An integrating sphere is an instrument which collects light through its input port and through multiple diffuse reflections produces a homogeneous optical power density over its inner surface. The power loss of the sphere is through leakage out of the input and exit ports as well as absorption at each internal reflection. At equilibrium, the power loss will equal the power input.
Since the inner surface of the sphere is designed to have low absorption loss (high reflectivity), the power density on the inner surface of the sphere will be higher than the input power density. This ratio is the so-called sphere multiplier “K”. Theoretically, this factor is given by
K=ρ/[1-ρ(1-a)], where “ρ” is the reflectivity of the sphere inner surface and “a” is the ratio of the total area of the input and exit ports to the total area of the sphere inner surface. Since “ρ” has a value close to 1, the ratio “a” has a sensitive impact on the K-value.
Consider placing a black anodized plate onto the input port of a 20mm integrating sphere with a 7mm input port. The ratio “a” including exit ports is about 3.5% leading to a K-value of approximately 18. The black plate may reflect up to 30% in the NIR [Marschall, et al; SPIE 2014]. Placing such a plate over the input port is effectively the same as reducing the port area to 70% of the original value: 5.8mm which gives a K-value of approximately 22. This means the integrating sphere will miscalibrated by about 20% when the plate is mounted.
Artifex Engineering has addressed this issue by designing fibre port adapters which reflect so little light back into the integrating sphere that the miscalibration is less than 1% when the adapter plate is mounted.
Will the installation of the fibre adapter affect the calibration of the detector head?
Any light impinging onto an integrating sphere will be reflected to some extent back towards the source. The calibration of the head was performed with no adapters installed. Thus, by mounting an adapter, there will be some affect on calibration since part of the back reflected light is reflected again by the adapter back towards the integrating sphere, thus increasing the amount of optical power impinging on the detector.
The fibre adapter is manufactured with a proprietary design to minimize this effect such that the calibration will be affected by less than 1%.
Will the detector receive all of the (divergent) light emitting from the fibre?
When used with integrating spheres, the detector will receive all of the light emitted from the fibre since the sphere has a wider field of view than divergence of optical fibre.
This is not necessarily the case with free beqam heads. Before choosing a fiber adapter with a free beam head, the user should ensure that the beam size and divergence do not overload the detector. This point must be given particular attention when using APC-assembled fibers, since the beam also emits at an oblique angle.