What Does the Integrating Sphere Do?

Although we are unfamiliar with integrating sphere, but it is indispensable. What is the integrating sphere used for? 

1. What is an integrating sphere?

The integrating sphere is a hollow sphere with its interior coated with a diffuse white reflective material. Diffuse-reflection factor of the covering material is close to 1. The common materials for this purpose are magnesium oxide and barium sulfate, which are sprayed onto the inner surface after being well mixed with rubstick. 

2. The theory of integrating sphere

After the incidence of lights through entrance ports, they are evenly scattered and diffused, so the exit ports get highly uniform diffusing light beams. Moreover, the incident angle, spatial distribution and polarization of incoming lights have no effect on beam intensity and uniformity of output lights. As lights are integrating inside the sphere before getting out, an integrating sphere can be regarded as a light intensity attenuator. The ratio of output lights intensity and incoming lights intensity equals to the area of entrance ports dividing spherical inner surface area. 

3. What is an integrating sphere used for?

(1) Optical receiver

The lights being tested enter an integrating sphere through entrance ports. One or two light detectors are placed on the inner spherical wall. As the light current exported by the detector is proportional to illuminance of spherical inner wall, then the variation of light current exported can lead to the light quantity's variation of incident lights. 

(2) Evenly illuminated object surface

Place a few bulbs (normally 4 to 6 bulbs) symmetrically and evenly on the spherical inner surface and exit ports. Lights emitted by these bulbs, after being diffused repeatedly by the inner surface, generate a uniformly bright and luminous sphere. Such a sphere is used to measure vignetting factors of photographic lens and uniformity of image plane illuminance. 

(3) Spherical collimator

An integrating sphere with collimator objective, bulb as well as white and black plugs is called Spherical collimator. It is employed to measure veiling glare index, one of telescope indexes. The measuring process is as following. Photoelectric detector measures the black and white targeted images respectively. These are corresponding indicator values measured by the photoelectric detector. Then veiling glare index of the tested telescope can be calculated. That is because if the imaging of a black object by the telescope is not totally black, that means there are parasitic incident lights on image plane besides the targeted object imaging.