The integrating sphere is a hollow, spherical device, usually coated with a white reflective material on its inner surface. The device, also known as Ulbricht sphere, is used for optical applications such as measuring light flux outputs of light sources. Integrating sphere introduction and further reading>>
The basic integrating sphere has a minimum of two ports, one for light entry and the other one for attaching a photo detector. However, customizable device have three or four ports that allow adding of more functions.
How the device works?
The device is based on the light diffusing effect and uniform scattering properties of reflective surfaces. When the light enters the sphere, it is scattered and uniformly distributed by the reflective walls.
After several diffuse reflections, the light flux becomes uniform as well as independent of the spatial properties and polarization of the incident light. The uniform light falls on the detector which can now measure the power of the incident light.
The sphere can directly measure the flux that originates from lasers, lamps or the flux density from hemispherical illumination.
Customizing
The sphere comes in different sizes, coating material and ports.
Reflective materials
The inner surface of the sphere is usually coated with a reflective material depending on the intended application and wavelength of the light flux. The commonly used materials, each with different reflective characteristics include barium sulfate, diffused gold and PTFE.
Barium sulfate (spectraflect)
This material is suitable for a majority of radiation measurement applications. The effective range of the coating is between 300 to 2400nm. Integrating spheres with Barium sulfate coatings and suitable for measurement applications for both the visible and near infrared (NIR) light spectrums.
Diffuse gold coating
This is a metallic coating that is electrochemically plated onto the surface. The coating is highly reflective over the infra-red as well as near infrared wavelengths. The coating remains stable even at temperatures over 100C, unlike the barium sulfate which may lose the reflectance at elevated temperatures.
This type of coating is suitable for the mid to far IR applications such as infrared and laser. Its effective range lies between 0.7 and 20 µm. Due to the coating’s high thermal limit, the gold plated integrating spheres are a good choice for high power measurements.
PTFE (Teflon)
PTFE (spectralon) PTE has a high diffuse reflectance within the 250-2500 nm range; the reflectance is over 99% in the 400nm to 1500nm range. The coating high reflectance makes it suitable for low level light applications. It is a reliable coating with good temperature stability.
In addition, the PTFE material is hydrophobic; chemically stable up to a temperature of 350°C can be cleaned without degrading its integrity which also does not deteriorate with age. In addition, the coating material is chemically inert; making PTFE based integrating spheres more suitable for harsh environments such as underwater, low and high temperature environments.
Ports
Some manufacturers include extra ports to customize the integrated spheres and meet the customers’ needs. Available variations include the port size, location, stands, number of ports, etc. Each port has a specific function and cannot be used for other functions; otherwise, the devices will give inaccurate measurements.
The ports allow adding a wide variety of accessories, such as fiber optic adapters, port frame reducers, port plugs, port reducers mounting assemblies, port expanders, etc.
The port plugs help to seal the port when not in use for certain applications; these will usually have a coating similar to the integrating sphere’s inner surface coating.
Even though the ports increase the functionalities of the sphere, they may reduce the overall area inside the sphere as well as the uniformity of the distribution of the light inside the sphere.
Applications
Used for a variety of optical applications. This includes;
- Measuring the total flux outputs of lasers, fiber optics, LEDs, tungsten lamps, and other light sources.
- Testing and calibrating cameras and sensors, such as those used for night vision, radiometers, UV-vision enhancement systems, optical detectors, and more.
- Studying optical properties, such as absorption and reflection of materials and surfaces.