The word “thermograph” often used to describe both an instrument and the product of that instrument has as its roots the Greek words thermos and gruphos. The latter means a sketch or pictorial representation; thus, the graphic arts, scientific graphs, photographs, etc. The former relates to heat or temperature. At first reading, then, the word “thermograph” is most appropriate for a graphical representation of the heat or temperature distribution of any body. But an infrared instrument does not measure either of these; it measures the distribution of the infrared radiation (more properly, radiant emittance or radiance). Thus a more appropriate and precise name for the instrument is an infrared radiograph. By analogy the photographic output could be called an infrared radiogram, and the technique, infrared radiography. This paper describes some of the instrument design principles. Instrument Design