Here - PMOD/WRC

Goniometric Realisation of Reflectance Scales in the Ultra Violet to Near
Infrared
M. J. Shaw & C. J. Chunnilall
National Physical Laboratory, Tedddington, TW11 0LW, U.K.
Abstract. The National Physical Laboratory (NPL)
realizes UK diffuse reflectance scales from 350 nm to 1
µm using a gonioreflectometer and from 2.5 µm to 56 µm
using a hemispherical technique. This paper presents
work carried out to extend the spectral range of
goniometric measurements made using the National
Reference Reflectometer (NRR) to realize scales for
radiance factor and hemispherical reflectance in the near
infrared from 1 µm to 2.5 µm. The NRR is also used to
realize a scale for multi-angle regular reflectance from 300
nm to 2.5 µm. Detailed uncertainty analyses covering the
various capabilities over the full spectral range will be
presented.
The National Reference Reflectometer (NRR)
The NRR is built around a pair of concentric rotary
turntables, one which rotates the sample to set the angle of
incidence and the other which moves the detector to set the
receiver angle. Using these two turntables the light
reflected off the sample can be measured at any position
within the plane of incidence. The light source is a
tungsten ribbon lamp the output from which is collimated
and then passed through a rotatable linear polariser. The
wavelength of the incident radiation is set using various
bandpass interference filters with a bandwidth of
approximately 15 nm [1, 2]. For measurement of regular
reflectance the interference filters are removed and the
light from the tungsten source is coupled into a double
grating monochromator used in subtractive mode for high
spectral resolution.
For earlier scale realization measurements in the visible
the incident beam was brought to a focus at the sample by
a pair of silica lenses. To reduce aberrations when using
the NRR in the near infrared, this lens pair has been
replaced by a new system of reflective optics. The
collimated light beam is deflected away from the optical
axis onto a spherical mirror that steers the light off a plane
mirror and brings it to a focus at the sample plane (figure
1).
Light reflected from the sample is viewed through a
precision circular aperture mounted in front of the detector.
For measurements from the UV to 1 µm a silicon
photodiode detector is used and from 0.9 µm to 1.6 µm an
InGaAs detector is used. For near infrared measurements
from 1.5 µm to 2.5 µm the incident light is chopped and
the reflected flux measured using a cooled InSb detector
and lock in amplifier for phase sensitive detection.
Figure 1. Optical design of the NPL Reference Reflectometer
with reflective launch optics.
Diffuse Reflectance
For several years NPL has independently realized UK
diffuse reflectance scales from 350 nm to 1000 nm using
the NRR. The 0º/45º scale is realized directly by
measurement of the directional radiance factor and the
hemispherical scale is determined by spatial integration of
goniometric values [1, 2]. From 2.5 µm to 56 µm, an
absolute hemispherical technique [3, 4] is used to establish
a scale for hemispherical reflectance. In the near infrared
from 1 µm to 2.5 µm NPL has until recently relied on
published values of barium sulphate [5] modified by
measurements made at 2.5 µm using the hemispherical
technique.
Using the infrared detectors and reflective optics, the
NRR has now been used to independently realize diffuse
reflectance scales in the range 1 to 2.5 µm. Measurements
at either end of this range show good agreement with
existing near and mid infrared scales completing a single
continuous scale from the UV to the far infrared.
Measurement of glossy samples
The NRR is also used to measure the reflectance of
glossy reference standards. In order to determine the
reflectance of such materials under 8˚/t (specular included)
and 8˚/d (specular excluded) geometries a series of
measurements are made with an increased angular
resolution around the specular peak. The specular and
diffuse components of reflection are then separated
mathematically allowing the hemispherical reflectance to
be determined under different measuring conditions.
Regular Reflectance
The source optics of the NRR may be reconfigured to
Proceedings NEWRAD, 17-19 October 2005, Davos, Switzerland 339