As dog whistles and whale murmurs cannot be heard by humans, X-rays and infrared cannot be seen—except by using special cameras. The photographs produced penetrate beneath the surface of the painting to reveal what lies underneath.

X-ray photographs

Almost since the first discovery of X-rays by Wilhelm Röntgen in 1895, innovative researchers have used them for studying paintings. In the latter 20th century, many museum labs purchased industrial or medical X-ray sources for studying sculpture and paintings. To obtain pictures, X-ray film is fixed in contact with the painting's surface, and exposed from behind. The X-rays are rapidly absorbed by heavier elements in paint pigments—particularly by lead white used in flesh tones. We typically look at X-ray photographs as negatives because of the useful coincidence that lead white is light-colored in both visible and X-ray negatives.

Infrared photographs

In the 1930s, art objects and materials were first examined in the near infrared spectrum, and in the 1960s, introduction of the first infrared cameras brought widespread usage by art conservators, particularly for visualizing sketched underdrawings. Infrared penetrates paint layers effectively at wavelengths ranging 1.0 to 2.5 mm depending on the painting (although a few pigments, such as ultramarine, are more reflective in infrared).