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National Institute of Standards and Technology

NIST Technical Note 1280

Absolute Specular Reflectometer with an

Autocollimator Telescope and

Auxiliary Mirrors

Tie Ming Wang, Kenneth L. Eckerle, and Jack J. Hsia

NIST Technical Note 1280

Absolute Specular Reflectometer with an

Autocollimator Telescope and

Auxiliary Mirrors

Tie Ming Wang

Kenneth L. Eckerle

Jack J. Hsia

Radiometric Physics Division Center for Radiation Research

National Institute of Standards and Technology

Gaithersburg, MD 20899

August 1990

° IT, > *

~afe^

U.S. Department of Commerce

Robert A. Mosbacher, Secretary

National Institute of Standards and Technology

John W. Lyons, Director
National Institute of Standards U.S. Government Printing Office For sale by the Superintendent

and Technology Washington: 1990 of Documents

Technical Note 1280 U.S. Government Printing Office

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

Tech. Note 1280

31 pages (Aug. 1990)

CODEN: NTNOEF
TABLE OF CONTENTS

Page

ABSTRACT v

1. Introduction 1

2. Instrumentation 1

3. Reflectometer with V-W Accessory and ACTAM 2

4. Adjustment of V-W Accessory in the IR Spectrophotometer 4

4.1 The Position of the V-W Accessory 4

4.2 Adjustment Procedure for the V-W Accessory 5

5. Use of the ACTAM to Align the Sample in the V-W Accessory .... 6

5.1 Principle of Operation of ACT 6

5.2 Adjustment of the Sample Mirror with the ACTAM 6

5.3 Angular Accuracy of the V-W Accessory with ACTAM 7

6. Uncertainty and Results 8

6.1 Alignment Error of the V-W Accessory 8

6 .

2

Uncertainty 9

6.3 Comparison with Previously Published Values 10

7

.

Summary 10

8. References 11

9. Acknowledgment 11

APPENDIX A 23

ill
LIST OF TABLES

Page

1. Polarization Study for Two Setups of the Reflectance Attachment . . 12

2. Results and Uncertainty for Measurements of an Electroplated

Gold Mirror 13

3. Reflectance Values for a Freshly Evaporated Gold Coating [Ref. 4] . 14

LIST OF FIGURES

1. Optical Diagram of the Spectrophotometer showing the ACTAM with

the V Geometry and W- Geometry. The Key Elements are Defined

in the Text 15

2. Equivalent Optical Diagram Showing Critical Points in the

Spectrophotometer. The Dimensions are Expressed in mm 16

3. Slit and Light Beam at Point E 17

4. The V and W Geometries with Angle of Incidence a 18

5. Diagram Showing How the ACTAM Interacts with the V-W Accessory

in the Spectrophotometer 19

6A. Autocollimator Telescope Interacting with a Mirror Tilted

by an Angle A9/2 20

6B. Field of View of Autocollimator Telescope

"a" is the Reference Bright Spot and

"b" is the Reflected Bright Spot 21

7. Diagram Showing ACTAM with Mirrors in the V and W Geometries ... 22

Al . Mirrors Ma

' and Ms Mounted on Holder 24

A2. Arrangement for Making Ma

' Parallel to Ms 25

IV
ABSOLUTE SPECULAR REFLECTOMETER WITH AN AUTOCOLLIMATOR TELESCOPE

AND AUXILIARY MIRRORS

Tie Ming Wang+, Kenneth L. Eckerle, Jack J. Hsia

Radiometric Physics Division

National Institute of Standards and Technology

ABSTRACT

The Strong method for measuring reflectance is used in many laboratories.

However, an uncertainty in the angular position of the sample can lead to large

errors. A novel use of an autocollimator telescope with two auxiliary mirrors

(ACTAM) can reduce this angular uncertainty and lead to more accurate measurement

of reflectance. The instrumentation, its adjustment, measurement procedure,

results, and uncertainties are discussed. The total uncertainty for the

reflectance measurement of a gold mirror was found to be less than 0.0025 over

the wavelength region 2 to 22 /zm.

Key words: alignment; autocollimator telescope; auxiliary mirrors; specular

(regular) reflection; strong (vw) method; uncertainty

+Guest scientist from Shanghai Institute of Technical Physics, Academia Sinica,

China.

1

.

INTRODUCTION

The Strong [1] or "V-W" method of measuring absolute specular reflectance

is well known. In this method, the light is reflected twice from the sample;

and thus, the absolute reflectance is a square root of the ratio of signals of

sample in and sample out. However, it has the disadvantage that the light beam

must make three reflections in the "W" geometry. Thus, the device is very

sensitive to angular settings. The "V-W" accessory is usually an accessory which

is inserted into the sample compartment of a spectrophotometer. Therefore, the

light path relative to the optic axis of the host spectrophotometer can be

different for the V geometry and W geometry. The method used in the present

apparatus uses an autocollimator telescope and two auxiliary mirrors (ACTAM) to

align the mirrors of the V-W accessory when changing from the V geometry to the

W geometry. This method is more accurate than the conventional one, especially

when using apparatus which does not have a radiation averaging detection system.

2

.

INSTRUMENTATION

A commercial IR spectrophotometer is used, and a diagram of the optical

system with ACTAM is shown in figure 1. The spectrophotometer is a double beam

instrument. The optical system consists of three sections: the source, the

sample area, and interior optical system,

(a) The source:

Beam source - S

Chopper - Cx Concave mirrors - Mx

, Mx

'

(b) The sample area

(c) The interior optical system:

Concave Mirrors - M<, , M7

, and M9 Flat Folding Mirrors - M2 , M2

' , M5

, M6, and M8 Semi -circular Rotating Flat Mirror - C2 Optical Filter to Remove High Orders of the Gratings and Reduce Stray

Radiant Energy - OF

Entrance Slit - S x Four Gratings for Different Spectral Regions - Grating Table

Exit Slit - S2
Detector - D

First, consider the alignment of the spectrophotometer when the sample area does

not contain the V-W accessory and ACTAM. The source S is imaged at E by the

ellipsoid mirror Mx

, and E is imaged at the entrance slit S x by the toroidal

mirror MA. The plane mirrors M2 and M5 are used to fold the beam paths

.

Figure 2 shows an equivalent diagram of the optical path for mirrors Mx and M4 in the horizontal plane. For alignment purposes, a visible source B is placed

between the slit S 1 and the parabolic mirror M7 with B placed as close as possible to the entrance slit S x (shown in fig. 1) . A bright image of the slit

S x is formed at E . The distance between E and the cover of the interior

optical system is found to be about 14 mm. The images of both S and S x are at

E , but the image sizes are quite different, since S and S 1 are magnified by

different amounts. Since the magnification of S x is 1, the image size S x

' is

the same as the size of S x. The height of S x is 12 mm. The width of S x is about

0.5 mm at 1050 cm" 1 wavenumber (the largest width of S x is 2.184 mm at

400 cm" 1 wavenumber). (See fig. 3.) The height of source S is 8 mm and the

width of S is 4 mm. The magnification of S is 236/95 or 2.5. The height of

S ' is 20 mm, and the width of S ' is 10 mm. (See fig. 3.)

Thus, the image S ' of the source S at E is larger than the image S x

' of

the slit S 1 at E . Therefore, the position of S ' is not critical with these

optical parameters, and the effect on the signal to noise will be small with a minimal measurement error for reflectance. The reason for this will become

evident later when it is shown that the beam path is the same for the cases of

sample in the beam (W geometry) and sample out of the beam (V geometry) . The

beam flux through E is the same even if the center of S ' is moved away from

the optical axis (or E ) by approximately 3.9 mm. This conclusion assumes the

beam flux at S ' is nearly uniform. The spatial relationship between the

entrance slit S 1 to the gratings and IR receiving system was not changed after

the IR spectrophotometer was calibrated. Therefore, we adjust the V-W accessory

with S x rather than with S ' , the former case being more accurate. (See sec . 4.

)

3. REFLECTOMETER WITH V-W ACCESSORY AND ACTAM

The V-W accessory is placed in the sample compartment of this IR

spectrophotometer. The V-W accessory consists of flat mirrors M10 , Ma , and H11 as well as a concave mirror M12 . MT and MT

' , are auxiliary mirrors used with
the autocollimator telescope (ACT) and together comprise ACTAM. In figure 4,

the beams drawn with dashed lines illustrate the W geometry and the solid lines

illustrate the V geometry. In figure 4, only the mirror Ma is used for the V

geometry, but two mirrors, Ma

' and Ms

, are used for the W geometry. By using

a rotatable mount, Ma and Ma

' can be the same mirror used at two positions. It

will be shown in section 4 that the optic axis and beam paths of the IR

spectrophotometer can be the same with and without the V-W accessory in the

sample area. Also, when the V geometry and W geometry are interchanged, ACTAM

reduces angular error in alignment.

The signal Sv with the V geometry and the signal Sw with the W geometry are given by

Sv = K 4> K (1)

Sw = K <f> Ra Rs (2)

where K is the instrumental response function, <f> is the incident flux, Ra is

the reflectance of the same mirror used as Ma and Ma

'

, and Rs

, is the

reflectance of the sample mirror Ms

,

. From eqs (1) and (2) , we obtain

Rs = (Sw /Sv ) 1/2

. (3)

It is necessary that Ms and Ma

' be parallel to Ma to keep the error of the

incident angle, a, as small as possible. The mirrors Ma

' and Ms have been made

parallel external to the ACTAM. For a discussion of this procedure, see appendix

A. The mechanical mount that is used can degrade the measurement accuracy of

Rs. The ACTAM is an optical feedback system to monitor and to correct the

angular positions of the mirrors. This decreases the measurement error in Rs

since the positioning accuracy with the ACTAM is better than the mechanical

positioning accuracy of the rotatable mirror mount.

In section 4, adjustment of the V-W accessory after insertion into the

spectrophotometer is outlined. This includes positioning and alignment of the

mirrors in the V-W accessory. In section 5, alignment of the auxiliary mirrors

and the autocollimator telescope are presented as well as how to use ACTAM to
align test samples in the V-W measurement.

4. ADJUSTMENT OF V-W ACCESSORY IN THE IR SPECTROPHOTOMETER

The V-W accessory is placed in the sample compartment. The resulting beam

paths in the interior optical system, after insertion, must be the same with

and without the V-W accessory in the sample compartment. If the adjustment of

the V-W accessory meets this requirement, light throughput, the S/N (signal to

noise ratio) , and measurement accuracy will be optimum. The adjustment procedure

is now described. There are two rectangular holes, Hx and U1

'

, on the cover of

the interior optical system. Hx

' allows the reference beam to pass through the

cover, and Hx allows the sample beam to pass through the cover. According to

the conjugate principle of optical image theory, the object and the image can

be interchanged. Let us assume that the entrance slit S 1 and source S are

geometrical points. The point E is not only an image of the entrance slit S a formed by the mirror MA but also an image of the source S formed by mirror Mx

.

When the V-W accessory is placed in the sample compartment, it must reimage the

beam at point E .

4.1 The position of the V-W accessory.

An optical diagram of the V-W accessory is shown in figure 5. It consists

of two folding mirrors, M10 and Mx x

, an off-axis parabolic mirror M12 > a sample

mirror Ms and a narrow mirror Ma

. Since this accessory uses first surface

mirrors, its focusing effects are the same at all wavelengths e.g., visible and

infrared. The optical axis passes through S , D and E without the V-W

accessory. When the V-W accessory is in the IR spectrophotometer, the optical

axis passes through S , D , D x

, D2 , D3, DA and E in the V geometry. The optic

axis in the W geometry passes through points S , D , Dx

, C x, D2

'

, C2, D3

, DA and
E . The V geometry is discussed first. Mirrors M10 , Mlx and M12 are adjusted

until the length DX D2 equals the length Dj^E,, and the length D2 D3 equals the

length D3 D4 E . Mirrors M10 and Ma are adjusted until the image of S is at the

surface of Ma. In a similar way, M12 and MX1 are adjusted until the image of

S is at E . Let the mirror Ma turn 180° around the axis which is vertical to

the horizontal plane through C . The mirror Ma is moved to position Ma

'

. Then,

the sample mirror Ms is inserted. Ma

' and Ms are separated by the length D2 'C .

The V geometry is thus changed to the W geometry. The reflecting surface of the

sample mirror Ms coincides with the plane "P" which is perpendicular to the

horizontal plane. The size of S ' and the position E are not changed when the

V geometry is changed into the W geometry. The reflecting surfaces of Ma and

Ma

' are equidistant to the plane P and also the reflecting surface of Ms

coincides with the plane "P."

4.2 Adjustment procedure for the V-W accessory.

The adjustment procedures stated in sections 4.1 are summarized here.

4.2a A visible source B is placed between the slit S x and the mirror M7

, as

close as possible to slit S x

. (See fig. 1.) The position E which is

the image position of the slit S x is found.

4.2b Ms and Ma

' are mounted on opposite sides of this holder, whose faces are

nearly parallel. Ma

' is aligned using the ACT and three mirrors external

to the instrument until it is parallel to Ms

. (See appendix A.)

4.2c The image position of the source S can be found at D2 on the surface

of the mirror Ma

. (See fig. 5.) The image length (D3 DA + DA E ) of M12

is equal the object length D2 D3 of M12 . The object size at D2 and the

image size at E on the reflecting surface of Ma will then be equal. (Of

course the focal length of mirror M12 equals D2 D3 /2.)
4. 2d The light flux from source S which passes through the V-W accessory and

E is imaged on the entrance slit S x

. Ma is moved along the 00' axis and

the mirror Mx x rotated about the point DA slightly until the image of the

source S on the entrance slit S x is of maximum brightness, and the center

of the image of S lies on E as well.

4.2e The mirror Ma is rotated to position Ma

' and the sample mirror at position

Ms is inserted (see fig. 5) i.e., the V geometry is changed to the W

gerometry

.

5. USE OF THE ACTAM TO ALIGN THE SAMPLE IN THE V-W ACCESSORY

5.1 Principle of Operation of ACT.

There are two bright image spots which can be seen in the field of view of

the autocollimator telescope. (See figs. 6A and 6B.) One bright spot "a,"

formed by a plane mirror in the autocollimator, in the middle of the field of

view is fixed. Spot "b" is formed by the reflected beam from an outside mirror

M. The collimated beam which comes from the autocollimator telescope is

projected onto the surface of the mirror M. If the reflecting surface of M is

perpendicular to the incident beam, the spot "b" will coincide with spot "a."

The autocollimator telescope can be used to determine the angular position of

a mirror and to keep its orientation fixed when this mirror is moved.

5.2 Adjustment of the Sample Mirror with the ACTAM.

There is only one mirror Ma in the V geometry. But in the W geometry,

there are mirrors Ma

' and Ms (shown in fig. 7). The length C D2 is equal to the

length C D2

' when the V geometry and the W geometry are interchanged since Ma ,

Ma

'

, and Ms are mounted on the same holder.
5.2a The mirror MT

' in the V geometry is shown in figure 7. The center rays

from the ACTAM are reflected to D2 on the mirror Ma

. The angle I in

figure 7 must be larger than the angle a in figure 5 so that the mirrors,

MT

' and MT

, do not block the source beam from S .

5.2b The mirror MT is positioned such that it is perpendicular to the collimated

beam which comes from the autocollimator telescope and is either folded

by the mirror Ma

, or by the combination of mirror Ma

' and the sample Ms

.

This will occur when the two bright spots coincide.

5.2c When the two bright spots coincide, the beam direction and the beam path

for the folding mirrors MT

' and MT are unchanged when the V geometry and

W geometry are interchanged. If the two bright spots do not coincide,

the movement error of the mechanical mount of the V-W accessory is the

cause. Therefore, the adjustment of the mechanical mount of the V-W

accessory must be meticulous in order to achieve high accuracy specular

reflectance measurements

5.3 Angular accuracy of the V-W accessory with ACTAM.

If the V geometry and the W geometry are interchanged without using the

ACTAM to monitor and to adjust their positions, one cannot assess how much error

in the reflectance Rs is due to position errors. After alignment of the V-W

accessory in the sample compartment of the IR spectrophotometer with the ACTAM

is completed, measurement of the reflectance Rs of the specular sample according

to eq (3) can be performed and the error in Rs due to misalignment is known.

If Ma in the V geometry is tilted by an angle 15, the reflected light is rotated

by an angle 2fi. The autocollimator telescope has a resolution of 10 s of arc

so that the beam direction passing through the accessory and into the interior

optical system is accurate to 10/2 or 5 s of arc.
6. UNCERTAINTY AND RESULTS

After the V-W accessory was operational, tests were performed using

an electroplated gold mirror Standard Reference Material (SRM) No. 16 which has

been certified from 0.6 to 2.5 /zm c 3 k ]

, and a test aluminum mirror. The absolute

reflectance of the gold mirror SRM was measured between the wavelength 2 /zm to

22.22 /xm. The test aluminum mirror (Ms

) is not to be confused with the aluminum

mirror (Ma

) which is used as a part of the V-W accessory. Besides being used

to compare at two wavelengths, the gold mirror SRM was also used to evaluate

setup and alignment errors. The test aluminum mirror was used to determine the

error caused by misalignment between the V and W geometries, and the error caused

by nonuniformity of the other aluminum mirror (Ma ) used in the V and W geometries

to measure the gold mirror SRM. The measurements are made in a convergent beam

with the central ray incident at an angle of 12.8 degrees. The "equivalent f/no"

(image distance divided by mirror diameter) is 4.5.

6.1 Alignment error of the V-W accessory

Four pairs of spectral reflectance measurements of the gold mirror SRM were

made. Each pair consists of one value with the polarizer (P in fig. 1) in the

0° position, R and the other value with the polarizer in the 90° position, R90 -

The V-W accessory was then removed from the sample compartment after the four

pairs were measured (Setup 1) . The V-W accessory was inserted again and four

more pairs of measurements (Setup 2) were made after the V-W accessory was

realigned. The results are shown in table 1. There was a systematic difference

between the measurements resulting from the two alignments. This systematic

difference was attributed to the fact that the light beam is not perfectly

uniform and the slits which open and close depending on the sensitivity function

of the spectrophotometer, may not reproduce the exact settings.

8
For each Setup, the four values of spectral measurements for each

polarization were averaged to give the R values in table 1. The individual

pairs of values for the 0° and 90° polarizations for Setups 1 and 2 were

averaged to yield Rx and R2 with standards deviations ARX and AR2

. Rx and

R2 with respective standard deviations, AR± and AR2 are shown in table 2. The

value |RX -R2 |/2 represents the maximum difference from the average, and is an upper bound for the systematic error (3a) for the average of Setup 1 and Setup

2, (R1 +R2 )/2. The total uncertainty, ART , is discussed below.

6 . 2 Uncertainty

Factors which were considered in the error analysis were the alignment

error after insertion of the V-W accessory, angle sensitivity as determined by

the ACTAM, linearity, stray light, wavelength errors, scattering, random

uncertainty, and nonuniformity of the small aluminum mirror used in the

measurements

From section 6.1, the whole V-W accessory alignment uncertainty is

estimated by |R1 -R2 |/2 in table 2 (3a).

The angle sensitivity uncertainty was estimated by measuring the test

aluminum mirror with a deliberate misalignment of the ACTAM. A misalignment of

one spot diameter in the ACTAM in the up, down, left and right directions

caused variations of 0.00077, 0.00028, 0.00001, and 0.00029 in R when compared

to the alignment spot centered. For this test R was again measured by four

pairs of R and R90 for each condition i.e., centered down, left, and right.

Therefore upper bounds for the vertical uncertainty were estimated to be

0.0008(3a) and the lateral uncertainty was estimated as 0.0003(3a). This test was done at 1000 cm" 1 or 10 /zm wavelength.

The linearity was measured by the double aperture method [2]. The

reflectances of the gold and test aluminum mirrors are higher than 0.98. There

is little error caused by nonlinearity , since the signal in the V geometry is

very near to the signal in the W geometry. This uncertainty was estimated to

be 0.00013(3a).

Instrumental stray light and wavelength errors were negligible since these

mirrors are spectrally neutral in the mid infrared range.
Quality mirrors are known to scatter light several orders of magnitude less

than the main specular component and the error due to scattering is assumed to

be negligible.

The standard error (standard deviation of the mean) for Rx and R2 was calculated from the four pairs of polarized measurements

.

4

SE = [ S (Rji - R) 2 ]/[n(n-l)] 1/2

i=l

with n = 4 and j = 1 or 2

,

and standard error for the average R is propagated as:

SE = (1/2) [(SE) 1

Z + (SE) 2

2

] 1/2

where (SE) X and (SE) 2 are the standard errors calculated above. Three times

SE is the random uncertainty with a confidence level of 99.7%.

The image of the illuminating beam is reversed for the V and W geometries.

Therefore, if the small aluminum mirror is nonuniform, an error is introduced.

This error was estimated by rotating the small aluminum mirror 180° about its own axis and comparing signals at 0° and 180°. The range was 0.00020, and this was taken to be an estimate of the uncertainty (3a).

These uncertainties were added in quadrature (square root of the sum of

squares) with the result ART in table 2. ART varies between 0.0014 to 0.0024

in the wavelength range of 2 to 22.22 /zm. The uncertainty of instruments which

do not use ACTAM are typically 0.01 to 0.02.

6.3 Comparison with previously published values.

Two values for 12.8 degree incidence in parenthesis in table 2 result from

interpolation from values at 6° and 30° angle of incidence for the SRM No. 16

gold mirror. The agreement is within the combined uncertainty. The values for

a freshly evaporated quality gold coating, which are always greater than those

for an electroplated mirror, serve as an upper bound and are shown in table 3 for this wavelength region. [5]

7 . SUMMARY

It has been shown that the ACTAM can be used to position the V geometry

and W geometry in a very reproducible way. Therefore, one of the most uncertain

10
features of this type of measurement is reduced and quantified. An example of

measuring a gold mirror standard gives results within the combined uncertainties

Finally, the uncertainty of the measurements of the gold mirror SRM using this

reflectance accessory is calculated and shown to be less than 0.0025 at all

wavelengths tested from 2 to 22 /zm.

8REFERENCES

[1] Strong, J., Procedures in Experimental Physics Prentice Hall, Inc.,

Englewood Cliffs, NJ 1938. p. 376

[2] Mielenz, K. D., and Eckerle , K. L. , "Spectrophotometer Linearity Testing

Using the Double Aperture Method," Appl . Opt. 11 (10) (Oct. 1972).

[3] Weidner, V. R. and Hsia, J. J. "NBS Specular Reflectometerspectrophotometer, " Appl. Opt. 19 (8) (April 1980).

[4] NIST Certificate for Standard Reference Material (SRM) 2011.

[5] Hass, G. in Kingslake, ed. "Applied Optics and Optical

Engineering," vol. Ill, pp. 309-330, Academic Press, Inc., New York, NY

(1965).

9ACKNOWLEDGMENT

This work was partially supported by the Electro-Optics Working Group

of the Calibration Coordination Group (CCG) of the U. S. Military Services.

11
Table 1. Measured specular reflectance at 12.8° of gold mirror SRM No.

16 for two orientations of the polarizer for two Setups of the reflectance

attachment. Each value is the average of four independent runs.

Setup

1

Setup

2

Setup

1

Setup

2

No. f(cm_1

) A(/zm) R

90: ^9

1. 5000 2.000

2. 4800 2.083

3. 4600 2.174

4. 4400 2.273

5. 4200 2.381

6. 4000 2.500

7. 3200 3.125

8. 2800 3.570

9. 2000 5.000

10. 1500 6.666

11. 1000 10.00

12. 800 12.50

13. 700 14.30

14. 500 20.00

15. 400 22.22

0.9831

0.9822

0.9824

0.9828

0.9828

0.9828

0.9847

0.9847

0.9870

0.9868

0.9879

0.9889

0.9890

0.9890

0.9900

.9855 .9819

.9862 .9832

.9864 .9833

.9860 .9832

.9857 .9836

.9868 .9839

0.,9884 0,.9850

0,.9883 0,.9852

0.,9879 0,.9870

0,,9885 0,,9878

0,,9890 0,,9879

0..9915 0,,9888

0,.9923 0,,9896

0.,9908 0, 9893

0. 9931 0. 9898

0.9865

0.9873

0.9868

0.9871

0.9873

0.9880

0.9883

0.9889

0.9895

0.9895

0.9897

0.9934

0.9939

0.9917

0.9938

12
Table 2. Rx and R2 are the specular reflectances at 12.8° of the gold mirror

SRM No. 16 for the average of the paired combinations of 0° and 90°

polarizations for Setups 1 and 2. ARX and AR2 are their standard deviations.

(R1 -R2 )/2 is the magnitude of the systematic difference from the average

(R1 +R2 )/2. ART is the total uncertainty as defined in section 5. The two

values in parenthesis are taken from the certificate for NIST SRM 2011.

No. »/(cm" ) A (/an) Ri ARi R2 AR2 |Ri-R2 1/2 (R1 +R2 )/2 ART 1. 5000 2.000 0.9825 0.00030 0.9860 0.00010 0.0017 0.9843

(0.9812)

0.0023

2. 4800 2.083 0.9827 0.00050 0.9867 0.00053 0.0020 0.9847 0.0024

3. 4600 2.174 0.9828 0.00063 0.9866 0.00022 0.0022 0.9847 0.0020

4. 4400 2.273 0.9830 0.00063 0.9866 0.00059 0.0018 0.9848 0.0023

5. 4200 2.381 0.9832 0.00078 0.9865 0.00062 0.0017 0.9849 0.0022

6. 4000 2.500 0.9834 0.00020 0.9874 0.00038 0.0020 0.9854

(0.9803)

0.0023

7. 3200 3.125 0.9848 0.00034 0.9883 0.00076 0.0018 0.9866 0.0022

8.. 2800 3.570 0.9850 0.00074 0.9886 0.00079 0.0018 0.9868 0.0024

9. 2000 5.000 0.9870 0.00022 0.9887 0.00026 0.0008 0.9878 0.0014

10. 1500 6.667 0.9873 0.00018 0.9891 0.00034 0.0009 0.9882 0.0014

11. 1000 10.00 0.9879 0.00088 0.9894 0.00042 0.0008 0.9887 0.0016

12. 800 12.50 0.9888 0.00044 0.9923 0.00067 0.0017 0.9906 0.0022

13. 700 14.30 0.9893 0.00036 0.9931 0.00037 0.0019 0.9912 0.0023

14. 500 20.00 0.9891 0.00031 0.9913 0.00076 0.0011 0.9902 0.0017

15. 450 22.22 0.9899 0.00065 0.9935 0.00036 0.0018 0.9917 0.0022

13
Table 3. Reflectance values for a freshly evaporated gold coating [Ref. 4]

A(/im) R(%)

2 99.1

3 99.3

4 99.4

5 99.4

6 99.4

7 99.4

8 99.4

9 99.4

10 99.4

15 99.4

20 99.4

14
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4-1 13

00 QJ a >-i •H ccj SO CO 43 4-1 CO a

QJ

M e

QJ QJ

4-1 .H

(1) QJ eo >»

4-1 QJ o A!

4=

(Hi QJ o 42 u H 4J

CJCU • P, !^

C/> !-i 4-1 QJ QJ 4= e

4-1 o

QJ

m 00 o 1 es 6

cd T3

J-i C2 00 cd

CO •H >» Q u

4-1 i-\ QJ cd

CJ

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4-1 CJ P. O > p

CJ) H

15
CD 43u • c g

•H c

cn •H

•Uc 'O

•H CD

Cfi

a CO

a)

H j-i CO (X

CJ

X

H Q) 4J •H 0) H V-i CJ CO

t>0 CO a c

•H o

13 •H O W

43 c

in 0)e e •H

cO T3

>-< GO CD

CO 43

•H

H

T3rH . « M o CD •H 4J

4-1 0) a S o o

4-1

u o C 43

0) Q* rH O

crj

U > 4J •H a 3 cd cr a

flj CO

CN

S3oH

16
I

E

E

CO

i

so

mm

/

S

0-5mm

EO

FIGURE 3. Slit and light beam at point E

I

E

E

O

17
FIGURE 4. The V and W geometries with angle of incidence a.

18
•u • •H J-i ^ (U

4-)

W 0)

4-> S a o

CO 4-) n o

a) x:o

S-i < <u h a u en <

JZ 4-1 4J

5s -H

00 o o

en cO B 3

CO I U >

60

CO <D •H 43 Q J-> OM

fX4

19
UJ

>-

CM c o •H < •u

CJ 0)

CO rH

4-> CO C

•H

C

CO a;O ^5 o

a; a) iH 4J O CO

CJO X.

4-J 4-1 < J5

<!VOW

PiOM

Pn

20
FIGURE 6B. Field of view of autocollimator telescope;

"a" is the reference bright spot and

"b" is the reflected bright spot.

21
FIGURE 7. Diagram showing ACTAM with mirrors

in the V and W geometries.

22
APPENDIX A: ADJUSTMENT OF MIRROR HOLDER

Mirror Ma

' and sample Ms are held on two sides of the same holder (see

fig. Al).

The procedures for checking and adjusting parallelism of Ma

' and Ms are as follows

:

(a) A test arrangement (See fig. A2) is set up with mirrors Kx

, K2 , K3 and

autocollimator telescope (ACT) without Ma

' and Ms

. The beam from the

ACT is reflected from mirrors Kx

, K2 and K3 and then back to the ACT

forming one bright spot at the center of the field of view of the ACT

in addition to the reference spot. This test arrangement is shown in

figure A2

.

(b) Next, the holder with mirrors Ma' and Ms is placed in the beam path of

the test arrangement (See fig. A2) with the coated surface of Ms against the holder. The holder is then adjusted until Ms is

perpendicular to the beam from K3. The image spot reflected by Ms and

by mirrors K3

, K2 and Kx is superimposed on the reference spot at the

center of the field of view of the ACT.

(c) After Ms is removed, the beam from the ACT is directed to Ma

'

. This

beam is reflected by Ma

' back to the ACT. Ma' is adjusted (See fig.

Al) so that the image spot is superimposed on the reference spot also.

Thus Ma

' is parallel to Ms since both images are superimposed on the

reference spot.

23
FIGURE Al. Mirrors M ' and M mounted on holder, 3 s

24
K3

M

ACT

K2

FIGURE A2. Arrangement for making M ' parallel to M

a. S

25
INSTRUCTIONS

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FC*M NBS-1 1 4A (REV 1 1 84) USCOMMOC 85-6006
FOOM NBS-1 MA IREV 1 1 gj|

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions;

1. PUBLICATION OR

REPORT NO. NIST/TN-1280

2. Performing Organ. Report No. 3. Publication Date

August 1990

4. TITLE AND SUBTITLE

Absolute Specular Ref lectometer with an Autocolliraator Telescope and

Auxiliary Mirrors

5. AUTHOR(S)

Tie Ming Wang, Kenneth L. Eckerle, Jack. J. Hsia

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instruct/on s)

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

(formerly NATIONAL BUREAU OF STANDARDS)

U.S. DEPARTMENT OF COMMERCE

GAfTHERSBURCL MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

SAME AS ITEM # 6

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

The Strong method for measuring reflectance is used in many laboratories.

However, an uncertainty in the angular position of the sample can lead to large errors. A novel use of an autocollimator telescope with two auxiliary mirrors

(ACTAM) can reduce this angular uncertainty and lead to more accurate measurement

of reflectance. The instrumentation, its adjustment, measurement procedure,

results, and uncertainties are discussed. The total uncertainty for the

reflectance measurement of a gold mirror was found to be less than 0.0025 over

the wavelength region 2 to 22 nm.

12. KEY WORDS (Six to twelve entries; a/phobeuco/ order; capitalize only proper names; and separate <ey words by semicolons)

alignment; autocollimator telescope; auxiliary mirrors; specular (regular)

reflection; strong (vw) method; uncertainty

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