This standard practice describes the calibration of standards for hemispherical reflectance in relation to the theoretically perfect reflecting diffuser with an assigned value of unity. The calibration of an instrument standard is made by means of a standard coated flat plate. The absolute reflectance of the flat plate is determined with a spectrophotometer equipped with an integrating sphere to which has been added an auxiliary sphere. The flat plate is coated at the same time, and in the same manner as, the interior of the auxiliary sphere, so that it is essentially a flat segment of the sphere's interior surface. The flux reflected from the interior of the auxiliary sphere through its port is compared with that reflected from the flat plate. The absolute or total diffuse reflectance of the flat plate is calculated by comparing the two reflected fluxes and the ratio of the port area to the wall area of the sphere.