Integrating Sphere for Specular and Diffuse Samples

An integrating-sphere system has been designed and constructed for multiple optical properties measurement in the IR spectral range. It is used for both diffuse and specular samples, the absolute transmittance and reflectance can be measured directly and the absorptance or emittance can be obtained from these by simple calculation. These properties are measured using a Fourier transform spectrophotometer as the spectrally tuned light source. Measurement procedures allow the absolute values of the properties to be determined for both specular and diffuse samples.

The inside wall of the sphere is coated with a material that is nearly a Lambertian diffuser and at the same time has a high directional hemispherical (diffuse) reflectance – about 0.9 for the IR spectral range: plasma sprayed Cu on a brass substrate, electroplated with Au. The sphere has entrance, sample, and reference ports, all centered on a great circle of the sphere. The detector’s field of view is centered on the same normal and corresponds to the bottom region of the sphere. The sample and the reference ports are located symmetrically with respect to the entrance port. The Hg:Cd:Te (MCT) detector Dewar is mounted on the top of the sphere. A pair of rotation stages underneath the sphere is used to move the sphere into positions for both reflectance and transmittance measurements.

Heated samples can also be measured with the integrating sphere. For temperatures below 600 K the sample temperature is determined from PRT or TC sensors embedded in the sample mount, or in the sample itself. The sample heater/mount can also be used to measure transparent samples. There are four interchangeable internal heat exchangers. Two use electrical heating for higher temperatures to 200 °C: one solid holder for opaque samples, and one ring holder for transparent samples. Two similar heaters use liquid from a temperature-controlled bath recirculator unit for temperatures between 270 K and 350 K. The ring shape of two of the heaters does not obscure the central region of the sample, allowing light to pass through for a transmittance measurement, as well as allowing the transmitted component to be diverted and absorbed in a reflectance measurement. For opaque samples, the solid heaters are used to obtain the best temperature uniformity.

Details of the absolute measurement methods used with the integrating sphere can be found in the references below.

Specifications

• Wavelength Range: 1.0 µm - 18 µm
• Temperature Range: 270 k - 570 k
• 6" Diameter
• Gold-Electroplated Plasma-Sprayed Metal Coating
• MCT detector with hyperbolic concentrator optics
• Baffling in sphere
• 8° Incidence Angle

Capabilities

• Reflectance, Transmittance & Absorptance (Emittance)
• absolute & relative specular R, T & A (E)
• absolute & relative diffuse R, T & A (E)
• uncertainties (2s):
  • specular: ≤ 0.3%
  • diffuse: 1.5 - 3%
  • arger for angle dependent structure
• can measure R of transmissive samples
• can sort out scatter from total R & T

The integrating sphere for absolute IR spectral transmittance
and reflectance at ambient temperature.

References

• "Integrating-sphere system and method for absolute measurement of transmittance, reflectance, and absorptance of specular samples,"
  Hanssen L.M.,
  Appl. Opt. 40, 3196-3204 (2001).


• "Integrating Spheres for Mid- and Near-Infrared Reflection Spectroscopy,"
  Hanssen, L.M. and Snail, K.A.
  in Handbook of Vibrational Spectroscopy, ed. by J. Chalmers and P. Griffiths (2001), p. 1175-1191.