REFLECTION-INCREASING AND -REDUCING FILMS

utilizes Airy's methods but credits Airy only with solv-ing the problem of determining the reflectance of a planeparallel plate with identical media on both sides. Theseequations are used in most optics books for predicting theintensity distribution in Haidinger ring systems seen inplane parallel plates. In fairness to these early workerswho labored both with inadequate theories and inferiorexperimental equipment, it would seem appropriate forothers in this field to study once again these early papersfor they contain much having a direct bearing on the

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

understanding of the optical properties of these films.Where possible, this work ought also to be more freelyacknowledged, for these papers are excellent examples ofthe advanced type of thinking which was possible eventhen and which we of later generations sometimes tendto overlook.

The author is indebted to Mr. A. J. Devlin of theOptical Shop at the Naval Gun Factory who providedthe prisms and films used for the experimental measure-ments connected with this paper.

VOLUME 42, NUMBER 4 APRIL, 1952

A Reflectometer for Measuring Diffuse Reflectance in the Visibleand Infrared Regions

WILLARD L. DERKSEN AND THOMAS I. MONAHANNaval Material Laboratory, New York Naval Shipyard, Brooklyn, New York

(Received June 25, 1951)

A simple reflectometer to measure diffuse spectral reflectance in the visible and infrared spectral regions isdescribed. The reflectometer is employed in conjunction with the monochromator of a Perkin-Elmer Model12B spectrometer. A lead sulfide photoconductive cell with a simple bridge circuit is employed as the de-tector. The long wavelength cutoff is at 2.8 microns while the short wavelength limit is defined by the spectralemittance of the tungsten lamp used as the source of radiation. The spectral reflectance of several commonmaterials, including human skin, have been determined.

INTRODUCTION

THERE is at the present time no commercial in-strument available for the measurement of diffuse

spectral reflectance in the wavelength region above onemicron. Moreover, published data on the reflectancesof materials in this region are meager.

The Naval Material Laboratory has constructedand is using an infrared reflectometer similar in designto that employed by W. W. Coblentz' at the Bureauof Standards in 1912, and later by Sanderson.2 Thereflectometer is used in conjunction with the mono-chromator of a Perkin-Elmer Model 12B infraredspectrometer. A 6-volt, 50-candlepower automobileheadlight lamp, operated at about 7 volts, is used as thesource. Lack of energy from this source below about0.4,q sets the lower wavelength limit of the reflectometer.The lead sulfide cell used as the receiving elementhas considerable response in the range from 0.4.t to2.8/..

THE REFLECTOMETER

A spherical mirror has the property that energyemanating from any point on a diameter near the centeris focused at a conjugate point opposite and equidistantfrom the center of curvature. In the reflectometer,a hemispherical mirror is employed to collect the energyreflected from the sample whose reflectance is beingdetermined. The sample is irradiated at a point nearthe center of the hemisphere and the reflected energy

1W. W. Coblentz, Bull. Natl. Bur. Standards 9, 283, 1912.2 J A. Sanderson, J. Opt. Soc. Am. 37, 771, 1947.

is then focused on the receiver at the conjugate point.The incident energy is measured by substituting thereceiver for the sample.

The reflectometer's optical elements are shown inprofile in Fig. 1. The achromatic glass lens with a 4.7 cmfocal length, which is mounted in the window behindthe exit slit of the Perkin-Elmer monochromator,focuses the energy from the exit slit through a hole inthe bottom of the hemispherical mirror to the point,P, in the aperture plane of the mirror about 0.7 cmfrom the center of curvature. The first-surface planemirror, M, directs the beam upward so as to allow thereflectometer to be operated with the sample in a hori-zontal plane. The sample holder and receiver are rigidly

POS CELL

HEMISPHERICALREFLECTOR

MONOCHROMATOREXIT SLIT

SAMPLE

M

GLASS LENS

FIG. 1. The optical diagram of the infrared reflectometer.

April 1952 263

W. L. DERKSEN AND T. I. MONAHAN

FIG. 2. Top view of the infrared reflectometer.

mounted on a sliding brass block so that either thesample or the receiver may be placed at P. When thesample lies at P, the receiver is located at the conjugatepoint, C. The push rods used to move the sample andreceiver holders are threaded into the sliding block sothat by turning the knurled knob and maintaining aslight pressure against the stops, a fine positioningadjustment at either of the two locations of the blockmay be obtained.

The angle of incidence of the energy on the sampleis approximately 9. The size of the slit image on thesample is 1 cm by 2 mm, the length being limited byreducing the slit length of the monochromator. Theallowable size of the image is set by the size of thesensitive area of the lead sulfide cell used as receiver.The cell has a sensitive area of about 7 by 11 mm, anda resistance at room temperature of about 0.6 meg.

The hemispherical mirror was cut from a large lampbulb, and has a diameter of 14.5 cm. The inside surfacewas coated by the evaporated aluminum process. Itis mounted in a machined copper and brass housing.The reflectometer assembly is shown in Fig. 2.

The change in resistance of the lead sulfide cell,which is linear with change in incident illumination inthe range of illuminations used, is determined byreading the unbalance in a simple resistance bridgenetwork by means of a sensitive galvanometer.

CALIBRATION

The energy incident upon the sample, I(X), is re-duced in several ways before it reaches the receivercell. First, it is reduced to p2(X)Io((X) by reflection uponthe sample, where p 2 is the reflectance of the sample.This remaining portion of the energy is then reducedto Pr(PxflIo) by reflection upon the mirror. Finally, theenergy actually entering the sensitive surface is KxprpzIo,where 1.- K. is the fraction of energy lost through thehole cut in the hemispherical mirror, by reflection bythe glass wall of the receiver cell, and by poor focusingbecause of imperfections in the mirror. The factor, K,is considered separately from p,; because it will varywith the polar reflecting qualities of samples, it is as-sumed to be constant with wavelength.

The reading of the reflectometer at any wavelength,which is the ratio of the received energy to the incidentenergy, is the R 2=Kpp 2. These readings are takenthroughout the range from 0.4 jt to 2.8 A. In order toobtain the value of the reflectances with reference toto magnesium oxide, as is the convention, readingsthroughout the same range are taken with a properlyprepared' specimen of magnesium oxide. Since thereflectance in the visible region of magnesium oxideis well established to be about 0.98,4 and since it hasbeen found that crystals of magnesium oxide have noabsorption bands in the entire wavelength range ofinterest, 5 reflectances referred to magnesium oxide inthis range will lie close to true reflectance values. Afairly thick coating of the oxide was used to assureagainst any transmission for the longer infrared wave-lengths. The reflectometer readings for magnesiumoxide, at each wavelength, will be equal to Rm= KmpprPm,where the subscript m refers to magnesium oxide. Therequired value, p, is then equal to RzKm/RmKx.

In order to find the value of Km/Km the reflectance ofthe sample in the range 0.4,u to 1.0,u is found by means

ILz MAOEI I 11O Ao I

I N. -

0 UK k DIG 0

A .6 .8 1.0 1.2 1.4 1.6 1.8 2.0 22 2.4 2.6WAVE LENGTH IN NICRONS

FIG. 3. Reflectance of calibration substances.

of General Electric recording spectrophotometer. Thereflectances found by this method are equated toRzKm/RmKz at sufficient wavelengths to permit anaccurate determination of Km/K2c.

In practice, an aluminized plane mirror and a mag-nesium carbonate block are used as calibration stand-ards, since their surfaces are more stable than that ofmagnesium oxide. Moreover, since reflection by thealuminized mirror is specular, the energy enters thereceiving cell normally, as it does when the cell is in theposition for measuring incident energy, and hence itsvalue of K is near unity. With this fact and employingthe spectral reflectance of the mirror as found with theG. E. spectrophotometer, the spectral reflectance ofthe hemispherical mirror in the range O.4 to 2.8,4may be found.

Figure 3 shows the values of Rm, the reflectometerreadings for magnesium oxide plotted against wave-length (curve 1), the spectral reflectance of the hemis-spherical mirror (curve 3), and the reflectance of mag-

a Natl. Bur. Standards,. Letter Circular, LC547 (1939).4 Benford, Loyd, and Schwartz, J. Opt. Soc. Am. 38, 445, (1948).8 J. C. Willmot, Proc. Phys. Soc. (London) 63, 389, (1950).

264 Vol. 42

REFLECTOMETER FOR INFRARED REGION

nesium carbonate at several wavelengths (curve 4),all as determined with the reflectometer. It will be notedthat the values of curve 3 are lower than is usual fora newly made sample of aluminized glass. This is knownto be the result of some lapse in the aluminizing tech-nique, since some flat samples which were treated atthe same time also showed unusually low reflectances.

Repetitive measurements of the reflectance of themagnesium carbonate and aluminized surfaces indicatea scattering of less than i five percent for measure-ments obtained over a period of several months. Re-petitive determinations on cloth and wood samplesyield reflectance values over the entire wavelengthrange, which differ by not more than three or fourpercent. The polar reflecting diagrams of samples mayvary with wavelength, giving rise to errors in reflectancedeterminations. This could be checked by taking meas-

'6 ,, LO L2-_ ~~~~_ _ _ _ .~iPi

WAVE LENGTHLEAD FOILDOUGLAS FIR…COTTON TWILL.....__

IN MICRONSWOOL SERGEWHITE PAINT ON DOUGLAS FIR

FIG. 4. Reflectance of some common substances.

urements with the acceptance angle of the receivercell limited to the peripheral regions of the hemisphere.

TYPICAL MEASUREMENTS

The results for several common substances whichhave been measured are given in Fig. 4. Included arevalues for samples of lead foil, a light-colored portionof Douglas fir, dark blue serge, and bleached cottontwill cloth. In Fig. 5 are given the reflectance valuesfor human skin. Below one micron the values wereobtained from the palm and back of the subject's hand,using the G. E. Recording Spectrophotometer. Thevalues above one micron were obtained from the ballof the thumb and the back of the index finger, usingthe infrared reflectometer. Because of positioningdifficulties the values above one micron were not asreproducible as those for quiescent samples.

I - -~~~~~~~~ I- ~ Jj

0' _ t _ w W) A _ _ _ TEE . -- ' -_

. Bto 12 IA4 L8 2 2. 2 A2*s.WAVE LENGTH IN MICRONS

IEDIUM NEGRO SKIN - PALM AND BALL OF THUMBMEDIUM NEGRO SKIN - BACK OF HAND AND FIRST FINGERFAIR CAUCASIAN SKIN - PALM AND BALL OF THUMBFAIR CAUCASIAN SKIN - BACK OF HAND AND FIRST FINGER--------

FIG. 5. Reflectance of human skin.

Of interest is the effect of pigmentation in the com-parison of the values obtained from a medium Negrosubject and an untanned, fair, Caucasian subject. Thevalues for reflectance of skin in the visible region cor-respond closely with those which have been previouslypublished. In the near infrared (0.75 to 1.2 microns)the reflectance values have their highest value, whilefor longer wavelengths there is relatively low re-flectance. The differences between the palm and theback of the hand are noticeable for both subjects inthe visible and near infrared regions, and for the fairsubject at longer wavelengths. For the Negro subjectthis difference was not apparent above one micron.

POSSIBLE MODIFICATIONS

There are several modifications which would serveto increase the usefulness of the apparatus. A secondlead sulfide cell can be placed in the bridge measuringcircuit in a position near the present cell but shieldedfrom radiation, so as to act as a drift compensator forresistance variation resulting from temperature changes.

The sensitivity of the reflectometer can be increasedwithout increasing the monochromator slit widthabove its present value of less than 50 mu, by a suitablechoice of the resistance bridge constants, and bychopping the incident beam and using an ac amplifieras an indicator.

ACKNOWLEDGMENT

It is a pleasure for the authors to express their ap-preciation to J. M. McGreevy for his encouragementin the prosecution of the problem, to H. Korbel for aidin preparing this paper for publication and to H. Judinfor obtaining the numerous measurements required toestablish the reproducibility of the instrument.

April 1952 265

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