Integrating Sphere Fundamentals and Applications

An integrating sphere collects electromagnetic radiation from a source completely external to the optical device, usually for flux measurement or optical attenuation. Radiation introduced into an integrating sphere strikes the reflective walls and undergoes multiple diffuse reflections. After numerous reflections, the radiation is dispersed highly uniformly at the sphere walls. The resulting integrated radiation level is directly proportional to the initial radiation level and may be measured easily using a detector.

Sphere Diameters

The smaller diameter, less expensive spheres necessarily have smaller utility ports and very high throughput. In fact, the throughput may be so high, depending on the light source, that filters or fiber optic cables are required to prevent detector saturation. The port fraction of the smaller spheres, however, is very high. Consequently, the measurement data generated from a small integrating sphere will be less accurate than the same application using a large sphere.

The larger integrating sphere exhibits less throughput than the smaller spheres and high optical attenuation, thereby introducing a higher signal-to-noise ratio. These spheres are more flexible but, at the same time, are more expensive to manufacture.

Sphere Materials

The cost effective barium sulfate coated GPS integrating spheres are constructed of two aluminum hemispheres. The hemispheres are joined by an anodized flange cover with screws. The effective spectral range of barium sulfate is 350 - 2400 nm, although the hemispherical reflectance falls off slightly above 1850 nm. This type of sphere is adequate for most radiation measurement applications in the visible and NIR spectrums.

Diffuse gold coating is an electrochemically plated, diffuse gold-metallic coating that exhibits high reflectance over the near-infrared and infrared wavelength regions from 0.7 to 20 ?m. The gold spheres are constructed in the same manner as the barium sulfate spheres, except that the external flat surface and port frames are gold-plated as well. A gold GPS is well-suited for infrared laser applications. Whereas a barium sulfate coating may lose its reflectance at high temperatures, the diffuse gold remains stable at temperatures well above 100C.

PTFE material exhibits very high diffuse reflectance over the 250 - 2500 nm spectral range with reflectance greater than 99% between 400 nm and 1500 nm. Although PTFE’s temperature stability is suitable for laser applications, its high reflectance property is best suited for low level light applications. Another distinct advantage of the PTFE spheres is reliability: The material does not deteriorate with age and can be cleaned without destroying the mechanical integrity of the material.

A PTFE integrating sphere is immediately recognizable through a sphere port by the 7 mm thickness of the reflective material along the inside sphere wall. A PTFE GPS is constructed of two machined hemispheres, fitted together to form the interior sphere cavity and held together by an aluminum outer shell. A PTFE sphere is more expensive than a barium sulfate GPS due to the required machining and assembly. Since the walls have thickness, the size options for the PTFE spheres are different as well. Due to its high reflectivity and diffusivity, the optical throughput of a PTFE GPS is high, and hence more care is required when selecting port attachments and fixtures.

Typical reflectance characteristics for PTFE, barium sulfate, and diffused gold sphere coatings
Figure 1. Typical reflectance characteristics for PTFE, barium sulfate, and diffused gold sphere coatings.

Sphere Port Sizes and Locations

The port size and locations on an integrating sphere are important considerations when selecting a sphere for the intended applications. A sphere port adds functionality to an integrating sphere, but at the same time, diminishes the uniformity of the light distribution inside the sphere. The ratio of the total port area to the area of the internal wall of a GPS is called the port fraction. The port fraction parameter constitutes a measure of sphere performance in accuracy. An integrating sphere with a low port fraction provides better performance than a high port fraction sphere.

3-Port Config
A 3-Port sphere port locations. Note that a 2" sphere shell is cubic.
3-Port Config
A 4-Port sphere port locations. Note that a 2" sphere shell is cubic.

Each port on an integrating sphere has a specific function and the improper use of any port will generate inaccurate measurement results. The port locations are identified as the 0 degree, 90 degree, 180 degree, and the north pole. All Sphere ports are machined at 90-degree intervals into the outer hemispherical shell. The dimensions of each port depend on the size and the series of the GPS. The functions of each port on a GPS are pre-determined during sphere design. Some ports provide a single function; some ports provide multiple functions. All GPS Series integrating spheres can be used for uniform source and light measurement applications. The 4-port integrating spheres provide a diffuse reflectance/transmittance measurement capability.

All Newport GPS are equipped with a baffle installed between 0 degree and 90 degree port. The purpose of this baffle is to prevent direct path radiation at 0 degree from reaching a detector installed at the 90 degree port. Direct path radiation is the main source of an error in the total luminous or radiant flux measurement. The baffle on a barium sulfate and diffuse gold GPS is aluminum plate, coated with the appropriate reflectance material and fastened to the outer shell of the sphere. The baffle on a PTFE sphere is a machined with the same PTFE material.

The proper use of any port on a GPS depends on the integrating sphere application. Some ports can accept certain optical components during one application but not during another application. Some ports are not designed for certain optical components at any time. During some applications one port configuration is preferred over another, although either configuration might provide acceptable results.

Port Accessories

Each port on a Newport integrating sphere is fitted with an aluminum port frame so that an accessory fixture can be mounted on it. Port accessories enable the integrating sphere to perform the user’s specific tasks and include items such as port plugs, port reducers, port frame reducers, and fiber optic port adapters. These accessories allow a single general purpose sphere to be configured for various applications such as a uniform source, light measurement, reflectance measurement or laser power measurement integrating sphere.

At Newport, various accessory items with the PTFE, barium sulfate, diffuse gold, and flat black are available. Normally, an accessory should be coated with the same reflectance material as the sphere. However, not all fixtures are available in all reflectance materials. For example, due to machining limitations of the material, only port plugs are offered as standard products for the PTFE material. The appropriate tool kit is also included.

Various sphere accessories available at Newport
Figure 2. Various sphere accessories available at Newport.

Collimated Laser Beam Power Measurement

Measuring total collimated laser beam power, independent of polarization or beam alignment, is straightforward. The beam is admitted into the sphere through the 180 degree port so that the hot spot is formed at the 0 degree port. When a detector is placed at the 90 degree port, the baffle prevents direct radiation from the hot spot from hitting the detector, which allows the spatially integrated beam power measurement. The north port can be used as a light pick-off for the wavelength measurement. Newport offers standard integrating sphere detectors calibrated as a single unit.

Divergent Light Source Power Measurement

An integrating sphere and calibrated detector setup is suitable for accurate, absolute value light power measurement of divergent beams from laser diodes, lensed LEDs and lensed lamps. Your measurements will be insensitive to problems associated with overfilling of the active area of the detector. The source is located at the 0 degree port and the detector is mounted on the 90 degree port. The baffle, positioned between the input port and the detector port prevents the detector from directly viewing the emitting aperture of the laser or the direct area of illumination. The north port can be used as a light pick-off for the wavelength measurement.

In an integrating sphere, the detected flux is always a small fraction of the incident flux. This attenuation, caused by light reflecting many times before reaching the detector, makes the integrating sphere an ideal tool for measurement of output light power of high-power lasers.

Fiber Optic Power Output Measurement

An integrating sphere is also ideal for measuring the output of optical fibers. Because the typical output from an optical fiber is slowly diverging, the first reflection spot at the opposite side of the source is not highly concentrated. Therefore, often either collimated beam configuration or the divergent configuration is fine. However, in case of lensed fiber, the divergent beam configuration is recommended due to the increased NA of the fiber. In case of using a fiber collimator, the collimated beam configuration is recommended.

Transmittance Measurement

Transmittance can be measured by using a 4-port integrating sphere to collect transmitted radiation from a sample held in the 0 degree port. The sample is irradiated, and then compared with a direct source measurement made outside the sphere. A baffle is used to shield the detector from non-integrated transmission, and a light trap mounted on the 180 degree port can be used to remove the unscattered component. Measurements of total integrated scatter, fluorescence, bulk scatter and forward and back scatter can also be made. The detector is mounted on the 90 degree port.

Reflectance Measurement

To measure reflectance, a sample is held in the 0 degree port and irradiated by an incident beam through the 180 degree port. Total reflected radiation is spatially integrated by the sphere and measured by a baffled detector. Using the normal-incidence sample holder, which reflects the specular beam back out of the input port, can eliminate the specular component of the reflective radiation. An 8°-incidence sample holder allows measurement of the “specular plus diffuse” reflectance. The reflectance of a sample relative to a known standard can be calculated by measuring both and taking their ratio. The sample and standard should have a similar reflectance to avoid errors caused by sample reflectivity. A dual-beam system can be used to eliminate this potential source of measurement error. The detector is mounted on the 90 degree port.

Uniform Light Source Sphere

A general purpose sphere can be configured as a crude uniform light source by introducing illumination into the sphere from an external source. The setup requires an illuminator, a detector, and a power meter or a radiometer. A three port sphere is better than a four port sphere, because the unused fourth port with a port plug might interfere with output uniformity. The light source is connected to the 90 degree port, while the detector is mounted on the north pole. The large 0 degree port serves as the uniform illumination output.

The detector connected to the power meter or the radiometer provides an accurate indication of the sphere illumination. The output will vary in linear fashion with the power reading as long as the detector is not saturated.

Newport Integrating Spheres

Newport’s general purpose integrating spheres are designed as cost-effective spheres to be configured in various ways for a variety of applications. With an extensive line of accessories available, a single sphere can perform various integrating sphere tasks such as uniform illumination, light measurement, and reflectance measurement with reasonable accuracy. For those users who do not require strict uniformity or precise measurements, Newport’s spheres offer a convenient choice for integrating sphere light measurement and light characterization.