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Spectral Reflectivity Measurements Using Fiber Optics
L. A. Lott and D. L. Cash
A simple technique utilizing a bifurcated fiber light guide for obtaining the reflectivity spectra of specu-
lar and diffusely reflecting surfaces is described. The technique can be used in certain situations where
conventional
reflectometers are not practical.
Introduction
Spectral reflectivity measurements can be accu-
rately and easily measured on samples that are small
enough to be accommodated by commercial reflec-
tometers. However, for large objects, objects with
nonplanar surfaces, or surfaces of toxic or radioactive
materials that must be handled in remote glovebox
situations, reflectivity measurements can be very dif-
ficult.
This paper describes how a commercially available
fiber optic device can be easily used for making ac-
curate reflectivity measurements on specular and
diffusely reflecting surfaces without the practical dif-
ficulties referred to above.
Description of Apparatus and Experimental
Procedure
A bifurcated fiber light guide' 120 cm long with a
main bundle diameter of 0.64 cm was adapted to the
sample compartment of a visible-near infrared spec-
trophotometer 2as shown in Fig. 1. Two front sur-
face plane mirrors Ml and M2are positioned in the
normal light path of the instrument. The mirrors
are oriented at angles of 45° with the direction of the
beam in such a way that mirror Ml intercepts the
light exiting from the monochromator section and
directs it into one branch of the bifurcated light
guide that is mounted in the side of the sample com-
partment. Light that reenters the sample compart-
ment through the other branch of the light guide is
directed by M2into the phototube section of the
spectrophotometer. A lens L corrects for the defo-
cusing produced in the beam.
The intensity of light reflected from a sample sur-
face is measured by placing the end of the main bun-
The authors are with Dow Chemical U.S.A., Rocky Flats Divi-
sion, P.O. Box 888, Golden, Colorado 80401.
Received 2 October 1972.
dle of the, light guide against the surface.
Monochromatic light entering the first branch of
the light guide passes through the main bundle and
onto the sample surface. The guide is positioned
against the surface by a hand-held Teflon probe
head in such a manner that the light is reflected
from the surface back into the guide. The end of the
guide and the Teflon head are shown in cross section
in Fig. 2. The Teflon head provides firm support so
that the probe can be held motionless against the
surface and is soft enough not to scratch or mar the
surface. The fibers in each branch of the light guide
are randomly distributed in the main bundle; there-
fore, half the reflected light passes into the second
branch of the guide and into the phototube compart-
ment.
The reflectivity spectrum of the sample is obtained
by alternately placing the probe against the sample
and a standard aluminum reference mirror. The
controls of the spectrophotometer are operated in the
normal manner. By first adjusting the slit and sen-
sitivity controls to obtain a reading of 100% for the
reference mirror, the relative reflectivity of the sam-
ple at a particular wavelength is read directly from
the instrument when the probe is placed on the sam-
ple. This is done point by point over the desired
wavelength range to generate the reflectivity spec-
trum.
Measurements can be made on surfaces located
anywhere within reach of the 120-cm-long guide.
Measurements can be made in the spectral range of
from 0.4 , to 1.5 over which the light guide has a
transmittance of approximately 40-50%.
As an alternative to using this type of spectropho-
tometer, one branch of the fiber light guide can be
attached to a separate light source and the other
mounted against the entrance slit of a single-beam
spectrophotometer. Reflectivity measurements are
made in the same manner as described. This sys-
tem was tried in a number of cases and gave essen-
tially the same results as the other.
April
1973 / Vol. 12, No. 4 / APPLIED OPTICS 837