Integrating spheres are important lab tools for evaluating electro-optical systems. By dispersing radiation in a homogeneous fashion, they improve the accuracy of sensor calibration and make it possible to quantify the power emitted by light sources without multiple measurements.
Like many optical components, integrating spheres vary in size, design and efficacy. Ideal models include some assumptions that don’t always hold true in the real world, and of course, your experimental design has a huge effect on the results you attain. Here are a few of the factors that determine how well integrating spheres function.
Coating Properties
The reflective coatings inside integrating spheres don’t exhibit completely uniform reflectance across all wavelengths. As a result, you must make up for spectral distortions associated with the sources you’re measuring by adjusting your equations.
Most commercial designs account for such factors, but that doesn’t mean they’re all equal. Some minimize these errors by selecting specially formulated optically diffuse materials that deliver heightened uniformity. In other words, your choice of integrating sphere determines the success of your measurement and calibration activities.
Physical Design
Ideal integrating sphere equations also assume that components like entry and exit ports, light sources and baffles are infinitesimally small. In practical applications, however, baffles and ports have noteworthy impacts on the amount of light reflection that occurs. So does the light source that you intend to measure.
Effective integrating sphere designs work around these problems in various ways. For instance, some include features like auxiliary lamps. Others simply provide multiple ports where you can install your own additional light sources, which can also make it easier to improve dynamic range.
Physical design also has a significant effect on utility. For instance, motorized illumination control systems and well-designed user interfaces may ease the measurement process and make it possible to gather more samples. Modular designs that lend themselves to easy configuration can potentially streamline data acquisition, which is of benefit to small labs.
Stability
The optical properties of integrating spheres often depend on your test conditions. For instance, exposure to UV radiation can degrade some coating materials, and at high temperatures, coatings and sphere geometries may change.
When not controlled properly, these factors can introduce significant uncertainty into measurements. Manufacturers of paired sphere-detector systems are known to overcome such hurdles through calibration, and as a lab user, you also bear some responsibility. Keeping your integrating sphere in good condition is vital to getting more out of your measurements.
Support and Sample Data
Although many of their operating principles are relatively straightforward, integrating spheres are complex devices. As assessments of NASA calibrations have demonstrated, factors like the design of a given aperture might introduce greater measurement variability than the actual object you’re measuring does. [1]
Effective integrating sphere engineering can definitely help you overcome these kinds of hurdles. Still, it’s vital that you go into any measurement endeavor armed with a complete understanding of how your equipment performs under practical conditions. Manufacturer support could make all the difference in your next experiment or test.
