Intercomparison of absolute radiometer standards

Project Description

A cryogenic absolute radiometer based on the principle of electrical substitution radiometry is constructed. The design goals of the device were:

a) flat response for visible and near-infrared light,

b) uncertainties less than 5x10-5 at 0.5 mW optical power level,

c) portable and suitable for a standard storage liquid helium dewar with a neck diameter of 50 mm,

d) for lighting technology purposes the laser light source of the system operates near the frequency at which the candela is defined.

The radiometer absorption cavity was made of high-purity copper and placed in high vacuum. For the light source of the system a He-Ne laser at 543.5 nm giving linearly polarized light beam at 0.1 mW was frequency and intensity stabilized.