by the filter itself and therefore the infrared light is the only one that can contribute to the red channel. This filter provides
images that are analogous to the IRFC because the infrared and visible lights are blended together and thus the BP1 is capable of distinguishing between paints which feature different infrared reflectance. Compared to typical IRFC, BP1 is less effective since the infrared is also detected by the blue and green photosensors reducing the capacity to render pigments with different false colors. The advantage of BP1 over IRFC is that no editing is needed. Therefore, this method is much faster and it is particularly useful for the study of large artworks, such as
mural paintings, since their documentation with IRFC.

References

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