Integrating Sphere Photometer
The geometry for a typical integrating sphere is shown below,
Constructional Details of Integrating SphereThe integrating sphere is equipped with–
- A photometer head
- A baffle
- An auxiliary lamp
- A lamp socket
Integrating Sphere Size
- The size of the baffle is determined by the size of the test lamp.
- The sphere size should be appropriate to avoid difficulty of self absorption by the lamp and sphere responsivity.
- Sphere coating should not be hampered by heat generated by the lamp.
Integrating Sphere Inner Coating
- Sphere coating is based on barium sulphate and poly tertra fluoro ethylene (PTFE).
- The reflectance of the coating is controlled by the mixing light absorbing material into coating.
- CIE recommends 80% reflectance for sphere coating.
Integrating Sphere Photometer Head
- The photometer head is V(λ) corrected silicon photo diode, with a cosine corrected angular responsivity.
- The responsivity of a photometer head can change with ambient temperature or due to head of the lamp in the sphere.
- By utilizing a careful designed amplifier (current to voltage converter) incorporate in the photometer head, a large dynamic range can be achieved.
Baffle of Integrating Sphere
- Baffle head is needed to shield the photo meter from direct illumination by the lamp.
- In order to minimize the spatial non uniformity, the baffle should be located at 1/3 or 1/2 the sphere radius from the photometer head.
- The angle (α) subtended by the baffle from photometer head should be kept to minimum, may be less than 30o.
Auxiliary Lamp inside the Integrating Sphere
- Integrating sphere photometer is equipped with an auxiliary lamp to allow measurement of self absorption by lamp.
- It must provide illumination to entire sphere wall.
- It must be shielded from the photometer head and the lamp to be measured.
Measurement of Luminous Flux by Integrating SphereBefore measurement we have to go for some assumptions, they are-
- Sphere is empty
- Inner surface is perfectly diffused in nature
- Reflectivity is spectrally non selective
Application of Integrating Sphere
- Light scattered by the interior of the integrating sphere is evenly distributed over all angles. The integrating sphere is used in optical measurements. The total power (flux) of a light source can be measured without inaccuracy caused by the directional characteristics of the source. Reflection and absorption of samples can be studied. The sphere creates a reference radiation source that can be used to provide a photometric standard.
- Integrating spheres are used for a variety of optical, photometric or radiometric measurements.
- They are used to measure the total light radiated in all directions from a lamp.
- An integrating sphere can be used to measure the diffuse reflectance of surfaces, providing an average over all angles of illumination and observation.
- An integrating sphere can be used to create a light source with apparent intensity uniform over all positions within its circular aperture and independent of direction except for the cosine function inherent to ideally diffuse radiating surfaces (Lambertian surfaces).
- Since all the light incident on the input port is collected, a detector connected to an integrating sphere can accurately measure the sum of all the ambient light incident on a small circular aperture. The total power of a laser beam can be measured, free from the effects of beam shape, incident direction, and incident position.