Integrating Sphere
I. Description
An integrating sphere is a hollow sphere with an inner surface coated with a diffuse reflective material, and its outer structure is generally made of metal. It has several small holes. For the coating of the integrating sphere, domestic options include MgO, BaSO₄, and PTFE, while foreign ones include Spectraflect, Spectralon, Duraflect, and Infragold. The small holes on the sphere are typically used as incident light apertures, for installing detectors, light sources, etc. Accessories include aperture reducers, hole plugs, collimating lenses, detectors, fiber optic adapters, bracket bases, and so on. Common applications include camera calibration, laser power spectrum measurement, transmittance spectrum measurement, reflectance spectrum measurement, spectral radiation measurement, and more.
II. Classification
Based on different application scenarios, integrating spheres are divided into various categories. Currently, the Weidu integrating sphere series products are classified as follows:
Standard universal integrating spheres: They can be equipped with optical components such as light sources, PD detectors, filters, aperture reducers, fiber optic interface parts, and collimating lens groups, meeting the application needs in various optical fields such as laser power measurement, transmittance measurement, reflectance measurement, and spectral radiation calibration.
Integrating sphere uniform light sources: Widely used in flat panel display detection, camera calibration (including large field of view cameras, various vehicle-mounted cameras, infrared cameras), calibration of imaging luminance meters/colorimeters, and calibration of cameras and ambient light sensors in various consumer electronics such as mobile phones.
Laser power integrating spheres: Applied in fields such as VCSEL, laser power measurement, detection of output characteristics of lasers and laser diodes, optical detection platforms, high-power infrared laser measurement, water-cooled laser power measurement, measurement of output characteristics of CO₂ lasers and Nd:YAG lasers, and high-power laser diode arrays.
Transmission-reflection integrating spheres: Can be used for measuring the optical transmittance of optical material lenses, suitable for spectral transmission measurement of mobile phone screens, displays, glass lenses, glues, inks, coated lenses, etc. They can also be used for reflectance measurement of samples with angular incidence.
Spectral radiation integrating spheres: Applied in spectral radiation measurement. When matched with a spectrometer, they can realize the measurement of photometric parameters of LEDs, such as luminous flux, color coordinates, luminance, illuminance, peak wavelength, and FWHM.
III. Working Principle
As shown in the figure below, after light enters through the input port, it is uniformly reflected and diffused inside the sphere. Therefore, the light obtained at the output port is a fairly uniform diffused beam. Moreover, the incident angle, spatial distribution, and polarization of the incident light will not affect the intensity and uniformity of the output beam. The integrating sphere can also function as a light intensity attenuator, where the ratio of the output intensity to the input intensity is approximately equal to: the area of the light output port / the internal surface area of the integrating sphere.
IV. Coatings
Coatings are one of the most important factors in evaluating the quality of an integrating sphere. Domestic coatings include magnesium oxide (MgO), barium sulfate (BaSO₄), and polytetrafluoroethylene (PTFE). Compared with MgO and BaSO₄, PTFE has the following advantages:
(1) In the wavelength range of 0.2-2.5 microns, its spectral reflectance is higher than that of MgO and BaSO₄;
(2) It has good neutral reflectance;
(3) The coating has good adhesion, is not easy to crack or wrinkle, and dust can be removed with a clean brush without damaging the coating;
(4) It is resistant to moisture and will not turn yellow when exposed to ultraviolet light.
Foreign coatings include Spectraflect, Spectralon, Duraflect, and Infragold, each with the following characteristics:
(1) Spectraflect: Economical and practical, with high reflectance in the visible light spectrum;
(2) Spectralon: High reflectance and wide spectral range;
(3) Duraflect: Waterproof, moisture-proof, and corrosion-resistant;
(4) Infragold: Suitable for near, mid, and far-infrared bands.
V. Selection
(1) Select according to the functional type of the integrating sphere
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Integrating sphere uniform light source: Creates a uniform light source
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Laser power integrating sphere: Used for laser power spectrum measurement
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Transmission-reflection integrating sphere: Used for transmittance spectrum measurement and reflectance spectrum measurement
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Spectral radiation integrating sphere: Used for spectral radiation measurement
(2) Select according to the coating of the diffuse reflection layer
The diffuse layer generally includes magnesium oxide, barium sulfate, and polytetrafluoroethylene, with their reflectivity in descending order as follows: polytetrafluoroethylene > barium sulfate > magnesium oxide.
(3) Select according to size
Generally, the total opening size of an integrating sphere should not exceed one-third of its diameter. Usually, as the diameter of the sphere increases, the "integrating" property of the sphere improves, allowing for larger port sizes and the ability to accommodate higher input power.
VI. Accessories
In practical applications, integrating spheres usually need to be paired with many accessories to complete relevant testing projects. These accessories generally include: light sources, detectors, radiometers, photometers, aperture reducers, fiber optic adapters, optical filters, filter adapters, cosine diffuser assemblies, collimating lens assemblies, mounting poles and base assemblies, opening adapters, etc.
VII. Precautions
(1) Operate strictly in accordance with the product instruction manual.
(2) When the instrument is not in use, please disconnect the power supply.
(3) Some products may reach high temperatures during use; do not touch them directly to avoid burns.
(4) Do not measure strong light sources for a long time, as this may affect the light-sensing performance of the instrument.
(5) Some precision measurement products need to be preheated.
(6) It is essential to keep the inside of the sphere clean, protect it from moisture and corrosion, and do not touch the inner wall coating of the integrating sphere with hands or other objects.
