The document describes the construction and working of an optical pyrometer. The optical pyrometer is a non-contact device that measures temperature by matching the brightness of a heated object to the brightness of an adjustable electric filament. It consists of a lens to focus radiation from the object onto the filament, and the current passing through the filament is adjusted until its brightness matches that of the object. The pyrometer allows temperature measurement of furnaces, molten metals, and other overheated materials without direct contact.