Why is the integrating sphere an important optical instrument? Integrating spheres have been known since the beginning of the 20th century when Richard Ulbricht published his work in 1900 on their implementation.
Ulbricht sphere statue at the University of Dresden:
Typically, it is difficult to build a visible or near-infrared source, to be used as a test and calibration standard for certain sensor characteristics, because for this purpose it has to be a very uniform spatially extended source, radiating uniformly in all directions hemispherically. This is especially true for very high color temperature sources of visible light because they are made of hot filaments. A very hot filament may produce a high intensity of light, but by nature, it is a spatially non-uniform source.
High spatial uniformity of light intensity of an extended source is required for example when testing the spatial response of a detector array or when the cross-section of an optical projection system exit pupil must be illuminated uniformly.
Typical CI test system including an integrating sphere subsystem
In figure 2 a typical test system is shown. It includes the projection optics represented by the parabolic mirror on the right side, usually of a large diameter exit pupil to fill the entrance aperture of a unit under test (UUT, typically a visible camera), the secondary mirror, a wheel including a number of test targets, such as a USAF target set, pinholes, knife edges, etc., for different standardized tests, the blackbody infrared source (it can be an extended area or a cavity source, used for FLIR’s) and finally an integrating sphere subsystem (IS).
The target wheel is on the focal plane of the projection system; the IS includes the visible light source on its entrance port, a control light detector on a second port at a right angle with the entrance port and with the exit port, a third exit port near the target wheel at the focal plane. The IR source and IS system are placed on a motorized stage, so that they can be exchanged automatically by computer control, according to which source must be used and which test must be done.
In figure 3 three examples of CI standard integrating spheres are shown, with the controller. As can be seen, the source on the top, the monitoring detector on the right, and the exit port in the front are arranged at 90 degrees to each other; this arrangement, with special baffles built in the interior of the sphere, prevent the detector and the exit port to be illuminated directly by the source, ensuring almost perfect randomization, as required.
These arrangements are most convenient for CI’s sources, with a mechanical fixture on the bottom, when they are used on the focal plane of a collimator as in figure 2, and the light exit is in the horizontal direction. In reflectance and transmittance measurements the light port may be set with light exiting vertically up or down, with different illumination directions on the sample to be measured. In these cases, the standard CI IS may have to be modified.
CI integrating spheres are spectrally measured and calibrated in the plant before shipment, by measuring the spectral radiance output with a NIST traceable spectroradiometer. In addition, in order to ensure the stability of calibration with time, despite changes of source output due to aging or line fluctuations, the integrating sphere is equipped with a monitoring detector and feedback loop control on a shutter.
During subsequent years of use of the system, once the user selects the needed radiance value at the output, the system automatically adjusts the shutter to the corresponding position according to the previously built calibration table.
The sphere is used with a broadband source,
The sphere used with the source spectrally filtered in specific narrower spectral ranges.
In figure 4 we show a typical spectral radiance measurement of the light exiting a CI integrating sphere as a function of wavelength, when illuminated with a quartz-halogen lamp, in different shutter positions.
Figure 5. Calibration table of output radiance integrated into the 400 to 720-nanometer wavelength range versus monitoring detector signal. The shown R2 value is the standard deviation from a linear fit.
The CI IS’s are provided with a motor-controlled filter wheel that can accommodate eight different filters. These are used when the light output must be limited to a specific wavelength range. A similar radiance calibration in each filter range, together with shutter feedback control for stability, can be done in each spectral range separately. Following is an example of such calibration using three filters (red, green, and blue ranges) and respective calibration tables analogous to figures 4 and 5 above.
The SR300 series VIS-SWIR based integrating sphere sources are used in testing and calibrating DayTV and SWIR imagers. They can be set up to operate as a standalone source or can be integrated into an Optical Test Bench based on either a METS or ILET Collimator for performing electro-optical test qualification of the DayTV and/or SWIR imagers.