Portrait of a gentleman, possibly Caspar Calthoff (d.1664), three-quarter-length, in a brown coat, his right arm resting on a water pump

Caspar Calthoff, a Dutch engineer from Dordrecht, was formally employed by Charles II to make 'guns and divers Engines and works for the King's service'. He taught lens grinding to Christiaan Huygens, a telescope maker from the Netherlands, and received a patent for a water pump on March 19, 1649. This may be the pump displayed in this portrait.

Samuel van Hoogstraten and Caspar Calthoff were friends. In his treatise on Hoogstraten, Thijs Weststeijn notes that the 'scientists with whom he [Hoogstraten] was in touch had a particular interest in optics: most notable was the Dordrecht mechanic and opticist Caspar Calthoff, whose portrait Van Hoogstraten probably painted in 1650.'