integrating spheres (original) (raw)

Definition: devices with a diffusely reflecting inner surface, used for certain optical measurements

Alternative term: Ulbricht spheres

Categories: article belongs to category photonic devices photonic devices, article belongs to category optical metrology optical metrology

Related: photodetectorsphotometers

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What are Integrating Spheres?

An integrating sphere (also called Ulbricht sphere after Friedrich Ulbricht) is an optical device for various purposes such as

integrating sphere

Figure 1: Integrating sphere for measuring optical powers independent of the spatial beam properties.

An integrating sphere for optical power measurements (see Fig. 1) is a hollow sphere with a diffusely reflecting (scattering) internal surface, typically two or more small openings (ports) for introducing light or attaching a photodetector, and often some so-called baffles, which are light barriers used to prevent direct illumination of a detector by a light source. The arrangement causes many diffuse reflections of the introduced light before it reaches a detector, so that the light flux becomes very uniform at the detector, and nearly independent of the spatial and polarization properties of the introduced light: the detected radiant flux (optical power) depends only on the total introduced power. In that way, the total output power of a light source can be measured, even if the beam divergence is fairly large. Such a device, consisting of an integrating sphere and a photodetector, is called an integrating sphere photometer.

Under certain simplifying assumptions, such as Lambertian scattering characteristics of the inner sphere surface with a uniform reflectivity ($\rho$) and a negligible fraction of the sphere surface covered with ports or other obstructions, the irradiance obtained at a small output port can be calculated as E = \frac{\Phi }{{4\pi {R^2}}}\;\frac{\rho }{{1 - \rho }}$$

where ($\Phi$) is the radiant flux injected through another port. The ($\rho$)-dependent second term can become quite large in cases with high reflectivity, e.g. 99 for ($\rho$) = 0.99. This is essentially because of a large number of reflections of light in such situations. However, the more closely ($\rho$) approaches unity, the larger is the influence of the ports, i.e., the less accurate the estimate is.

Apart from such measurement purposes, an integrating sphere can be used to illuminate a device very uniformly. This can be important e.g. for testing the homogeneity of digital imaging equipment (e.g. CCD arrays).

Ideally, the coating on the inner side of the integrating sphere has a very high reflectivity over the required wavelength range, and the reflection is very diffuse. If the optical losses in the sphere and through the small ports are low, the multiple reflections can lead to a fairly high optical intensity inside the sphere and consequently to a high optical efficiency, even if the sphere is much larger than the light source and the detector.

Spheres with rather different sizes are available. The larger ones may offer more and/or larger output ports, and there are even versions which can handle very high optical powers in the multi-kilowatt region, suitable for measurements on high-power lasers.

Frequently Asked Questions

What is an integrating sphere?

An integrating sphere, also known as an Ulbricht sphere, is a hollow optical component with a highly reflective and diffusing inner surface. It is used for various purposes like measuring optical power, characterizing material reflectance and transmittance, and creating uniform light sources.

How does an integrating sphere work for power measurements?

Light entering the sphere undergoes many diffuse reflections on the inner surface. This process thoroughly homogenizes the light, so that the power measured by a photodetector becomes nearly independent of the input beam's spatial profile, direction, and polarization state.

What are the main applications of integrating spheres?

Key applications include measuring the total radiant flux from light sources like LEDs and lasers, determining the transmittance or reflectance of scattering samples, providing highly uniform illumination for testing imaging sensors, and for depolarization of light.

What is an integrating sphere photometer?

An integrating sphere photometer is a complete measurement device consisting of an integrating sphere combined with a calibrated photodetector. It is designed to measure the total optical power of a light source.

Why is the inner coating of an integrating sphere important?

The inner coating must be highly reflective to ensure that light undergoes many reflections, which leads to high optical throughput and uniform light distribution. The coating's diffuse (Lambertian) scattering characteristic is crucial for homogenizing the light.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 26 suppliers for integrating spheres. Among them:

Artifex Engineering

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integrating spheres

Artifex Engineering manufactures high quality polymer and gold integrating spheres for laser applications. These spheres can be combined with our optical power meters to realize high power measurement units. High power measurements are often performed using thermopile detectors. Thermopiles however, have the disadvantage of reacting very slowly — typical risetimes leading to measurement periods of 1 second at best. The combination of an integrating sphere and a photodiode based power meter opens the possibility of measuring high power fluctuations on a microsecond time scale.

AMS Technologies

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integrating spheres

AMS Techno­logies offers a series of general-purpose integrating spheres with 3 or 4 ports. Out of a broad range of models, choose your sphere by selecting size, ports, and reflectance material/coatings. Three durable and highly stable coating options combine a highly reflective surface with nearly perfect Lambertian reflectance.

Interchangeable accessories like light sources, assemblies or port reducers allow for an endless number of application-specific solutions:

Instrument Systems

⚙ hardware🧩 accessories and parts🔧 maintenance, repair📏 metrology, calibration, testing💡 consulting🧰 development🎓 training💻 software

integrating spheres

The right integrating sphere for every application: Instrument Systems manufactures high quality spheres (50–2000 mm) in Berlin and Munich with barium sulfate or PTFE coatings to meet diverse technology and customer requirements:

Edmund Optics

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integrating spheres

Our general purpose integrating sphere systems are designed to spatially integrate radiant flux in order to measure optical radiation. They can be coupled with a sensor to create radiometers, photometers, or spectroradiometers in order to measure the total geometric flux emanating from a light source or the flux density of an illuminated area. Additionally, these sphere systems can be used to measure the output of high power lasers and laser diodes or to measure the reflectance and transmittance of materials.

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