Developed in the late 1960s by Dutch physicist J. R. J. van Asperen De Boer, infrared reflectography (IRR) is a technique used to look through the paint layers. Though seemingly similar to x-radiography, infrared reflectography reveals slightly different information. When the longer wavelengths of infrared radiation penetrate the paint layers, the upper layers appear transparent. The degree of penetration depends on the thickness of the paint, the type of paint used, and the length of the wave of infrared radiation. The longer the wavelength of the infrared and the thinner the paint layers, the easier it is to penetrate to the layers beneath. Many paints will appear partially or completely transparent while others, such as black, will absorb the infrared radiation and appear dark. A special infrared camera captures the light reflecting off the surface of the painting. The resulting image, known as an infrared reflectogram, is digitized by a computer and appears as a black-and-white image on the computer monitor. The contrast of absorption of various materials reveals layers of the painting not visible to the naked eye, such as the underdrawings and changes in the paint layers.
The infrared reflectogram of The Old Guitarist reveals several interesting facts about the composition and Picasso's technique. The thicker brushstrokes of the second composition of the young mother that were partially visible with raking light become dark when examined with infrared. It is now obvious that the second figure was a young mother seated in the center of the composition with her left arm outstretched. The head of a calf or sheep can be seen to the right of the mother's outstretched arm. We now see that the young woman has a thoughtful expression and long dark hair. Careful examination of the guitar reveals that Picasso sketched the strings before painting them. The earliest composition, that of the old woman, is no longer visible. This indicates that the second composition was painted relatively thickly and with paint that the infrared radiation is not able to penetrate.