Measuring the Optical Properties of Materials

Optical Proprties

Making measurements using light. From sunscreen SPF and UVA protection factor (both of which are derived in-vitro from spectral transmittance) to the reflectance of butterfly wings. Surface "appearance" is guaged by measuring BRDF or scatter.

Reflectance Measurements

Reflectance spectroscopy is a versatile analytical technique which can give information about a material’s chemical composition or formulation, as well as quantification of colour in biological, pharmaceutical, and many other applications. Reflectance is measured in a number of standardised geometries, generically termed "directional" or "diffuse". For glossy materials, reflectance is normally measured in a directional geometry, in which the sample is illuminated at 0° and reflected light collected at 45°. In optics, the Helmholtz principle of reciprocity means that you can measure in the opposite geometry and still obtain the same result. This directional geometry is referred to as 0/45, and its reciprocal as 45/0.

For scattering materials, diffuse reflectance is measured using an integrating sphere. An 8/H geometry (or its reciprocal of H/8) indicates that the sample illumination is at an angle of 8° to normal with collection by the integrating sphere of the scattered light over the complete hemisphere. The terms ‘specular-included’ and ‘specular-excluded’ refer to variations on these basic themes. For example, in the case of the 8/H geometry, a light trap can be positioned at the point on the sphere wall where the specular reflection from the sample first strikes. A light trap normally comprises a conical, black cavity which serves to fully absorb the specularly reflected component. Measurements with the light trap in place are termed ‘specular excluded’ and the sphere then measures diffuse reflectance only. If the trap is replaced with a white painted port plug, the specular component is included and the sphere will measure total reflectance. Specular reflectance can then be calculated from the total reflectance minus the diffuse reflectance.

A spectrometer configured for spectral reflectance measurements is a is a modular instrument that can be used for both directional and diffuse reflectance measurements in the UV-VIS-NIR spectral range. For diffuse reflectance, the ISP-REF is a 38mm diameter integrating sphere with 10.3mm measurement port. An internal tungsten halogen lamp providing diffuse illumination with an 8° directional (H/8) viewing geometry. The sphere includes a selectable gloss trap for performing specular included and specular excluded reflectance measurements. Reflected light is coupled to the spectrometer via optical fibre. Calibration of the instrument is performed using a Labsphere Spectralon standard of diffuse reflectance.

Other integrating spheres are available for more specilaised applications, including the Labsphere RT-060-SF and RTC-060-SF which are 150mm diameter spheres which provide an 8/H measurement geometry and include the facility for correction of the sample substitution error. The RTC version provides for "trans-reflectance" measurements with a variable angle, centre-mounted sample holder with the option of clip, jaws and cuvette holders.

For directional reflectance, the ISP-REF integrating sphere is replaced with the RPH-1 reflectance fixture. This is an SMA-905 fibre optic holder for performing 0/45 directional reflectance measurements. An optical fibre delivers illumination at 0° from an Ocean Optics light source (e.g. LS-1 tungsten halogen lamp) and a second optical fibre couples the reflected light to the spectrometer. Calibration of the instrument is performed using a Labsphere Spectralon standard of diffuse reflectance.

Transmittance Measurements

Transmittance spectroscopy can also yield information about a transparent or translucent material’s chemical composition or formulation, as well as quantification of colour. "Regular" transmittance is the analogue of specular reflectance in which the sample is illuminated at 0° and light collected along the same axis. This geometry quantifies the total transmittance of non-scattering materials.

For scattering materials, diffuse transmittance is measured using an integrating sphere. Haze can be measured using an integrating sphere by subtracting the regular component of transmittance from the total transmittance. Haze represents the diffuse component of transmittance.

A spectrometer configured for spectral transmittance measurements is a modular instrument that can be used for both regular and diffuse transmittance measurements in the UV-VIS-NIR spectral range. The 74-ACH is an adjustable collimating lens holder that provides a flexible mounting system for transmittance measurements. An optical fibre delivers illumination at 0° from an Ocean Optics light source (e.g. LS-1 tungsten halogen lamp) and a second optical fibre couples the transmitted light to the spectrometer. Each fibre is equipped with a 74-UV or 74-VIS collimating lens.

For diffuse transmittance, the collecting fibre is connected to an integrating sphere which collects all of the scattered light transmitted through the sample. A simple, general-purpose Labsphere integrating sphere will normally suffice for this type of measurement. For more specialist measurements, the Labsphere RTC-060-SF is a 150mm diameter sphere which provides for "trans-reflectance" measurements with a variable angle, centre-mounted sample holder with the option of clip, jaws and cuvette holders.

BRDF/BTDF Scatter Measurements

Colorimeters are instruments which are commonly used to measure the colour or spectral reflectance of materials. A variety of standardised measurement geometries are employed in such instruments, including directional illumination (e.g. 8°) with diffuse (hemispheric) collection using an integrating sphere or directional illumination (e.g. 0°) with directional collection (e.g. axial or annular 45°). Whilst such instruments provide measurements which conform with internationally accepted conventions, they suffer from severe limitations when used to measure the colour of materials whose spectral
reflectance varies greatly with viewing or illumination angles, such as certain metallic finishes and special effect paints. To fully quantify the appearance of such materials, it would be necessary to measure the reflected colour at all angles of illumination and viewing. Certain manufacturers have attempted to address this problem with so-called “multi-angle” colorimeters. These measure the reflected colour in several directions (e.g. 15, 25, 45, 75 & 110°) for a single illumination direction (e.g. 15°). Such instruments can only be a partial solution to the problem of measuring exotic paints and finsihes though.

Single or multi-angle colorimeters are simple, inexpensive and fast tools but can only report colour for a fixed angle of illumination and a single or limited number of viewing directions in one plane. As such, they cannot provide a meaningful measurement of appearance for complex materials. A much more sophisticated (and expensive) device is the scatterometer. This is a generic name for the type of
goniometric instrument which measures angular reflectance. A scatterometer is normally configured to measure the BRDF (bi-directional reflectance distribution function) of a surface, the ratio of incident irradiance to reflected radiance for defined angles of illumination and viewing. The light source is usually a laser, and BRDF data is given at specific wavelengths. A scatterometer is a very powerful device, providing high resolution and accuracy. However, measurements are very slow (reflected radiance is measured sequentially at one angle of elevation and at one azimuth angle for each angle of illumination), and scatterometers are very costly and complicated to use.

The Imaging Sphere IS-SA from Radiant Imaging is a unique tool that can be used to measure the angular luminous intensity and colour variation of light sources, the view angle performance of displays, and the light scatter from surfaces (bi-directional reflectance distribution function or BRDF). Combining a CCD imaging photometer with a hemispherical reflecting chamber, the Imaging Sphere is based on novel technology jointly developed by Radiant Imaging and Royal Philips Electronics. For light source and display characterisation, the Imaging Sphere functions as a far-field goniophotometer, except there are no moving parts, the measurements take just a few seconds and the source is characterised in all directions in a single measurement. For surface appearance testing, the Imaging Sphere functions as a BRDF instrument, capturing the full 2 pi hemispherical reflected intensity distribution for a given angle of illumination in a matter of seconds. In addition, the Imaging Sphere costs a fraction of the price of traditional goniometer and BRDF systems, allowing its use in routine production testing as well as R&D.

The Imaging Sphere can also be configured to measure the BTDF of a material - the bi-directional transmissive distribution function.