Introduction
Many optical sources emit light in multiple directions rather than in a narrow beam. Measuring the total power from these sources can be challenging because conventional detectors capture only a portion of the emitted light.
An integrating sphere solves this problem by collecting and redistributing light inside a highly reflective spherical cavity. This allows detectors to measure the total optical power emitted by a light source regardless of beam direction or spatial distribution.
Integrating spheres are widely used in photonics laboratories for measuring lasers, LEDs, optical sensors and other light sources.
Table of Contents
What Is an Integrating Sphere?
An integrating sphere is a hollow sphere with a diffusely reflective inner surface. This may be a porous polymer or a roughened and highly reflective metal surface.
When light enters the sphere through an input port, it reflects multiple times from the interior walls. These reflections create a uniform light distribution inside the sphere.
A detector mounted in the wall of the sphere measures the average light intensity inside the sphere, which is proportional to the total optical power entering the cavity.
This design allows accurate measurement of sources with complex emission patterns.
How Integrating Spheres Work
The inner surface of an integrating sphere is composed of a material that reflects light diffusely rather than specularly.
When light strikes this surface:
- it scatters in many directions
- the intensity becomes evenly distributed
- directional information is lost
- polarization information is lost
After many reflections, the light inside the sphere becomes nearly uniform. This uniform distribution enables precise measurement of total optical power.
Integrating Sphere Components
A typical integrating sphere consists of several components.
Sphere Body
The spherical cavity is usually made from aluminium or copper coated with a highly reflective diffuse layer, or from a porous, highly reflective polymer material.
Input Port
Light from the source enters the sphere through an input port. The size of this port must be chosen to allow the complete light beam to enter. However, it should not be too large, as this influences measurement accuracy and sphere efficiency.
Detector Port
A photodiode or other optical detector is mounted at a separate port in the wall of the sphere to measure the internal light intensity.
Baffles
Baffles are small barriers placed inside the sphere to prevent direct light from reaching the detector. They ensure that the detector only measures diffusely reflected light after many internal reflections.
Advantages of Integrating Spheres
Integrating spheres provide several important benefits for optical measurement.
Directional Independence
Direct photodiode sensors have a protection window which acts as a weak etalon. Furthermore, the reflectivity of the photodiode surface with its high index of refraction is angle dependent. Therefore, the amount of light reaching the photodiode’s active region is dependent on the angle of incidence. This effect can be as much as several percent.
In an integrating sphere, the measurement result does not depend on the direction of the incoming light beam as the beam is randomly scattered and homogenized within the sphere.
Reduced Alignment Sensitivity
Again, the etalon effect of the protection window of a direct photodiode sensor results in lateral alignment sensitivity.
Using an integrating sphere, small changes in beam position or orientation have negligible effect on measurement accuracy.
Polarization Independence
Direct photodiode sensors can show some level of polarization dependence.
The multiple diffuse reflections within an integrating sphere create a random polarization state, thus eliminating this parameter from affecting the measurement result.
Accurate Total Power Measurement
The sphere collects nearly all emitted light from the source. This may not be the case for a direct photodiode sensor.
Power Attenuation
Direct photodiode detectors can only measure up to several milliwatts of power before they are saturated.
Integrating spheres attenuate the optical power by spreading the captured light over a surface area which is much larger than the surface area of the photodiode. Furthermore, optical fibres can be used to couple an even smaller amount of light to an external detector. In this manner, power levels of Watts to kilowatts can be measured.
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Applications of Integrating Spheres
Integrating spheres are widely used in many areas of photonics and optical engineering.
LED Measurement
LEDs emit light over a wide angular range. Integrating spheres capture the full emission pattern, enabling accurate measurement of total output.
Laser Diode Characterization
Laser diodes without collimation optics produce divergent beams. Integrating spheres ensure that the complete optical output is measured. Furthermore, the attenuation of the integrating sphere allows measurement of high power laser diodes up to the kW range.
Spectroscopy
Integrating spheres are used to measure diffuse reflectance and scattering properties of materials.
Optical Calibration
Calibration laboratories use integrating spheres to create uniform light fields for camera chip or detector calibration.
Integrating Spheres in Optical Power Measurement
When used with optical power meters, integrating spheres allow accurate measurement of total emitted optical power.
This is especially important for sources where direct measurement with a small detector would capture only part of the emitted light.
Combining integrating spheres with calibrated detectors provides highly reliable measurement results.
Several design factors influence the performance of an integrating sphere.
Reflectivity
Higher reflectivity improves measurement sensitivity and uniformity.
Sphere Size
Larger spheres provide better spatial averaging but may reduce optical efficiency.
Port Size
Large ports allow more light to enter but reduce the number of internal reflections.
Balancing these factors is essential for optimal measurement performance.
Integrating Sphere Solutions from Artifex Engineering
Artifex Engineering designs integrating spheres optimized for high-precision photonics measurements.
These systems are engineered to provide:
- high reflectivity and uniform light distribution
- compatibility with optical detectors
- stable measurement performance
They are used in research laboratories and industrial environments for accurate optical power measurement. Artifex Engineering provides compact spheres with as small as 10mm internal diameter for low power measurement, up to 200mm internal diameter, gold coated metal spheres with internal water cooling for high power measurement.
The range of operation is:
- 250 – 2490nm
- 100nW – 12kW
Conclusion
Integrating spheres are indispensable tools in modern photonics measurement. By redistributing light inside a reflective spherical cavity, they allow detectors to measure total optical power from sources with complex emission patterns.
Their ability to eliminate alignment sensitivity and capture nearly all emitted light makes them essential for applications such as LED characterization, laser diode testing and optical calibration.
As photonic technologies continue to advance, integrating spheres will remain a key component of accurate optical measurement systems.
About the Author:
Dipl. Ing. Berndhard Neumann
CTO & Master of Science (M.Sc.)
Bernhard Neumann is the CTO of Artifex Engineering and holds a degree in Applied Laser Technology (M.Sc.). With 20 years at the company, he develops the measuring instruments that define Artifex’s work — covering everything from software and hardware to design and programming. Over the years, he has come to know many applications and the challenges of working in photonics- inside and out.
