integratingsphere

An integrating sphere, also known as an Ulbricht sphere, is a highly reflective hollow cavity used to measure light and optical properties. Its interior is coated with a diffuse, highly reflective material so that light entering the sphere is scattered many times and distributed uniformly over the surface. This diffuse, near-Lambertian illumination makes the measured signal largely independent of the light source geometry and the angular distribution of the light.

The sphere typically has multiple ports for a light source, a detector, a sample, and reference measurements. A diffuse interior coating, often made of barium sulfate or PTFE (such as Spectralon), ensures that light is reflected in many directions with minimal angular bias. Light entering the sphere is redistributed through multiple reflections, creating a uniform radiance field. A detector placed at a dedicated port then measures a signal proportional to the total light power inside, enabling radiometric or photometric determinations.

Applications include measurement of total radiant flux, spectral power distribution, and luminance. It is also used to assess sample reflectance and transmittance, including diffuse reflectance factors and colorimetric properties. Calibration against known standards is essential, addressing stray light, port geometry, and detector responsivity to achieve accurate results.

Variants exist for different wavelength ranges and measurement goals, with coatings and geometry chosen to optimize reflectance uniformity. Integrating spheres are common in laboratories for lamp and LED photometry, color science, solar and optical coatings testing, and quality control in lighting and display industries.