The geometry for a typical integrating sphere is shown below,
Constructional Details of Integrating Sphere
The 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 Sphere
Before 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
Now for standard lamp, the luminous flux ФS is premeasured.
The standard lamp luminous flux ФS is proportional to its Illuminance ES.
Now for a test lamp, Luminous Flux ФT is to be measured.
We measure Illuminance of the test lamp ET with the help of integrating sphere photometer head.
Here both lamps are fed to the DC supply.
Step 1. When test lam is ON, auxiliary Lamp is OFF. Measure the test Lamp Illuminance ET.
Step 2. When the Test Lamp is OFF, the auxiliary Lamp is ON. Measure the Illuminance ES.
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.