NATL INST. OF STAND & TECH R.I C

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NIST United States Department of Commerce

Technology Administration

National Institute of Standards and Technology

NIST Technical Note 1413

45°10° Reflectance Factors of Pressed

Polytetrafluoroethylene (PTFE) Powder

P. Yvonne Barnes and Jack J. Hsia

QC

100

U5753

NO. 1413

1995
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'At Bould 103.

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NIST Technical Note 1413

45°10° Reflectance Factors of Pressed

Polytetrafluoroethylene (PTFE) Powder

P. Yvonne Barnes and Jack J. Hsia

Radiometric Physics Division

Physics Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-0001

July 1995

-<0F ><

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"^TESO*

U.S. Department of Commerce

Ronald H. Brown, Secretary

Technology Administration

Mary L. Good, Under Secretary for Technology

National Institute of Standards and Technology

Arati Prabhakar, Director
National Institute of Standards U.S. Government Printing Office For sale by the Superintendent of and Technology Washington: 1995 Documents

Technical Note 1413 U.S. Government Printing Office

Natl. Inst. Stand. Technol. Washington, DC 20402

Tech. Note 1413

23 pages (July 1995)

CODEN: NTNOEF
Table of Contents

Abstract 1

1

.

Introduction 1

2. Sample Pressing Method 2

3. 45°/0° Reflectometer 3

4. Measurements 3

4.1 Operator Variability 4

4.2 Material Variability 4

4.3 45°/0° Reflectance Factor and Uncertainty 4

5. Remarks 5

6. References 5

Tables and Figures 7

Appendix A. General and advance preparation information for pressing

PTFE samples 12

Appendix B. Reprint, NBS 45°/normal reflectometer for absolute

reflectance factors 13

in
List of Tables

1Operator Variability : Each laboratory returned two pressed PTFE

samples; alpha-numerics indicate the laboratory (A - J) or the

sample (1 - 2). The quantity reflectance factor minus the average

for the twenty samples is shown for each sample at the

measurement wavelengths. The Type A uncertainty (k = 2) for the

20 measurements is shown in the last row 7

2. Material Variability : Pressed PTFE samples from four drums;

each drum was manufactured at a different time. The quantity

reflectance factor is shown for each drum at the measurement

wavelengths. The mean and Type A uncertainty (k = 2) for the

measurements of the four drums are shown in the last two

rows 8

3. 45°/0° Reflectance Factors of Pressed PTFE Powder (1 g/cm3

) 9

List of Figures

1PTFE powder presser designed to produce a sample with spatial

uniformity in density and in appearance 10

2. 45°/0° reflectometer system (Top view) 11

IV
HIST TECHNICAL NOTE WORKFORM r? I ' f

Call number: QC100 .U5753 NO. XX 19

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Dencriptive cataloging:

090 QC100 j b.U5753 no. 19

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4570° Reflectance Factors of Pressed

Polytetrafluoroethylene (PTFE) Powder

P. Yvonne Barnes and Jack J. Hsia

Radiometric Physics Division, National Institute of Standards and Technology,

Gaithersburg, MD 20899, USA

Abstract

Pressed polytetrafluoroethylene (PTFE) powder is used for 45°/0° reflectance factor

standards. The radiometric and spectrophotometric measurement community such as the Council for Optical Radiation Measurements (CORM) has demonstrated the need

for such a standard and its application to quality control and quality assessment. This

publication briefly describes the instrumentation used for the 45°/0° reflectance factor

measurements of pressed PTFE powder from 380 nm to 770 nm. Also, the variations

of 45°/0° reflectance factor with sample preparation and materials are discussed. The

expanded uncertainty at a coverage factor of two for the 4570° reflectance factors of

pressed PTFE powder ranges from 0.009 to 0.017.

Keywords

Diffuse Reflectance, 4570° Reflectance Factor, Polytetrafluoroethylene resins, Pressed

PTFE powder, PTFE, Reflectance Factor

1. INTRODUCTION

Over the years, the Radiometric Physics Division at the National Institute of Standards

and Technology (NIST) has investigated [1,2] the reflection properties of

polytetrafluoroethylene (PTFE) resins, in a form of granular molding powder. NIST

has established the 6°/hemi spherical reflectance factor scale of pressed PTFE powder

[3] and provided general preparation information on pressing PTFE powder (Appendix

A). Recently, standards organizations, government agencies, national laboratories,

optical material industries, and instrument manufacturers have expressed the need for a 45°/0° reflectance factor standard in a report on critical problems and projected

national needs in optical radiation measurement by the Council for Optical Radiation

1
Measurements [4]. Although these requirements span the ultraviolet, visible and near

infrared spectral regions, most of the requests for calibration are 45°/0° reflectance

factors in the visible wavelength region.

The reflectance factor of a sample is defined [5] as "the ratio of the radiant flux

reflected in the directions delimited by the cone to that reflected in the same directions

by a perfect reflecting diffuser identically irradiated." This cone is formed by the

receiver limiting aperture and with apex at the center of the sample surface. A perfect

reflecting diffuser is an ideal (no loss) lambertian (uniform in all directions) diffuser.

The 4570° reflectance factor is the reflectance factor measured at 45° incident angle

and normal viewing angle. The governing equations and the description of how the

reflectance factor is measured are given in Reference 6 and a reprint is included in

Appendix B. The reflectance factor measurement method (see app. B) using a stepdown technique and a view factor calls for the measurements of the ratio of two fluxes

and, in addition, some linear dimensions.

In order to develop the pressed PTFE powder as an intrinsic standard, studies of the

major factors that affect the measured values were performed. This publication

describes the sample pressing method and briefly described the 45°/0° reflectometer

for reflectance factor measurements of the samples from 380 nm to 770 nm. Also, the

variations of 45°/0° reflectance factor with sample preparation and materials are discussed. Finally, a table is presented for 45°/0° reflectance factors and their

associated uncertainties of pressed PTFE powder from 380 nm to 770 nm at every

10 nm.

2. SAMPLE PRESSING METHOD

This section describes the method used to prepare a diffuse PTFE sample that is uniform in both appearance and density (approximately 1 g/cm 3

). After practice and

familiarity with the presser (see fig. 1) [7], a sample can be made quickly, frequently,

and with consistent results. For the best results and ease of processing, 25 g of

PTFE powder is placed in a blender with sharp blades. The PTFE powder is pulverized until it appears light, airy and powder-like.

Referring to Figure 1, the diameter of the poly(methyl methacrylat) plunger is 51 mm;

the height of the funnel is 75 mm; and the receptacle is 10 mm in depth. The end area of the plunger is pre-treated by pressing it on some amount of PTFE powder to reduce

the influence of hydrocarbons in the plunger material. The following steps are followed to press a sample: 1) the funnel and the receptacle are assembled; 2) 25 g of

pulverized PTFE powder is scooped into the funnel; 3) the pre-treated plunger is inserted into the funnel and manually pressed downward until it stops; 4) the plunger

is withdrawn slowly to overcome the suction created by the close fit of the funnel and

the plunger; 5) the receptacle is separated from the funnel by loosening the set screws;

6) the pressed sample is then imprinted with a sheet of pre-treated sandpaper by hand

with minimum pressure. Some general information on PTFE powder and preparation
is provided in Appendix A. The safety aspects in handling and use of PTFE powder

are listed in Reference 8.

3. 45°/0° REFLECTOMETER

The overall instrument consists of a source system and a 4570° reflectometer [6,9,10].

The source system consists of a tungsten ribbon-filament lamp, mirrors, polarizer,

predisperser, monochromator, and other mirrors. The source system provides a nearly

collimated, polarized output with 10 nm band pass. The configuration of the 4570°

reflectometer is shown in figure 2. Details of the receiver system are shown in

figure 2 of Appendix B.

Referring to figure 2a in Appendix B, the incident beam illuminates at 45° from the

sample normal an area 21 mm by 21 mm at the center of the sample (S). Due to the

change of the optical arrangement of the source system after the publication of the

paper in Appendix B, the larger light beam still provides a 10 nm band pass. The

receiver system views the entire sample at 0° from the sample normal. In figure 2b of

Appendix B, two 15 cm diameter averaging spheres (AS) are mounted together

vertically and the photomultiplier (D) is attached to the upper sphere. A precision

circular limiting aperture (LA) on the lower sphere subtends a half angle of 2.8° from

the center of the sample. This double-averaging sphere detection arrangement

provides a uniform receiver system for both of the required measurement geometries:

a small light beam reflected from a specular black glass plate and a larger diffusely

reflected light beam from a diffuse sample.

4. MEASUREMENTS

Due to the low signal levels associated with this type of bidirectional reflectance

factor measurement, a step-down technique is used to reduce the required dynamic

range for measurements and to reduce measurement uncertainty. A specular black

glass plate with a regular (specular) reflectance value of about 0.04 is employed. The

22.5°/22.5° regular reflectance of the black glass is measured, using a specular

reflectometer [11], with light polarized parallel and perpendicular to the plane of

incidence at each wavelength. The flux of the pressed PTFE sample is then measured

relative to the flux of the black glass plate with the 45°/0° reflectometer for each

polarization state. This relative value of the pressed PTFE sample, the specular

reflectance of the black glass plate and the projected solid angle of the receiver

limiting aperture are used to calculate the 45°/0° reflectance factor according to Eq

(9) in Appendix B.

The 45°/0° reflectance factors of pressed PTFE powder were performed on ten freshly

pressed samples. These samples were prepared by one operator, taken from one batch

of PTFE powder, and measured over the visible spectral range. The standard

deviation for these measurements for a coverage factor (k) of 2 was less than 001
indicating a good repeatability. There was no spectral dependence. The Type B

uncertainty [12] for k = 2 was 0.0027 for the visible spectral region [6]. The

contributors to Type B uncertainty were receiver system non-linearity and nonuniformity, scattered flux, and angular setting and view factor uncertainties.

4.1 Operator Variability

A study was conducted to investigate variability in the results when ten different

laboratories prepared samples. Each laboratory received an instruction sheet, a

presser, a container with pulverized PTFE powder, and a pre-labeled box with packing

material to return the pressed PTFE samples to NIST. Each laboratory pressed two

samples with the proper amount of PTFE powder needed to prepare a sample of

1 g/cm 3 in density, 51 mm in diameter and 10 mm in thickness. No measurements

were required from the laboratories and all twenty samples were returned to NIST for

measurements. Results of this study are reported in Table 1, which shows the

difference of the reflectance factor for each sample from the mean value of 20 samples

at each of the five wavelengths. The Type A uncertainty (k = 2) of these differences

for the 20 samples range from 0.006 to 0.012 in the wavelength region from 380 nm to 770 nm.

4.2 Material Variability

The variability in the results due to the composition of the PTFE powder was studied

by making samples from four different drums of PTFE powder. The Ausimont 1

, type

G-80 and type Algoflon F5 PTFE powder were used in this investigation. Each of

these powders was produced at a different time with different lot numbers. The

samples were made as described above and measured at seven wavelengths. The

quantity reflectance factor for each drum at the measurement wavelengths is shown in

Table 2. The Type A uncertainties (k = 2) for measurements of four drums range

from 0.013 to 0.001 for the wavelength range from 380 nm to 770 nm.

4.3 45°/0° Reflectance factor and uncertainty

The 45°/0° reflectance factors of pressed PTFE powder of samples from four drums in

Table 2 were averaged at each of the seven wavelengths and these values were

1 Certain commercial instrument or materials are identified in this paper in order to specify adequately the procedure. In no case does such identification imply

endorsement or evaluation by the National Institute of Standards and Technology.

4
interpolated by spline fit for other wavelengths. These values are reported in Table 3

together with the expanded uncertainties for a coverage factor of 2. These

uncertainties include the type B uncertainty, statistical measurement uncertainty,

operator to operator variations, and material to material variations. The standard

deviations (k = 2) range from 0.017 to 0.009 for wavelengths from 380 nm to 770 nm.

5. REMARKS

The results of this investigation show that the expanded uncertainties of the 4570°

reflectance factors of pressed PTFE powder mainly due to two factors, one is the

material variability which is larger at the shorter wavelengths and the other is the

operator variability. Furthermore, these expanded uncertainties are about twice those

of the 6°/hemi spherical reflectance measurements [2].

The following laboratories participated in the PTFE powder pressing study described

in this publication: Biospherical Instruments, San Diego, CA; Byk-Gardner, Silver

Spring, MD; Datacolor, Lawrenceville, NJ; Hughes, Danbury, CT; Hunter Lab,

Reston, VA; Jet Propulsion Laboratory, Pasadena, CA; Labsphere, North Sutton,

NH; Miles Lab, Elkart, IN; Munsell Color Science Laboratory, Rochester Institute of

Technology, Rochester, NY; University of Arizona, Tuscon, AZ.

Both the 45°/0° reflectometer and the specular reflectometer used in this investigation

were replaced in 1995 by a new Spectral Tri-function Automated Reference

Reflectometer (STARR) incorporating new detectors and electronics. Preliminary

intercomparisons between the old and new instruments on 45°/0° reflectance factor

show measurements agreeing to within 0.002.

6. REFERENCES

[1] V. R. Weidner and J. J. Hsia, J. Opt. Soc. Am., 71, 856 (1981).

[2] V. R. Weidner, J. J. Hsia and B. Adams, Appl. Opt., 24, 2225 (1985).

[3] F. E Nicodemus, J. C. Richmond, J. J. Hsia, I. W. Ginsberg and

T. Limperis, NBS Monograph No. 160 (1977) p. 9.

[4] Council for Optical Radiation Measurements: CORM Fifth Report on Pressing

Problems and Projected National Needs in Optical Radiation Measurements (1989).

[5] Commission International de l'eclairage: International Lighting Vocabulary, CEE

Publ.No. 17.4 (1987).

[6] J. J. Hsia and V. R. Weidner, Metrologia, 17, 97 (1981). (A reprint is attached as Appendix B.)
[7] American Society for Testing and Materials: Standard Practice E259-93, ASTM

Annual Book of ASTM Standards 06.01 (1994) p. 709

[8] E.I. Du Pont De Nemours and Company, Inc., Fluorocarbons Division,

Teflon Fluorocarbon Resins - Safety in Handling and Use (1970)

[9] W. H. Venable, Jr., J. J. Hsia and V. R. Weidner, NBS Tech. Note 594-1 1 (1976).

[10] V. R. Weidner and J. J. Hsia, NBS SP 250-8 (1987).

[11] V. R. Weidner and J. J. Hsia, Appl. Opt., 19, 1268 (1980).

[12] B. B. Taylor and C. E. Kuyatt, NIST Tech. Note 1297 (1994) p. 2.
Table 1. Operator Variability : Each laboratory returned two pressed PTFE

samples; alpha-numerics indicate the laboratory (A - J) or the sample (1 -2). The

quantity reflectance factor minus the average for the twenty samples is shown for each

sample at the measurement wavelengths. The Type A uncertainty (k = 2) for the 20

measurements is shown in the last row.

Wavelength [nm]

380 450 600 700 770

Al 0.002 0.004 0.002 0.003 0.004

A2 0.004 0.006 0.003 0.001 0.003

Bl -0.001 0.001 0.000 0.000 -0.007

B2 -0.005 0.000 -0.001 0.000 -0.001

CI 0.009 0.006 0.004 0.004 0.005

C2 -0.005 -0.005 -0.004 -0.006 -0.005

Dl 0.001 0.002 0.001 0.001 0.002

D2 0.007 0.004 0.003 0.002 0.002

El -0.003 0.002 0.002 0.003 0.004

E2 -0.004 -0.004 -0.005 -0.006 -0.006

Fl 0.000 -0.011 -0.004 0.008 0.004

F2 -0.001 0.000 0.001 0.001 0.001

Gl -0.002 -0.004 -0.004 -0.004 -0.005

G2 0.009 0.005 0.002 0.002 0.004

HI -0.012 -0.006 -0.006 -0.008 -0.007

H2 0.010 0.001 0.000 0.001 0.002

11 -0.003 0.000 0.002 -0.001 0.000

12 0.006 -0.002 0.002 0.002 0.005

Jl -0.010 -0.007 -0.005 -0.007 -0.004

J2 0.003 0.007 0.006 0.008 0.006

std dev 0.012 0.010 0.006 0.008 0.008

(k = 2)
Table 2. Material Variability : Pressed PTFE samples from four drums, each drum

was manufactured at a different time. The quantity reflectance factor is shown for each

drum at the measurement wavelengths. The mean and Type A uncertainty (k = 2) for the

measurements of the four drums are shown in the last two rows.

Wavelength [nm]

Drum 380 420 470 550 630 700 770

#

1 1.005 1.008 1.011 1.013 1.012 1.013 1.016

2 1.001 1.006 1.009 1.011 1.012 1.012 1.017

3 0.993 0.999 1.003 1.007 1.010 1.011 1.016

4 1.007 1.010 1.011 1.012 1.013 1.013 1.017

mean 1.002 1.006 1.009 1.011 1.012 1.012 1.017

stddev 0.013 0.010 0.008 0.005 0.002 0.002 0.001

(k=2)

8
Table 3. 45°/0° Reflectance Factors of Pressed PTFE Powder (1 g/cm3

) Wavelength Reflectance

390 1.003

400 1.005

410 1.006

420 1.006

430 1.007

440 1.007

450 1.008

460 1.008

470 1.009

480 1.009

510 1.010

520 1.010

530 1010

540 ion

550 ioil

560 ioil

570 ion

580 ion

590 ioil

600 ion

610 i.oil

620 1.012

Uncertainty*

[nm] Factor k = 2

380 1.002 0.017

0.017

0.016

0.016

0.015

0.015

0.015

0.014

0.014

0.014

0.014

0.013 490 1009

500 i.oio 0.013

0.012

0.012

0.011

0.011

0.011

0.011

0.010

0.010

0.010

0.010

0.009

0.009

630 1.012 0.009

640 1.012 009

650 1.012 010

660 1.012

670 1012

680 1.012

690 1.012

700 1.012

710 1.013 0011

720 1.014 011

730 1.015

740 1.015

0.010

0.010

0.011

0.011

0.011

0.011

0.011

750 1016 0011

?60 1.016 011

770 1.017 o.oil

The total uncertainty consists of the instrument uncertainty, material and operator variations
PLUNGER

FUNNEL 1^

POWDER

RECEPTACLE

Figure 1 . PTFE powder presser designed to produce a sample with spatial

uniformity in density and in appearance.

10
Signal

Averaging

Sphere

Detector Port

Top of Sphere

Specular

Component

Limiting

Aperture

/

Incident Beam

(45° From Normal)

Black Glass

Standard

Sample Standard

2-Position Slide

Sample

Turntable

Figure 2. 45°/0° Reflectometer System

(Top View)

11
APPENDIX A. General and Advance Preparation Information for

Pressing PTFE Samples

a. PTFE presser: Since there are no commercially available PTFE pressers that

met our specifications, a simple PTFE presser was designed and fabricated at NIST (Refer to fig. 1). The presser consists of three parts: a plunger that was

constructed of poly(methyl methacrylate), a funnel of sufficient size to hold

bulk PTFE powder before pressing, a receptacle designed to fit into the

specimen holder of the 45°/0° reflectometer. This plunger was preconditioned

by pressing against clean PTFE powder.

b. Commercial blender: PTFE powder naturally clumps together as aggregates.

An Osterizer [13] blender was used to pulverize PTFE powder into a fluffy

powder-like appearance. This type of commercial blender was chosen because

it could be attached to glass jars and the blades could be sharpened in the

machine shop.

c. Alternative devices: A stainless sifter or a flour sifter may be used instead of a commercial blender.

d. Static charge: PTFE powder acquires an electro-static charge throughout the

pulverization process and will coat the glass blending jar and blades.

e. Utensils: Only stainless steel, glass, and porcelain materials or containers

should be used to handle PTFE powder. The ultra-violet spectral reflectance

factors of a pressed PTFE sample are affected by the hydrocarbons in plastic

materials.

f. Containers of PTFE powder: PTFE powder is usually shipped by the

manufacturer in drums of either 20 kg or 45.5 kg. The powder is packed in a

plastic bag. Only the PTFE in the center portion of the drum is used, thus

avoiding the powder that came in contact with the plastic bag.

g. Preconditioned sandpaper: A sheet of 1 50 grit size sandpaper was rubbed with

a piece of sintered ( in an electric furnace at 370 °C for 1 hour) PTFE, and

shaken repeatedly so that the loose particles would fall off and not be

embedded to the surface of the sample.

h. Ceramic tile: Use a ceramic tile or a similar type of object to evenly distribute

the applied force over the preconditioned sandpaper on the sample area.

12
i. Laboratory conditions: PTFE powder is contaminated by airborne particles

and smoke due to the electro-static charge (see sec. d above).

j. Safety: Obtain a Material Safety Data (MSD) sheet from the manufacturer of

PTFE powder. Safety precautions and safety issues are the sole responsibility

of users.

k. PTFE sample: A pressed sample of PTFE is durable and will not fall out of the

receptacle. A sample may be positioned vertically, and if necessary it may also

be inverted and tapped lightly so that the loose particles would fall off.

1. Cleaning. A clean camel-hair brush can be used to remove pieces of lint on the

sample surface.

APPENDIX B. Reprint, NBS 45°/Normal Reflectometer for Absolute

Reflectance Factors

(see next six pages)

13

Metrologia 17, 97-102 (1981)

metrologia

© by Springer- Verlag 1981

NBS 45°/Normal Reflectometer for Absolute Reflectance Factors

Jack J. Hsia and Victor R. Weidner

National Bureau of Standards, Washington, DC 20234, USA

Received: March 25, 1981

Abstract

A 45°/normal reflectometer has been constructed and

tested for calibrating the absolute reflectance factor of

diffuse samples over the 380—770 run spectral range

using polarized radiation. The measurement equations

have been derived for the method used. The method

using a step-down technique and view factor calls for the measurements of the ratio of two fluxes and, in addition, some linear dimensions. The uniformity of the

receiver system is achieved by means of a double-sphere

signal averager. Uncertainties of the absolute-reflectancefactor measurements obtained with this system are estimated to be ±0.3% of the measured value. For all the samples that have been tested, the 45°/normal re- flectance factor was found to be higher than the 6°/hemispherical reflectance factor. The higher reflect- ance values for 45°/normal geometry were confirmed by

additional gonioreflectometer measurements.

1. Introduction

Measurements of the diffuse reflecting characteristics of materials are of basic importance in radiometry,

photometry and colorimetry as well as many other

fields of science, technology and industry.

The International Commission on Illumination

(Commission Internationale de l'Eclairage — CIE) re- commended that starting in 1969, in colorimetric

measurements of opaque materials, the perfect diffuser was to be taken as the primary reference standard. There

is no existing material having diffuse reflection properties corresponding to the perfect diffuser with sufficient approximation. Therefore, the characteristics of all physical standards actually employed in comparison measurements must be established by techniques

through which their absolute reflectance factors can be determined. The measurements involve not only the determination of the ratio of radiometric quantities but

also the determination of linear dimensions.

^The four geometries recommended by the CIE are 45°/normal, normal/45 °, diffuse/normal, and normal/

diffuse. The method and instrumentation to realize the NBS scale of directional-hemispherical reflectance

factor has been described in [1 ]. For the 45°/normal and normal/45° reflectance

factor measurements, there exists a large ratio of the incident flux to the reflected flux contained in a small

solid angle. Various step-down techniques have been

used by several National Laboratories [2—8] to insure

sufficient accuracy over this wide dynamic range. The purpose of this paper is to describe the method

and the instrumentation used to establish the NBS scale of absolute reflectance factor in the 45°/normal geo- metry.

2. Measurement Equations

An absolute reflectance factor is defined [9, 10] as the

ratio of the radiant flux reflected in the directions bounded by a given cone with the apex at a point of the

surface under test to that reflected in the same directions by a perfect reflecting diffuser identically irradiated. The term "perfect reflecting diffuser" means the ideal

(lossless) lambertian (uniform in all directions) diffuser.

If the solid angle of the cone approaches zero, or 2tt steradians, the reflectance factor approaches radiance

factor or reflectance, respectively.

The reflectance factor of sample x, Rx

, can be ex- pressed as

RX" r

f

5 L(U,X) Sx (Um,$ Ku.X) U-dA dfti/da dw dX

f

s L(U,X) Sid r(u,X) U-dA d£l uda dej dX (1)

where L is the incident radiance; 5X and 5id are the

scattering functions of the sample and of the ideal sur- face, r is the relative responsivity of the receiver; t/and u are the direction vectors of travel of the incident and

emergent radiation; dI2 and dej are elements of solid angle oriented in the direction U and u; dA and da are elements of area through which the energy passes on to the sample and emerges from the sample; dA is the ele- ment of wavelength; and the symbol /" indicates inte0026-1 394/81/001 7/0097/S01. 20
gration with respect to n variables. In general, the nomenclature (especially the symbols) used here is that of reference [11]. There it is more completely defined

and discussed.

If there is no interaction between the wavelength

dependence and the geometrical dependence of the incident radiance and of the responsivity, the functions

are separable. For any source function C(A) and

luminous efficiency function V(X), Eq. (1) can be

written as

Rx = say m

/GW V{X) (2)

[$*L(U)Sx {U,u,k)r{u) U-dA d£lu-da dco] dk

[S^LiU) K«) U-dA dfi uda da;] dA

/qA)K(A)Rx(A)dA

^

/a^) v(k) dx

The spectral reflectance factor RX

(A) is expressed as

*w>=

f*L(U) Sx(Uj4,\) K«) U-dA dn uda do

Sid fL(U) r{u) U-dA dft uda dco

(3)

(4)

For a receiver system with uniform responsivity

within the cone,

r(u) = r within co and riu) = outside of co. The scattering function of a perfect diffuser could be

written as

(5)

5id =5(7»-p)/7r (6)

where P and p are position vectors on the testing surface

for incident space and the emergent space, respectively.

Thus (4) can be expressed as

7T *X(A) W=7T (7)

O)P- f2

*i(A)

where cop = / u-da dco is the projected solid angle of

the cone onto the surface to be measured.

4>x = J"4 L(U) Sx (Uji,X) U-dA dHi/'da dco is the reflect- ed radiant flux for the sample that enters the limiting re- ceiver aperture, and

*i = J

2 L(U) 8(P - p) U-dA d£l is the incident radiant

flux.In radiation heat transfer [12, 13], coJn (or fx _2) is named the view factor (alternatively designated as the form factor, configuration factor, geometrical factor, or

angle factor) representing the fraction of the radiant flux leaving a diffuse surface that is entering the limiting

aperture. In other words, lx -2

is the fraction of the in- cident flux reflected from (1) a perfect diffuser that is entering the (2) limiting aperture. The spectral reflect- ance factor can now be expressed as Rm- l *xU) (8)

Equation (8) clearly indicates that the reflectance fac- tor is the ratio of the reflected flux (into the limiting

aperture) to that of a perfect diffuser. If another sample

S with known reflectance is used to step down1 the in- cident flux, the reflectance factor of sample x can be

calculated as RxW

l

fi- *ico'Ps

(9)

where <I> S and ps are the flux and the reflectance of the step-down sample S, respectively. The view factor will be expressed in detail below. For a circular limiting aperture (diameter D) which

is parallel to the sample surface at a distance d, and for AA 1 , a small portion of the illuminated area, and the

distance between a point in AA 1 and the central normal

(on the sample surface) to the circular area of the

limiting aperture being c, (see Fig. 1) the view factor can be expressed [14] as:

fAxi_^2 = [l-J/K]/2,

where J = 1 + G

2 - B2

(10)

2\2il/2 K = [G4 +2G2 (1-B2 ) + (1+B2

)

2

] B=£>/2d

G = c/d

AA1

Fig. 1. Schematic for view factor (Eq. 10) Determination

When the illuminated area is very small and is at the

central normal to the circular area of the limiting aperture, the view factor becomes

fi-2=fAA1 - A2 =[l+4d2 /D2 r

l (11)

and when the illuminated area is finite, the view factor is U - 2=

AT- f^Al-AlMl (12)

A/11

0rfl " 2=

~AT U

{^X-A2

if AA 1 is selected to be sufficiently small.

(13)

1 Procedure described in Sect. 5

98
3. NBS Reference Spectrophotometer

The 45°/normal reflectometer is an accessory to the NBS reference spectrophotometer for reflectance [15].

The light source, monochromator, electronics and other

associated equipment are located in a system control room. The exit slit housing of the monochromator is attached to a light-tight diaphragm in a wall, which

allows the exit light beam or sample beam to enter a second room where the various reflectance-measuring

devices, such as the specular reflectometer or the 45 / normal reflectometer, are installed. The advantages of

this arrangement are that the second room can be used

as an experimental dark chamber, while the light source,

electronics and control systems are isolated from the experimental area. The sample beam emerging from the

exit slit of the monochromator can be controlled to provide a spectral bandpass of 2, 5, 10, or 20 nm. For a 10-nm bandpass, the light beam has a cross section 10 by

18 mm at the sample position.

To measure spectral reflectance factor over the

visible wavelength range of the monochromator, the system employs a tungsten strip lamp and a photomultiplier detector. A complete description of the spectrophotometer is given in an earlier publication [15]. Data have been published [16, 1 7] on the large effect of polarization on the reflectance factor. Therefore, the NBS design includes a polarization-insensitive receiver system and the measurements are made with polarized

incident light. The final result is the average of measure- ments made with light polarized perpendicular and

parallel to the plane of incidence.

4. 45°/Normal Reflectometer

The light beam from the source system is incident on a sample at 45° from normal and the reflected flux at nor- mal to the sample is collected and measured. The sche- matic of the instrument is shown in Fig. 2. The 45°/normal rather than the normal/45 geometry was chosen,

because the distance between the limiting aperture and

Fig. 2 a and b. Schematic of the 45°/normal Reflectometer. a Top view with top sphere not shown, for clari- ty. S: Sample, LA: Limiting Aperture, DA: Detector

assembly, SA: Aperture between spheres; b Front view

of the two-sphere Receiver Assembly, AS: Averaging

Sphere, SH: Shutter, D: Detector, LA: Limiting

Aperture, SA: Aperture between spheres

the sample surface and, in turn, the view factor could

be determined more accurately. Because the step-down technique with a polished

black glass is used to obtain the absolute reflectance fac- tors of a diffuse sample, the instrument should be able

to measure accurately both the specularly and the

diffusely reflected light beams. This requires that the re- sponsivity of the receiver system be uniform over its aperture area. To fulfill the above requirement, the photomultiplier is equipped with a double-sphere signal averager. The spheres are approximately 1 5 cm in diameter. The

sphere coating is a fluorocarbon powder, which is a nearly perfect diffuser of very high reflectivity. The two

spheres are mounted together vertically and are designed

to allow the inside curvature of the sphere coating to meet at the center of the port between the two spheres.

The photomultiplier is mounted horizontally on the upper sphere. The spheres are attached to a steppingmotor-controlled vertical slide for alignment and scanning purposes. The lower sphere has a circular limiting

aperture facing the sample. To prevent scattered radia- tion, other than that directly from the sample, reaching

the limiting aperture, no component is placed between

the surfaces of the sample and the limiting aperture. A

black felt is placed on the top surface of the rotation arm to further reduce scattered radiation. The sample holder, which can hold two samples, is mounted on a two-position slide that moves 12.5 cm be- tween positions and is pneumatically driven. The slide

is mounted on a stepping-motor-controlled turntable. The detector assembly is on an arm attached to another

independently-controlled turntable having the same axis of rotation as that of the sample turntable. The signal processing of the output from the photomultiplier utilizes a current-to-frequency converter that can integrate the detector output over a selected time

interval, and a digital counter. The digital data from the counter are transferred to a computer for analysis.

5. Measurement Procedure

Before a measurement is performed, the instrument is first aligned. The sample holder can be adjusted normal

to the incident light beam with the aid of a flat mirror

in the sample position. The sample holder is then turned

22.5° from the normal position toward the limiting

aperture. The limiting aperture is adjusted to center on

the reflected light beam which is 45 from the incident

light beam. A thin mirror, with the reflecting surface

facing the limiting aperture, is held against the limiting

aperture. The limiting aperture is aligned so that the

light is reflected back to the exit slit of the monochromator. Thus the limiting aperture is aligned normal to the reflected light beam 45° from the incident beam.

The diameters of the limiting aperture were meas- ured at eight places by using a shadow profile projector

before the aperture was mounted on the lower sphere.

The circularity has been checked by a deviation plotter.

The average diameter is 38.238 mm. The distance be99
tween the surfaces of the limiting aperture and the sample is determined when the sample holder is turned

45° from the normal position. A thin glass plate is placed over the limiting aperture. The distance between

this glass plate and a flat surface at the sample position

is measured with an inside micrometer. The thickness of

the glass plate is added to the micrometer measurement

to give the total distance which in this case is 381.89 mm. The view factor can thus be calculated using (10)

and (13).

The spectral 22.5° specular reflectances of a piece

of polished black glass are first determined with the specular reflectometer [18] with light polarized per- pendicular and parallel to the plane of incidence. This

piece of black glass is then used to step down the in- cident flux to determine the 45°/0° reflectance factors of a sample. The flux of the sample in 45°/0° geometry

is compared to the flux of the black glass in the 22.5 / 22.5° geometry. The reflectance factor of the sample

can thus be calculated using (9). The measurements are performed with polarized light, and the final result is the average of measurements made with light polarized

perpendicular and parallel to the plane of incidence.

6. Performance

It is beyond the intent of this paper to describe in detail the various checks that were made on the performance

of the monochromator, other components of the spectrophotometer, and the reflectometer, except to mention briefly the magnitudes of errors associated with

this portion of the system, since they do influence the accuracy of measurements of reflectance factor. Infor- mation regarding some of these investigations is presented in [15]. Wavelength scale uncertainties, receiver linearity, receiver uniformity, scattered radiation, angular setting

uncertainty, and view factor uncertainty are involved

in checking the performance of the instrument.

The wavelength scale of the monochromator was

checked by measuring the emission lines [18] of several line-source lamps and the instrument function, then

determining the centroid wavelengths. Uncertainty in the wavelength scale is 1 nm or less for a 10 nm bandpass. Corrections for these errors are made by adjustment of the wavelength-scale setting to compensate for differences between the wavelength counter and the

true wavelength.

The linearity of the receiver system was measured by

the light addition method with a double aperture apparatus [20, 21]. The results indicate the receiver system is linear to better than ±0.1% and the nonlinearity is corrected for measured data. Even though the receiver system utilizes double

spheres as an averager device, there is still a slight nonuniformity causing 0.05% uncertainty in comparing the

fluxes of specularly and diffusely reflected beams.

Radiation scattered off the components between

the exit slit of the monochromator and the limiting

aperture of the receiver system causes errors in a

reflectometer. The room containing the reflectometer

is lined with a black felt material to absorb scattered

radiation. All mechanical components, including the sample holder and the limiting aperture, are black

anodized to reduce the scattered radiation. Some

interreflections between the limiting aperture and the sample and between the sphere coating and the sample

are unavoidable. The scattered radiation caused by the

interreflection is determined to be less than 0.05% of

the reflected radiation. The 0.05° uncertainty of the angular setting of the sample holder contributes to 0.1 5% uncertainty in deter- mining the ratio of the reflected fluxes of the sample

and the piece of black glass. The uncertainties in determining the diameter of

the limiting aperture, the distance between the sample

surface and the limiting aperture, and the dimensions of

the light image size on the sample surface result in an

uncertainty of about 0.05% for the view factor. An analysis of the various sources of error that may

affect the accuracy of reflectance factor measurements on the NBS 45°/normai reflectometer indicates that the

final uncertainty in these measurements will be of the order of ±0.3% of the measured value.

7. Ratio of R(45°/Normal) TO R(6°/Hemispherical)

with Specular Component Excluded

The hemispherical reflectance factor R(0°/h) of a diffuser for normally incident light can be calculated from

the angular variations of reflectance factors R(0 /0)

using the following expression:

7T/2 / R(O°/0) cos0 sin0 27rd0 R(07h)=^ (14)

/ cos0 sin0 27rd0

o

Divided by R(0°/45°), Eq. (14) can be written as:

R(0>) = 2 V W°ld ) sin0cos0d0 (15)

R(0°/45°) I R(0°/45°)

In terms of measurable quantities, (15) can be rewritten

as:

where

IT1

R(0°/h)

R(0°/45°)

BfQ

o, . = $(O»/cos0

} *(0°/45

o )/cos45°

— =2 f

B(O°/0) sin0 cos0d0 (16)

(17)

with 4> as the reflected flux from the sample.

The two working standards, vitrolite glass and porcelain enamel plate, one pressed BaS04 sample, and one

pressed fluorocarbon sample were measured on the gonioreflectometer at 550 nm with 10 nm bandpass.

The incident angle was 0° and the angles of viewing were from 5° to 85 at 10° intervals. Measurements were

100
made with light polarized parallel and perpendicular to

the plane of incidence. B(O°/0)'s were calculated for each polarization and the averages obtained.

Average B(O°/0)'s at the discrete intervals were fitt- ed by the least-squares method. Individual data did not

deviate from the fitted curve by more than ±0.001 and

were typically within 0.0005 of the fitted curve. The

curve fittings were done by using a six-parameter equation: B(0»= £ bjfli i=

Inserting (18) into (16), we have

R(o» ,; v h i R(0°/45°) ~ £Q

bi lj

where

l

i =Sol2 e

i sm6cosdde

(18)

(19)

(20)

8. Summary

A 45°/normal reflectometer spectrophotometer was constructed for calibrating the absolute reflectance factor of

diffuse samples over the 380-770 nm spectral range. The measurement equations were derived and the measurements were made as a function of wavelength with

a polarized light beam. The system performance was examined for such parameters as wavelength scale uncertainties, receiver system linearity and uniformity,

scattered radiation, angular setting and view factor uncertainties. Uncertainty in the measurements of reflect- ance factor will be of the order of ±0.3% of the measured value. Measured ratios of the 45°/normal re- flectance factor to the hemispherical reflectance factor with specular component excluded were found to be

greater than one for all the samples that have been test- ed. The higher reflectance values for 45 "/normal geo- metry were confirmed by additional gonioreflectometer measurements.

The values [1] of the integrals I, are (from i = to 5): 0.50000, 0.39270, 0.36685, 0.37990, 0.42147, and

0.49129. With the values of I; and b{

, R(0°/h)/R(0°/

45°) can be predicted using (19).

Six deg.-hemispherical reflectance factors, R(6°/h),

with specular component excluded and 45°/normal re- flectance factors, R(45°/0°), were also measured for the

four samples. R(6°/h) will not be different significantly

from R(0°/h), and R(45°/0°) is equal to R(0°/45°)

according to the Helmholtz reciprocity principle.

The measured values of R(45°/0°), R(6°/h) and the

ratio of the measured values and integrated ratios are given in Table 1 . The measured and integrated values of

the ratios agree to 0.005. These ratios are consistent

with references [4, 22, 23] in that the values of 45°/

normal reflectance factors are higher than the values of

the 6°/h reflectance factors. And in the cases of vitrolite and enamel samples the ratios are about four percent

higher than unity.

Table 1. Comparison of R(45°/0°) and R(6°/h)* (at 550 nm)

Measured Value R(45°/0°)/R(6°/h)

Sample R(45°/0°)R(6°/h)* (b)/(a) Integrated**

(b) (a)

BaS04 0.987 0.982 1.005 1.002

Fluorocarbon 1.009 0.994 1.015 1.011

Vitrolite 0.922 0.884 1.043 1.044

Enamel 0.875 0.839 1.043 1.040

* Specular component excluded

** Integration using normalized gonioreflectance data

References

1. W.H. Venable Jr, J.J. Hsia, V.R. Weidner: Establish- ing a scale of directional-hemispherical reflectance

factor. I: The Van den Akker method. National Bureau of Standards (US). J. Res. 82, 29-55(1977)

2. J.S. Preston, G.W. Gordon-Smith: A new determination of the luminance factor of magnesium oxide.

Proc. Phys. Soc. B65, 76-80 (1952)

3. F.J.J. Clark, G.E.V. Lambert, J.M.B. Sparrock, F.A.

Gargorth: Research on absolute reflectance stand- ards at the National Physical Laboratory. Proc. Symp. Color Measur. Ind. 25-40, 1967 (The Color

Group, London)

4. F.J.J. Clark, F.A. Garforth, D. Parry: Goniophotometric and polarization properties of the common

white reflection standards. Nat. Phys. Lab. Rep.

MOM 26(1977)

5. W. Erb: Bestimmung des spektralen LeuchtdichteKoeffizienten fur die Messgeometrie 45 /Normal.

Die Farbe 20, 1 79- 1 86 ( 1 97 1

6. L. Morren: Mesure absolute des facteurs de lumi- nance et de reflection. LUX No. 45, 448-453

(1967)

7. V.E. Kartahevskaya, S.P. Skachkova: New method

of the luminance factors and total reflectances for the standard plates of the Institute of Metrology.

Svetsteknika 1, 16-19 (1966); also available as translation NRC TT-1 265

8. Commission internationale de l'^clairage: Absolute

methods for reflection measurements. CIE Publica- tion Nos. 44 and 46 ( 1 979)

9. Commission internationale de re"clairage: Colori- metry, official CIE recommendations. CIE Publication No 15(1971)

10. F.E. Nicodemus, J.C. Richmond, J.J. Hsia, J.W.

Ginsberg, T. Limperis: Geometrical considerations

and nomenclature for reflectance. Nat. Bur. Stand. (US). Monogr. 160(1977)

11. W.H. Venable, Jr, J.J. Hsia: Describing spectrophotometric measurements. Nat. Bur. Stand. (US).

Tech. Note 594-9(1976)

101
12. E.M. Sparrow, R.D. Cess: Radiation heat transfer 18. (augmented edition) p. 121. New York: McGrawHill Book Company, 1978

13. F.E. Nicodemus (ed.): Self-study manual on opti- 19. cal radiation measurements: Part I — Concepts,

Chap. 4 and 5 p. 97. Nat. Bur. Stand. (US). Tech.

Note 910-2(1978) 20.

14. M. Jakob: Heat transfer. Vol. II p. 11. New York:

John Wiley and Sons Inc., 1957

15. W.H. Venable, Jr, J.J. Hsia, V.R. Weidner: Develop- 21. ment of an NBS reference spectrophotometer for diffuse transmittance and reflectance. Nat. Bur. Stand. (US). Tech. Note 594-1 1 (1976) 22.

16. A.R. Robertson: Effect of polarization on reflect- ance factor measurements. Appl. Opt. 11, 1936—

1941 (1972)

17. D.C. Cramer, M.E. Bair: Some polarization charac- 23.

teristics of magnesium oxide and other diffuse re- flectors. Appl. Opt. 8, 1597-1605 (1969)

V.R. Weidner, J.J. Hsia: NBS specular reflecto- meter-spectrophotometer. AppL Opt. 19, 1 268—

1273(1980)

C.E. Hodgman (ed.): Handbook of Chemistry and

Physics, 44th. edn, pp. 2892, 2931 , 2961

The Chemical Rubber Publishing Co. 1962-1963

K.D. Mielenz, K.L. Eckerle: Design, construction,

and testing of a new high accuracy spectrophotometer. Nat. Bur. Stand. (US). Tech. Note 729(1972)

K.D. Mielenz, K.L. Eckerle: Spectrophotometer

linearity testing using the double-aperture method.

Appl. Opt. 11 2294(1972)

V.E. Kartachevskaya, H. Korte, A.R. Robertson:

International comparison of measurements of luminance factor and reflectance of white diffusing

samples. Appl. Opt. 14, 2694-2702(1975)

A.R. Robertson, W.D. Wright: International comparison of working standards for colorimetry. J. Opt.

Soc. Am. 55, 694-706 (1 965)

102
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