Up until now, when the dynamic course of writing pressure in a handwriting trace - an important criterion - has been examined on the basis of the indentation of the trace, oblique white light has often been used to assess indentation depths. The impression of the shadows is frequently distorted, however, because the shadows are overlapped by the intense coloration of the writing trace. Other procedures for using indentation depths to assess writing pressure (mechanical or laser-assisted scanning/stacks of microscope images) are uneconomical or difficult to manage. The InfraRed Detection Apparatus (IRDA) is simple in design, relatively inexpensive to acquire, and low-maintenance. Little time is required to get best results. In infrared light, most writing media are at least partially transparent. When the writing medium is transparent, photos taken with oblique lighting allow precise, detailed analysis of the dynamic course of indentation depths in handwriting traces and facilitate its description in an expert opinion. The shadow in the indentations is no longer distorted by the intensity of the coloring. With low-angle ring illumination, the loss of information is minimized. This article describes the procedure and the layout of the InfraRed Detection Apparatus (IRDA). A validation is attached. Figures and references (Published Abstract)