Reflectance Transmittance Integrating Spheres

Labsphere RT & RTC Reflectance Transmittance Integrating Spheres

Labsphere RT and RTC reflectance transmittance integrating spheres are purpose designed for measuring the total hemispherical reflectance and transmittance of both specular and diffusely reflecting materials. The RT-060-xx integrating spheres provide for reflectance and transmittance measurement of a sample placed outside the sphere at a port opening. The RTC-060-xx integrating spheres provide in addition a centre mounted sample holder that allows a sample to be suspended vertically in the centre of the sphere. This can be rotated with respect to the sample beam for the measurement of total reflectance as a function of angle of illumination. Two sphere coatings are offered, Spectraflect (SF) for 300-2400nm and Infragold (IG) for 0.7-20μm.

Key Features of Reflectance Transmittance Integrating Spheres

• Integrating spheres purpose-designed for measuring reflectance & transmittance
• Measure total hemispherical reflectance (specular included), diffuse reflectance (specular excluded) & specular reflectance as well as total hemispherical transmittance
• Measure total hemispherical reflectance versus angle of illumination (RTC models only)
• 6-inch diameter integrating spheres with a choice of Spectraflect (300-2400nm) or Infragold (0.7-20μm) coatings

Reflectance Spectroscopy

Reflectance measurements that use an integrating sphere provide for absolute values that are independent of whether the sample is matte or glossy. Similarly, transmittance measurements performed with a sphere yield readings that are independent of whether the sample is clear of scattering. With the sample placed at the sphere port and illuminated with a collimated beam of light, the integrating sphere captures all of the reflected light over a complete hemisphere, which is then sampled by a spectrometer coupled to the sphere. The spectral reflectance recorded allows the object’s colour to be computed, while the spectral signature will be characteristic of the material’s formulation which provides an insight into the makeup of the material.

Purpose Designed for Reflectance & Transmittance Measurements

The Labsphere RT-060-xx integrating spheres feature five 1-inch diameter ports necessary to accommodate sample and reference beams necessary for a 8° double beam geometry, as well as port plugs required for a 8° single beam geometry. A 0.5-inch detector port is located at the north pole position on the sphere. A specular light trap is included for specular excluded reflectance measurements. A 1/4-20 tapped mounting boss is located at the south pole position on the sphere, and a post mount, post mount holder and square base plate are included.

The Labsphere RTC-060-xx integrating spheres provide for enhanced versatility with a 2.5-inch port at the north pole that accepts a vertically suspended, centre mounted, rotating sample holder that allows for measurements of reflectance versus incident angle of illumination. RTC spheres feature three equatorial 1-inch diameter ports and two 1.25-inch diameter ports necessary to accommodate sample and reference beams necessary for a 8° double beam geometry, as well as port plugs required for a 8° single beam geometry. A 0.5-inch detector port is located at the south pole of the sphere. A specular light trap is included for specular excluded reflectance measurements. RTC spheres are housed in a sturdy “H-frame” stand.

Both RT and RTC spheres are suitable for transmittance measurements. In transmittance mode, the sample is placed at the input port of the integrating sphere and illuminated with a collimated sample beam. The sphere then collects both the normal and diffuse components of the transmitted light, meaning that total transmittance is recorded for both clear and scattering materials.

The RT spheres can perform reflectance measurements in the specular included (8°/H) and specular excluded (8°/D) geometries. Subtracting the diffuse-only reflectance from the total hemispherical reflectance yields the value of specular-only (“gloss”) reflectance. In addition, RTC spheres provide for total hemispherical reflectance measurements as a function of angle of illumination.

All sphere ports are knife-edged to permit collection of wide-angle scatter and internal baffling is minimised to allow the detector a maximum view of the sphere wall. Spring-loaded sample holders are provided that attach to the rugged port frames surrounding the port openings to hold the sample at either the reflectance or transmittance port positions and also hold a reflectance standard.

Centre Mounted Sample Holders

The RTC-060 integrating spheres feature a 2.5-inch port opening at the north pole position that accepts a variable angle sample holder. As standard, RTC spheres are supplied with a jaws-style centre mount sample holder, while clip and cuvette-style centre mount sample holders are available as optional accessories. The jaws-style holder may be used for samples that can be clamped. The jaws of the sample holder can accommodate samples as large as 1 x 2-inches and up to 0.38-inches thick. The clip style sample holder is designed for thin films or samples which do not have the rigidity to be held by the jaws-style sample holder. Samples up to 1.50 x 2-inches and 0.125-inches thick can be held using the clip-style holder. The cuvette-style holder holds standard 12.5 mm square cuvettes and is used to hold liquids and powders for absorbance and scatter characterisation. A 2.5-inch port plug is included that seals the north pole port opening for those measurements that do not require a centre mount sample holder.

Reflectance Transmittance Integrating Sphere Calibration

For reflectance measurements, a reference standard of diffuse reflectance is required. This is placed first at the sample port and the spectrometer signal recorded (reference measurement). The reflectance standard is then swapped out for the sample under test and the spectrometer signal recorded for a second time (sample measurement). The reflectance of the sample material is then computed from the ratio of the two readings.

Please note that the multiple reflections inherent in an integrating sphere lead to sample reillumination and an error called the substitution error. This can be eliminated in one of two ways. Either the sample and reflectance standard are mounted on the sphere at the same time and their positions swapped between reference and sample measurements. This maintains the same sphere throughput and eliminates the substitution error. Alternatively, the sample beam can be split into sample and reference beams (called a “doubled beam” configuration) and each of the two beams illuminates the sample and reflectance standard respectively. This approach avoids the need to physically move the sample and reflectance standard between ports when performing calibration and sample measurements. Both the RT and RTC integrating spheres support each of these approaches.

In transmittance, the integrating sphere is self-calibrating. To calibrate the sphere, the sample beam signal is recorded with the input port open. The sample is then placed at the port opening and the transmitted signal recorded with the spectrometer. The transmittance of the sample material is then computed from the ratio of the two readings.

Integrating Sphere Coatings

Labsphere RT and RTC series integrating spheres are available with two diffusely reflecting coatings. Choose Spectraflect, a barium sulphate-based coating for the 300-2400nm spectral range with a nominal 98% peak reflectance. For the infrared band from 700nm to 20μm, electro-plated Infragold is available, a coating that features a reflectance of approximately 94%. Both coatings exhibit a near-perfect Lambertian (or diffuse) reflectance, which means that the reflected radiance from the sphere surface is equal for all directions of view, promoting a higher level of measurement accuracy and repeatability with both matte and glossy sample materials.