Use of the Ulbricht Sphere in measuring reflection and transmission factors
E. Karrer · Scientific Papers of the Bureau of Standards · 1921
A brief historical survey is given of the methods and instruments used in measuring the reflection factor of surfaces.The theory of the infinite luminous plane is discussed at some length in connection with the Nutting absolute reflectometer, which is based on this theory.Recent suggestions of the use of the Ulbricht sphere are noted and several new ways of using it are described.A reflectometer based on one of the latter ways is described in detail.This consists in a combination of the sphere with the Martens polarization photometer.The surface whose reflection factor is sought is laid over an aperture in the sphere.The brightness of it is compared with the brightness of the sphere wall.By screening the test surface from the first reflected light this comparison gives directly the unknown reflection factor.The theory of this use of the sphere is given in full.The reflection factor of a block of magnesium carbonate is shown to be 98.7 per cent by means of this instrument.A transmissometer based on the same theory is also described.It is also pointed out that the spectral reflection and transmission factors may be obtained in this way ,as a step toward standardization of methods.A simpler and less expensive reflectometer is also described, that will enable one to obtain the reflection factor of surfaces to within 10 to 15 per cent.