X-Ray and Infrared Imaging of Old Paintings

Infrared and X-ray imaging reveal what lies beneath an old painting’s surface: hidden sketches, earlier compositions, pentimenti (artist’s changes), underdrawing, and even the canvas’s original weave. These non-invasive techniques let conservators and art historians see the artist’s working process without touching the paint, often exposing details invisible to the naked eye.

Formula

The calculator works out your average pace and speed for a run or ride. Enter the total time (in minutes) and the distance (in kilometers). The formula for pace is:

Pace (min/km) = Total time (min) ÷ Distance (km)

Speed (km/h) = Distance (km) ÷ (Total time (min) ÷ 60)

Variables: t = total time in minutes, d = distance in kilometers. The pace is the number of minutes it takes to cover one kilometer; speed is how many kilometers you cover in one hour.

Worked Example

Using the default values: t = 50 minutes, d = 10 km.

• Pace = 50 ÷ 10 = 5.00 min/km
• Speed = 10 ÷ (50 ÷ 60) = 10 ÷ 0.8333 = 12.00 km/h

So a 50-minute, 10 km run gives a pace of 5 minutes per kilometer and an average speed of 12 km/h.

Why Imaging Matters for Old Paintings

Old paintings are not flat, static objects. They are built in layers: a support (wood panel or canvas), a ground (a preparatory coating), an underdrawing (the initial sketch), and then layers of paint and varnish. Over centuries, the upper layers can become opaque or darkened, hiding what lies beneath. X-ray and infrared imaging are the two most powerful ways to see through those layers without damaging the work.

Infrared reflectography (IRR) uses infrared light (typically 900–1700 nm) to penetrate the upper paint layers. Because many pigments (especially those containing carbon, like charcoal or ink) absorb infrared light while others (like lead white) reflect it, the underdrawing shows up as dark lines on a lighter background. X-radiography, on the other hand, uses X-rays that pass through the painting and are absorbed differently by heavy elements. Lead white (a common pigment) blocks X-rays strongly, so the X-ray image reveals the distribution of lead-based paint, including areas that were later overpainted.

How Each Technique Works

Infrared Reflectography

Infrared reflectography works by shining infrared light onto the painting and capturing the reflected light with a special camera that is sensitive to infrared wavelengths. The light penetrates the paint layers to varying depths, depending on the pigments and their thickness. Carbon-based underdrawing materials absorb the infrared, so they appear dark in the resulting image. This technique is especially effective for revealing underdrawings, preparatory sketches, and changes made by the artist during the painting process (pentimenti).

X-Radiography

X-radiography works like a medical X-ray but for paintings. The painting is placed between an X-ray source and a detector (film or digital sensor). X-rays pass through the painting, but heavier elements (like lead, mercury, and iron) absorb more of them. Lead white, a common pigment in old paintings, absorbs X-rays strongly, so areas painted with lead white appear white on the X-ray image. This reveals the distribution of lead-based paint, which can show the artist’s original composition, changes in the pose of a figure, or even a completely different painting underneath (a reuse of the canvas or panel).

What Each Technique Reveals

Infrared imaging excels at showing:

• Underdrawings – the artist’s initial sketch in charcoal, ink, or paint
• Pentimenti – changes the artist made while painting (e.g., moving a hand or an eye)
• Inscriptions or signatures hidden under paint
• Lines from a stylus or other drawing tools pressed into the ground

X-ray imaging excels at showing:

• The distribution of lead white paint – often reveals the main figures and drapery
• Underlying compositions – sometimes a completely different painting on the same canvas
• Canvas weave and panel grain – helps date the support
• Restoration patches or old repairs

For example, an X-ray of a portrait might show that the artist originally painted the sitter with a different hat or a different hand position, while infrared might reveal a detailed underdrawing that was completely covered by the final paint.

Choosing Between Infrared and X-Ray

Conservators choose the technique based on what they want to see. If the goal is to see the underdrawing or the artist’s initial sketch, infrared reflectography is usually the first choice. If the goal is to see the paint layers themselves, especially the distribution of lead white, or to detect a hidden composition underneath, X-radiography is preferred. Often both are used together, because they complement each other: infrared shows carbon-based materials, while X-ray shows heavy-metal pigments.

For example, in a study of a Renaissance panel painting, infrared might reveal a detailed underdrawing that shows the artist’s careful planning, while X-ray might show that the panel was originally used for a different painting that was later painted over. Neither technique alone would give the full picture.

Common Problems and Limitations

Both techniques have limits. Infrared cannot penetrate thick layers of paint, especially those with a high lead content, because lead white reflects infrared rather than letting it pass through. So if the underdrawing is beneath a thick layer of lead white, it may not be visible. X-rays, on the other hand, are absorbed by lead, so areas with heavy lead white may appear as solid white blobs, hiding what is underneath. Also, X-rays cannot show color, so they cannot tell you what pigments were used, only where heavy elements are.

Another common issue is that the underdrawing may be made with a material that does not absorb infrared (like a silverpoint or a red chalk), in which case infrared will not reveal it. Similarly, if the artist used a ground that contains lead, the X-ray image may be too dense to show fine details.

Costs and Accessibility

Infrared reflectography is generally less expensive and more accessible than X-radiography. Many museums have infrared cameras, and some portable models can be brought to the painting. X-radiography requires specialized equipment and safety precautions because of the radiation, so it is usually done in a museum’s conservation lab. The cost varies widely, but for a single painting, infrared imaging might cost a few hundred dollars, while X-radiography could cost several thousand, depending on the size and the facility. However, many museums offer these services for research purposes, and some academic institutions have open-access imaging labs.

Safety and Non-Invasiveness

Both infrared and X-ray imaging are non-invasive – they do not touch the painting. Infrared is completely harmless to the painting and to people. X-rays are also harmless to the painting, but they do involve radiation, so the painting is exposed to a low dose of X-rays. This is generally considered safe, but repeated or prolonged exposure could potentially cause fading in some pigments, so conservators limit the number of X-ray exposures. The painting itself is not damaged, and no samples are taken.

Alternatives and Complementary Techniques

Other imaging methods can provide additional information:

• Ultraviolet (UV) fluorescence – reveals varnish layers and some retouching
• Raking light – shows surface texture and craquelure (cracks)
• Macro photography – high-resolution detail of the paint surface
• Multispectral imaging – captures images at many wavelengths, including visible, infrared, and ultraviolet
• X-ray fluorescence (XRF) – identifies elements in the pigments without imaging, helping to date the painting

These techniques are often used together in a comprehensive conservation study. For example, UV fluorescence might show where varnish has yellowed, while infrared reveals the underdrawing, and XRF tells you which pigments were used.

Frequently Asked Questions

Can infrared imaging see through all paint layers?

No. Infrared can penetrate some paint layers, but not those with a high lead content. Lead white reflects infrared, so if the underdrawing is under a thick layer of lead white, it may not be visible. The depth of penetration depends on the pigments and their thickness.

Is X-ray imaging safe for old paintings?

Yes, X-ray imaging is considered safe for paintings. The radiation dose is low, and the painting is not physically touched. However, conservators limit the number of exposures to avoid any potential fading of sensitive pigments, and the process is done by trained professionals.

What is a pentimento?

A pentimento (plural: pentimenti) is a change made by the artist during the painting process, such as moving a figure’s arm or changing the position of an eye. These changes are often hidden under the final paint layers and can be revealed with infrared or X-ray imaging.

Can X-ray imaging show the colors of the original paint?

No. X-ray images are grayscale and show only the distribution of heavy elements, not colors. To identify pigments, conservators use X-ray fluorescence (XRF) or take tiny samples for analysis.

How do conservators decide which imaging technique to use?

They start with the question they want to answer. If they want to see the underdrawing, they use infrared. If they want to see the paint layers or a hidden composition, they use X-ray. Often they use both, along with other techniques like UV fluorescence, to get a complete picture.

Can these techniques be used on any old painting?

Yes, but the results vary. The effectiveness depends on the pigments used, the thickness of the paint layers, and the support. For example, a painting on copper may produce a very different X-ray image than one on canvas, because copper absorbs X-rays strongly.

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in t | Finish time | min | 50 | 0
in d | Distance | km | 10 | 0
out pace | Average pace | min/km | t / d | 2
out speed | Average speed | km/h | d / (t / 60) | 2