Why do I need port plugs, covers and / or reducers for my integrating sphere?

Integrating sphere port plugs and covers are used to close unused ports on the sphere.

Port reducers are used to reduce the size of ports in use. There are several possible reasons for using them:

• To prevent extraneous light from entering the integrating sphere and interfering with measurements to be made.
• To protect the interior of the integrating sphere from dust and other contaminants. This is very important for maintaining integrating sphere performance, especially the calibration on calibrated units.
• Maximizing the reflective surface area of the sphere. This will increase the uniformity of the light distribution in the sphere and also increase the sphere throughput (i.e decrease the effectective attenuation of the sphere). Note: If the sphere is calibrated this will change the calibration.

In its most basic configuration, an integrating sphere requires only a single port where the detector is attached. This configuration is commonly seen in very large spheres that are used to collect light from sources that emit in all directions (4p steradians) with the light source located in the sphere. In most cases however, integrating spheres have at least one input port for working with light sources external to the sphere. Additionally, auxiliary ports may also be provided to allow for the attachment of additional detectors, spectrometers, light sources or other equipment. The “North Pole” port at the top of many spheres is a good example of this.

Even though the ports increase the functionalities of the sphere, they reduce the overall surface area inside the sphere, reducing the amount of light that reaches the detector and also impacting the uniformity of the light distribution inside the sphere.

Port plugs/covers

Integrating sphere port plugs and port covers  seal the port when not in use and

Integrating sphere port plugs contain white reflectance material. Typically the same PTFE material that is used for the interior reflecting surface of the sphere is used. In some cases, barium sulfate white reflecting paint is used. When a port plug is used, it changes the working geometry of the integrating sphere by increasing the effective reflecting surface area.

Integrating sphere port covers on the other hand have black, low reflectance surfaces. Port covers have minimal impact on the sphere working geometry because the low reflectance is optically equivalent to an open port.

Port covers can be also be used by customers as blanks when fabricating customized port adapters needed to attached their own accessories to the integrating sphere.

In deciding whether to use a port plug or a port cover several aspects need to be considered:

• Using a black port cover is an easy way to close a port without changing the sphere performance.
• Sometimes using a white port plug is a must, such as when the port being closed is directly opposite from the input port.
• A white port plug will increase the sphere throughput if higher light efficiency is needed.
• If a detector is calibrated with the sphere, using a white port plug will change the calibration. This change needs to be taken into consideration when making measurements. How to compensate for this change will be explained in a future blog post.

In summary, closing unused integrating sphere ports is a good idea in general. Port plugs and port covers are two different means for closing ports, each with its own implications. Deciding which one to use depends on the application.