IRR is a complementary method to IR photography in order to reveal underdrawing and changes in a work of art. It is done with a specialized  and costly scientific camera, generally, an InGaAs camera.  A less-known variation of this method is IRRF, Infrared Reflectography Fluorescence.  Such as for IRF, Infrared Fluorescence photography, some pigments can be identified by their fluorescence emission in the IRR spectral region (1000-1700 nm). This note discusses the findings of this method tested on our 2 Pigments Checkers, "standard" and "modern & contemporary art". Some case studies are also discussed ranging from easiel paintings, historical prints, and pottery.

 

Infrared Reflectography (IRR) allows researchers to identify underdrawing and pentimenti in artworks. Scientists perform IRR using a specialized scientific camera that images within the spectral range of 900–1700 nm. Interestingly, pigments such as azurite, Prussian blue, and malachite become transparent only in the far-infrared region, around 1500 nm. Because these cameras have small imaging sensors, operators must stitch together numerous images to produce a final image with sufficient resolution.

Moreover, IRR complements standard infrared (IR) photography by revealing underdrawing and changes in a work of art more effectively. A less-known variation of this method, called Infrared Reflectography Fluorescence (IRRF), functions similarly. IRRF is conceptually close to Infrared Fluorescence (IRF), a method commonly included in standard Technical Photography documentation.

IRF enables researchers to detect pigments like Egyptian blue and cadmium-based pigments. Several molecules and minerals, including mineral pigments, exhibit infrared fluorescence. This phenomenon resembles ultraviolet fluorescence, where ultraviolet light excites visible light emission. In the case of infrared fluorescence, visible light or UV radiation stimulates emission in the infrared range. We refer to these cases as IRF-VIS or IRF-UV, respectively.

Operators perform IRF photography using either the visible light lamp ALICE or the UV lamp FABRIZIO. Therefore, we name the methods IRF-VIS and IRF-UV, indicating which lamp we use. The UV lamp enhances emission from cadmium pigments, while the VIS lamp works well for Egyptian blue.

Similarly, IRRF captures fluorescence emission, but researchers must use a more expensive InGaAs camera to detect emissions in the far-infrared region (1000–1700 nm).

Pigments Checkers “Standard” and “Modern & Contemporary Art”

We tested the two current Pigments Checkers: “Standard” and “Modern & Contemporary Art.” The Standard Pigments Checker includes the most commonly used pigments from prehistory through contemporary art, so it features only a few modern pigments. In contrast, the Modern & Contemporary Art Pigments Checker gathers the most important pigments used specifically in modern and contemporary artworks. This new checker focuses exclusively on modern pigments and complements those already present in the Standard Pigments Checker. We applied the colors using an acrylic binder on a cardboard support.

Lamps for Infrared Reflectography Fluorescence

We excited fluorescence in the IRR region using various light sources with higher energy than IRR: UV, VIS, and IR. The VIS-only lamp ALICE produced the best results.

UV region. We tested three UV lamps: 365 nm (UV lamp Fabrizio), 254 nm, and 222 nm. Only the UV lamp Fabrizio generated fluorescence in the IRR spectral range. We call this method IRRF-UV.

VIS region. The VIS-only lamp Alice produced the strongest fluorescence among all sources. We refer to this method as IRRF-VIS.

IR region. We tested LED lamps at 850 nm and 950 nm, but they did not generate any fluorescence. This method is known as IRRF-IR.

IRRF of Pigments Checker STANDARD

Figure [1] shows the IRRF-UV and IRRF-VIS images (IRRF-IR did not provide any fluorescence) of the 2 Pigments Checkers. The IRRF-VIS provides the same results that we are used to in IRF photography, while IRRF-UV does not produce strong fluorescence from any pigments.

Pigments showing IRRF with the VIS lamp Alice (in descending fluorescence-intensity order):
Egyptian blue
hun blue
cadmium red

This is what we already see in IRF photography, so there isn’t any advantage in using IRRF for this collection of pigments.

IRRF of Pigments Checker “Modern & Contemporary Art”

Pigments showing IRRF with the UV lamp Fabrizio (in descending fluorescence-intensity order): 
PY 53 – nickel titanium yellow
PB 33- manganese blue
cadmium red (TP–MSI calibration card)
PG 36 – phthalo green YS

Pigments showing IRRF with the VIS lamp Alice (in descending fluorescence-intensity order):
PB 33- manganese blue
PY 53 – nickel titanium yellow
cadmium red (TP–MSI calibration card)
PG 36 – phthalo green YS

The IRRF-UV makes the PY 53 – nickel titanium yellow brighter, but the IRRF-VIS is definitely better to increase the fluorescence of one more pigment, PG 36 – phthalo green YS, figure [2].

 

IRRF of PB 33 – manganese blue

PB 33 – manganese blue was in use from 1935 through the 1990s. Both artists and conservators valued this pigment, and it also achieved widespread success in various industrial applications. Notably, the famous artist Diego Rivera used manganese blue in his works. In conservation, professionals appreciate manganese blue for retouching because it provides an excellent non-metameric match for azurite.

In 2014, researchers investigated the IRRF response of this pigment [1], and its fluorescence emission is now well documented. This emission peaks around 1300 nm, which lies beyond the detection capabilities of standard IRF methods that rely on modified photo cameras. Therefore, capturing this fluorescence requires using an InGaAs camera capable of detecting in the farther infrared region.

We apply IRRF to map manganese blue in both modern artworks and conservation retouchings. Since we know the approximate timeframe during which this pigment was used, manganese blue can assist in dating a painting—or at least help determine when a conservation intervention took place.

IRRF of PY 53 – nickel titanium yellow

Developed in 1954, this more recent pigment also features strong IRRF emission. So far, we didn’t find any published study on the fluorescence of this pigment. It is commercialized as an artist color and it is considered a safer replacement for Naples yellow.

IRRF of PG 36 – Phthalo green YS

From 1957, this is the more recent and the less bright of these pigments emitting in the IRRF imaging. It is a bromated and chlorinated copper phthalocyanine. Must be noted that there are other 2 phthalocyanine pigments, in Pigments Checker STANDARD: PB 15 – phthalo blue, and PG 7 – phthalo green. These pigments do not show any IRRF emission.

Case study: Ireland map, 1604

We tested these imaging methods on an Ireland map. ORTELIUS, Abraham. Ausszug auss des abrahami Ortely Theatro Orbis…. Frankfurt, J Keerbergen and L. Hulsius: 1604, figure [3].

The investigation with IRF (both IRF-VIS and IRF-UV) did not reveal any fluorescence emission. We then tested IRRF using both VIS and UV excitation. IRRF-VIS revealed a strong fluorescence, as shown in figure [3]. In this image, we can clearly see the IRRF emission from the cadmium red swatch on our TP-MSI calibration card, which confirms that the image was captured correctly.

More interestingly, we also observed a strong fluorescence emission from the map’s yellow frame. We are currently working to identify this material using other analytical techniques. So far, the only yellow pigment known to produce IRRF emission is the modern PY53 – nickel titanium yellow. If further analysis confirms this identification, it would suggest that the coloring is a later addition—a fake—applied to what was originally an unpainted map.

Case study: Jerusalem map, 1702

We tested a large map of Jerusalem, dated 1702. Edit by Daniel Stoopendaal (35,5 x 46 cm). In excellent condition, split at the bottom, reinforced on the back, and beautifully colored by hand, figure [4]. Verso: Dutch text. 
IRF photo did not reveal any fluorescence, on the other hand, IRRF-VIS shows the fluorescence originating from large areas of the map painted light yellow and pale red, figure [5]. Also, these materials would need further analytical investigation for their identification.

Case study: 2 studio paintings, 1970′

This studio painting (60 x 43 cm) dated around the 70′ exhibits strong infrared reflectography fluorescence (IRRF) emission coming from the yellow pigment and the green areas (which as far as we know could be a mixture of the yellow IRRF emitting pigment), figure [6]. No IRF emission was detected.

The imaging analysis of the second studio painting shows that the red pigment used for the lady’s lips features both IRF and IRRF, strongly suggesting the use of cadmium red, figure [7].

Case study: photograph conservation

A preliminary examination of historical photos suggests that the IRRF method can have practical applications for photograph conservators. Figure [8] shows the IRRF image revealing the fluorescence originating from some of the white areas in a historical photo (CHSOS collection, item #55).

Similar observations can be made on another photo (CHSOS collection, item 59). Figure [9] reveals the IRRF emission in some white tones areas.

 

Case study: historical prints conservation

As an example of the infrared reflectography fluorescence possible applications for historical prints, we show the image taken on a historical postcard, figure [10]. There is an IRRF emission coming from the white areas, but not all of them. Indeed, some white spots do not have any IRRF output, indicating, likely the use of different materials, figure [11].

Case study: pottery conservation

Even pigments used for pottery can show IRRF emission. Figure [12] illustrates the case of a porcelain color sample plate. This kind of plate shows the individual pigments and it allows the evaluation of the manufacturer’s range of vitrifiable colors for porcelain. The orange color is bright in the IRRF photo and in the IRF, likely being a cadmium-based orange pigment.

Conclusion

Just like infrared fluorescence (IRF), infrared reflectography fluorescence (IRRF) also offers a valuable tool for easily mapping modern pigments on works of art. As the case studies show, several other artists’ materials and conservation pigments may exhibit IRRF emission beyond those already documented in the Pigments Checkers. Therefore, it is important to continue testing additional materials and include them in future reference charts.

One clear advantage of this method is its non-invasive nature. In fact, researchers can perform IRRF without unframing the painting. Both VIS (input) and IRR (output) radiation pass easily through glass, which means there is no need to remove the glass cover from framed canvases.

References

[1] Accorsi, G. et al. “Imaging, photophysical properties and DFT calculations of manganese blue (barium manganate( VI ) sulphate) – a modern pigment” Chem. Commun., 2014,50, 15297.

Resources

National Gallery (London) – Raphael’s Madonna of the Pinks
The National Gallery has pioneered the use of infrared imaging to study underdrawings in paintings. For example, the gallery’s infrared imaging study of Raphael’s Madonna of the Pinks (~1506–07), revealing an exquisite underdrawing beneath the paint layers.