Abstract: An instrument for measuring the retroreflectance properties of surfaces to angles including an angle of zero degrees of the incident beam. The instrument comprises a source of collimated radiation, a beam splitter such as a cube biprism, a beam chopper, a radiation sensor system operated in a coherent detection mode with the chopper, and indicating and/or recording means. A beam of radiation aimed at a normal incidence at the surface of the test specimen is passed through the biprism which has its 45.degree. diagonal semi-reflecting surface interposed in the beam. The undesirable part of the beam is reflected off the diagonal surface to and through one of the sides of the biprism and the reflection back therefrom passes through the diagonal surface and then travels to the radiation sensor. The desirable part of the beam from the radiation source goes directly through the diagonal surface and passes to the surface of the specimen.