Reflected light was measured using a pyrometer lamp as a photometer. Handheld pyrometer devices are often marketed as infrared thermometers. A modern pyrometer has an optical system and a detector. This establishes a second calibration point for the pyrometer. However, known temperatures must be used to determine the calibration of the pyrometer. The glass will remain in the closed kiln until the pyrometer reads room temperature. In 1904, Professor Bristol invented the first practical pyrometer for measuring high temperatures. The first disappearing-filament pyrometer was built by L. Holborn and F. Kurlbaum in 1901. An absorbing medium of known transmission is inserted between the pyrometer and this black-body. To get around these difficulties, the ratio or two-color pyrometer was developed.