Reflectance standardisation over the spectral range 2.5 µm to 55.6 µm (4000 - 180 cm^-1) is carried out at NPL using PE580B and PE983G spectrophotometers with special optical devices mounted in the large sample compartments. These devices consist of a regular reflectance accessory or a variable angle regular reflectometer or a hemispherical reflectometer / transmissometer devised at NPL^[1]. The regular reflectometer^[5] can be used validly only for measurements on optically flat polished surfaces that have no diffuse component of reflection, and the angle of incidence is 10°. The variable angle reflectometer^[5] can also only be used for optically flat non-scattering surfaces, and the angle of incidence may be anything from 20° to 85°. Rigorous control of polarisation is necessary in this case.

The hemisphere reflectometer / transmissometer^[1] can be used for a sample of any surface finish, ranging from a polished to a matt or roughened finish, and the optical flatness is not as critical as with the regular reflectometer. The diffuse and regular components of reflectance need to be separated in the hemisphere measurement procedure, due to the systematic uncertainties of measurement of the components requiring different correction procedures^[2-4], and measurements are usually made at 0° and at 17.5° to the normal. For both regular and hemispherical reflectance calibrations, any part of the spectral range 2.5 µm to 55.6 µm can be covered, and where appropriate the spectral emittance and the total emittance can be computed, the latter for any temperatures required. The resulting emittance values are known more accurately than would be possible if measured by near-ambient thermal methods.

Aluminium reflectance spectrum. 

Transfer standards of regular reflectance

Two types of front-surface mirror are supplied by NPL: either a non-overcoated vacuum evaporated aluminised mirror or a ruggedised aluminium mirror which has an overcoating of silicon oxide. Both types are on optically flat glass substrates, and the maximum and minimum sizes stocked are 50 mm x 50 mm and 40 mm x 25 mm respectively, with 50 mm x 40 mm as a preferred size. Sizes of 40 mm x 40 mm, 40 mm x 30 mm and 50 mm x 30 mm may also be stocked or can be specially procured. The ruggedised aluminium mirror can be cleaned repeatedly, if necessary, and should resist most damage from normal handling. However the ruggedised mirrors have a polarisation-sensitive absorption band at around 8.1 µm which becomes marked at medium to large angles of incidence.

Glass reflectance spectrum. 

In addition to the high reflection standards a choice of two uncoated glass mirrors is offered to provide reference values over a range of low to medium reflectances. The choice is either Float or BK7 glass, the latter covering a larger range of reflectances and having a smaller spectral range of transmission where second surface reflection occurs.

Transfer standards of hemispherical reflectance

These consist of coated metal plates, maximum size 50 mm x 50 mm and minimum size 30 mm x 15 mm. Three types of coating are currently available, all with diffusely reflecting properties. The high-reflectance coating is flame-sprayed aluminium, the medium- reflectance coating is the same but with a Dacrylate^™ overcoating, and the low-reflectance coating is either 3M Co. Nextel^® 101-C10 black paint or a matt black car finish (e.g. Simoniz^® Matt Black Spray Paint) which has not got the restrahlen peaks of the Nextel^® paint.

Customer supplied samples

These measurements can also be carried out on samples provided by customers, either to give information about the sample or to allow the customer to use the calibrated sample as a working standard.

References

1. CLARKE, F.J.J. Measurement of the radiometric properties of materials for building and aerospace applications. Proc. Soc. Photo-Opt. Instrum. Eng. 1980, 234, 40-47.
2. CLARKE, F.J.J. and LARKIN, J.A. Measurement of total reflectance, transmittance and emissivity over the thermal IR spectrum. Infrared Physics, 1985, 25, 359-367.
3. CLARKE, F.J.J. and LARKIN, J.A. Emissivity determined from hemispherical reflectance and transmittance throughout the thermal infrared spectrum. High Temp. - High Press., 1985, 17, 89-96.
4. CLARKE, F.J.J. and LARKIN, J.A. Improved techniques for the NPL hemispherical reflectometer. Proc. Soc. Photo-Opt. Instrum. Eng. 1988, 917, 7-14.
5. CLARKE, F.J.J. Infrared regular reflectance standards from NPL. Proc. Soc. Photo-Opt. Instrum. Eng. 1996, 2776, 184-195.