If the normal(法线) angle is maintained when building the instrument, any specular(镜面反射) reflection exits through either the source or detector port. However, integrating spheres(积分球) designed for color measurement offset(偏离) the normal angle between about 6∘ and 8∘. By placing a specular port at the opposite angle, the specular reflection can be either included or excluded from the measurement by placing material identical to the sphere’s interior or a black trap, respectively, at the specular port. This design was first implemented by Hardy (1935, 1936, 1938). He noted that when observers look at glossy samples, they always rotate them to eliminate any specular reflections. By excluding this light from a color measurement, the result would have better correlation to visual measurements.

Hardy, A.C. (1935). A new recording spectrophotometer. Journal of the Optical Society of America 25: 305–311.
Hardy, A.C. (1936). Handbook of Colorimetry. Cambridge: The Technology Press.
Hardy, A.C. (1938). History of the design of the recording spectrophotometer.Journal of the Optical Society of America 28: 360–364.