Laser-based spectrophotometry at NPL

ABSTRACT Traditional spectrophotometry, including regular and diffuse scattering measurements of reflected and transmitted light,employs lamp-based sources of radiation which are spectrally filtered by a spectrometer or by individual optical filters.Lasers offer distinct advantages to the traditional sources, most notably high spectral power and the possibility of verysmall irradiated area. NPL has established a new laser-based facility for reflectance and transmittance measurementswhich utilises NPL’s National Laser Radiation Facility, a suite of lasers covering a broad spectral range from the UV tothe IR. This paper will highlight the capabilities of this new facility, discuss several of its unique design features, andpresent an analysis of the factors which influence its ultimate accuracy.Keywords: diffuse reflectance, diffuse transmittance, regular reflectance, regular transmittance, BRDF, BSDF,spectrophotometry 1. INTRODUCTION The National Physical Laboratory has developed a facility for measuring mainly diffuse but also regular reflectance andtransmittance using laser radiation and a goniometer system. This facility is designed to exploit the key advantages oflaser radiation compared to lamp-based sources, namely high spectral power and the ability to produce very smalldiameter beams of collimated radiation. For example, the bi-directional reflectance distribution function (BRDF) fromareas of sample on the order of a millimetre in diameter can be measured only with a laser beam source. Thereforelasers provide a means of measuring both scatter from small samples as well as the uniformity of scatter across thesurface of larger ones. The high spectral powers available also enable relatively high signal-to-noise measurementseven for poor scatterers like windows, mirrors, and black coatings.A key design feature of the apparatus is its versatility. The core of the goniometer system is a double rotation stage,which can be easily transported between rooms. Various additional stages can be mounted on top of the goniometer toenable sample rotation within the plane of its surface, out-of-plane scatter measurements, and regular reflectance andtransmittance measurements. These will be described in section 2.Probing smaller sample areas makes alignment accuracy more critical. Therefore we have used numerical modelling,discussed in section 3, to investigate how the diameter and the alignment of the laser beam and detector aperture affectsmeasurement uncertainty. As this paper is intended as an overview of our new facility, details of the alignment andmeasurement procedures will not be discussed in depth.