A hole or "port" in the integrating sphere allows this uniform illumination to be used in an optical system

An integrating sphere is a hollow spherical shell coated on the inside with a highly reflecting diffuse coating. It is available with light traps, port reducers, 0°/8° sample adapters, sample holders and detector mounts. The integrating sphere produces illumination that has extremely uniform irradiance and radiance. A good working model of an integrating sphere is to consider the port to be a hole in a wall, and, at a totally arbitrary distance behind it, another wall of infinite extent and radiance. A hole or "port" in the integrating sphere allows this uniform illumination to be used in an optical system.

The projected solid angle from any point on a sphere to any element of area on the integrating sphere is the same, regardless of location. This fact combined with the diffuse coating and the multiple reflections cause any light introduced into the sphere to produce uniform radiance of and irradiance on the wall of the sphere. The irradiance at the wall of an integrating sphere is incident from a full hemisphere. The radiance at the exit of an integrating sphere extends to a full hemisphere (π projected steradians). The specular component may be included or excluded in the measurement, by the use of a trap or reflectance piece at the relevant port.

Bentham is a company which manufactures a range of integrating spheres for measuring diffuse transmission and reflection. The following part is writing about brief introduction fof two products.

The DTR6 or DTR4 is its product of integrating spheres, coupled with a monochromatic source and detection electronics, allow the determination of the diffuse transmittance and diffuse reflectance of a sample. The monochromatic probe is imaged onto the place of the port appropriate to the measured reflectance, transmittance or quantity. In the case of reflectance, the sample is placed at the reflectance port. In the case of diffuse transmittance, the sample is placed at the transmittance port, the integrating sphere collects all light transmitted into the hemisphere behind the sample.