Integrating Sphere in Optical Measurements: Structure, Application, and Maintenance

Introduction:

An integrating sphere, also known as a photometric sphere or luminous sphere, is a hollow spherical device used for optical measurements. Its design incorporates one or more ports on the sphere’s surface for light input and placement of light-receiving devices.

Structure and Characteristics of Integrating Spheres:

Sphere Shell and Deviation Requirements:

• An integrating sphere consists of a complete spherical shell, with its inner surface maintaining a good spherical shape.
• Deviation requirements for the ideal spherical surface generally do not exceed 0.2% of the inner diameter.

Inner Surface Coating Material:

• The inner surface of the integrating sphere is coated with an ideal diffuse reflecting material to ensure multiple reflections of light on the inner surface, resulting in uniform illumination.
• Common diffuse reflecting materials include magnesium oxide (MgO) and barium sulfate (BaSO₄), which are sprayed onto the inner surface after being mixed with a resin binder.
• The spectral reflectance of magnesium oxide coatings typically exceeds 99% within the visible spectrum range.

Aperture Ratio:

• To improve measurement accuracy, the aperture ratio of the integrating sphere should be as small as possible.
• The aperture ratio is defined as the ratio of the area of the ports on the integrating sphere to the total inner surface area of the sphere.

Applications of Integrating Spheres:

Integrating spheres find wide applications in the field of optical measurements, including:

• Light source measurements: Integrating spheres can accurately measure the total luminous flux of light sources by uniformizing their light intensity.
• Optical properties measurement of materials: Integrating spheres can be used to measure the reflectance and transmittance of materials.
• Photometry measurements: Utilizing integrating spheres, one can measure the luminous distribution of light sources, providing a uniform illumination environment.

Steps for Measurement Using an Integrating Sphere Spectral Analysis System:

Measurement Steps:

Standard Lamp Measurement:
• Place a standard lamp inside the integrating sphere and illuminate it.
• Read the spectral data of the standard lamp as a reference.

Sample Measurement:
• Replace the standard lamp with the sample to be tested and illuminate it.
• Read the spectral data of the sample lamp.

Self-absorption Correction:
Perform self-absorption correction on the measurement data to eliminate the absorption effects of the sample’s emitted light.

Self-absorption Effect and Its Impact:

During measurements, especially for LED lighting products, the lamp body or heat sink may strongly absorb light due to their inseparability from the light source. This phenomenon, known as self-absorption effect, refers to the absorption effect of the emitted light by the sample itself.

The self-absorption effect mainly impacts:

• Measurement errors: When the size and shape of the sample differ significantly from the standard lamp, the response constant of the integrating sphere system may change, leading to measurement errors.
• Spectral reflectance differences: The spectral reflectance of the integrating sphere coating is uneven at different wavelengths, resulting in differences in the self-absorption effect at different wavelengths.
• Methods to Eliminate Self-absorption Effect:

To eliminate the self-absorption effect, auxiliary lamps can be used. The auxiliary lamps should possess the following characteristics:

• Broadband spectral emission: Select bulbs capable of emitting broadband spectra, such as halogen bulbs with omnidirectional light intensity distribution.
• Uniform illumination: Ensure the auxiliary lamps can create uniform illumination inside the integrating sphere to effectively correct errors caused by the self-absorption effect.

Conclusion:

By following the above steps and methods, the measurement accuracy of integrating sphere spectral analysis systems can be effectively improved, reducing the impact of the self-absorption effect on measurement results and ensuring accurate spectral data acquisition.

Maintenance of Integrating Spheres:

Proper use and regular maintenance of integrating spheres can significantly extend their service life. Maintenance of integrating spheres mainly includes three aspects: coating maintenance, circuit maintenance, and fixture maintenance.

Coating Maintenance:

• Cleaning method: Due to the unevenness of the chemical coating inside the integrating sphere, fine dust and debris can easily adhere. When cleaning, gently wipe with a soft brush in one direction.

Operating precautions:

• Before testing, hold the bulb carefully to avoid dropping or touching the coating of the integrating sphere to prevent damage.
• After testing, immediately turn off the power of the tested lamp, remove the fixture, and promptly close the integrating sphere to prevent dust and debris from entering.

Circuit Maintenance:

• Grounding and power selection: Ensure that the integrating sphere system is properly grounded and choose the appropriate power supply to ensure the safe and stable operation of the equipment.
• Regular inspection: Regularly check the wires outside the integrating sphere and the terminal junction box for damage. If worn or damaged wires are found, replace and repair them promptly.

Fixture Maintenance:

• Lamp head contacts: Ensure that the contacts of the lamp head are working normally without damage or depression to ensure stable power supply to the bulb.
• Wiring check: Ensure that the wiring of the bulb fixture is correctly connected to avoid loose or incorrect connections, which may affect test results.

Summary:

By implementing the above measures, the long-term stability and measurement accuracy of integrating spheres can be ensured, providing reliable data support for various optical measurements. Here are some specific maintenance suggestions:

• Regular cleaning: Clean the inner wall of the integrating sphere before and after each test to prevent dust and debris accumulation.
• Regular circuit checks: Inspect the circuit system monthly or quarterly to ensure that wiring and power sources are normal.
• Fixture maintenance: Check the wiring and contacts of the bulb fixture every time a bulb is installed or removed to ensure proper operation.
• Through these measures, the service life of integrating spheres can be effectively prolonged, and their accuracy and reliability in optical measurements can be maintained.

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