NBSIR 73-232

A Survey of the Stability of

Opticai Flats

Charles P. Reeve

Ralph C. Veale

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

June 25, 1973

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NIST

PUBLlC/VTlONS I

100

.U56

#73-232

1973

NBSIR 73-232

A SURVEY OF THE STABILITY OF

OPTICAL FLATS

Charles P. Reeve

Ralph C. Veale

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

June 25, 1973

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Seaetary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

A Survey of the Stabillly of Optical Flats

by

Charles P. Reeve and Ralph C. Veale

1. Introduction

The National Bureau of Standards has occasionally been asked how

often optical flats should be calibrated. The motivation for this

question seems to be a concern on the part of the optical flat owner

about the expected long range stability of his flat. It would be

futile to try to answer this question directly because optical flats

are used in many different situations and different levels of accuracy

are required in each case. Perhaps the best way to approach this problem

is to study the case histories of several flats which have been regularly

calibrated over a number of years. The frequency of use for all of them

is not known, but is believed to range from virtually no use to almost

daily use.

2. Method of Sampling

The most commonly calibrated flats for which records exist between

1959 and 1972 are those of ten inches in diameter. Five of these flats

were calibrated at least three times during that period and are included

in this survey. Also included, are the most frequently calibrated four,

six, and eight inch flats. The four inch flat was coated and the coating

was noted to be wearing. The only flat which belongs to NBS is the ten

inch master flat #1-3. The frequency of calibration of these flats was

once every two to four years.
I

3. Graphical Display

In order to enhance the interpretation of the optical flat profiles,

the profiles are presented as graphs instead of numbers in figures 1

through 19* Each line represents the profile of the flat along the given

diameter. Measurements were made at the positions where a vertical slash

(|) appears, and then a smooth curve composed of weighted parabolas was

fitted to the measured values. This curve is believed to give a good

estimate of the actual surface profile.

The variations in the profiles can be attributed to three main sources:

(1) random and systematic error in a single calibration

(2) a between time component of error due to variation in the setup

procedures

(3) a long-term physical change in the surface profile of the flat

(instability).

The current method of calibration for a typical flat has a three standard

deviation limit for random error of measurement of about 0.10 microinch.

The limit for master -error is estimated not to exceed 0.25 microinch giving

a total uncertainty of 0.35 attributable to the first source. Methods for

separating errors from the second and third sources have not been devised.

It is believed, however, that errors from the second source do not exceed

the random component of error in the measurement process.

4. Conclusions

In all cases except figures 10 and 11, it is apparent that the long

term variation in profile of an optical flat is quite small. (Figures 10

2
and 11 give a strong indication that either the flat was refinished between calibrations or that a possible mistake in sign was made in reducing

the data.) This does not necessarily mean that optical flats should be

calibrated only once. Repeated calibrations serve as checks on previous

calibrations and also give an indication of the total variability of the

optical flat.

It is up to the user to weigh the above factors as he examines his own

particular needs and then conclude how frequently he should have his optical flat calibrated.

3
i

TABLE I

Years of Calibration for Selected Optical Flats

Optical Flat

Identification Diameter

Years of

Calibration Figure No

NBS #1-3 10" 50 54 64 68 1

//Q-0513 10" 68 69 71 2-5

#EPP-1 10" 61 66 67 69 6-7

#ACL82414 10" 63 64 66 8-9

#852 10" 60 62 66 10-11

#TI 3„ 59 61 62 64 65 66 67 69 12-13

#VK6224 6" 64 65 69 71 14-17

#VK4505 4" 64 65 66 67 68 18-19

4
Bibliography

1. Dew, G. D. , The Measurement of Optical Flatness, Journal of

Scientific Instruments, Vol 43, pp 409-415, 1966.

2. Emerson, Walter B. , Determination of Planeness and Bending of

Optical Flats, Journal of Research of the National Bureau of

Standards, Vol 49, No. 4, pp 241-7, October 1952.

3. Reeve, Charles P., The Calibration of an Optical Flat by Interf erometric Comparison to a Master Optical Flat (in preparation)

.

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FORM NBS-1 MA (1-71) U.S. d::pt. of ccmw. BIBLIOGRAPHIC DATA

SHEET

t. PUuLICATION OR REPORT NO. ' 2. Oov't Acccssi-.r^-. Uci ipicMi's Acclssio.-, No. NBSIR 73-232 No.

4. TITLE AND SUBTITLE

A Survey of the Stability of Optical Flats

5. Publication O.uc June 25, 1973

6. Performing Organisation Code

7. AUTMOK(S)

Charles P. Reeve f.nd Ralph C= Veale

8. Pc-rforniing ()rg.ini;'..itiu.i NBSIR 73-232

10. Project/Task/ v. ork Unit No. 9. PERFOR.MING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF ST.\NDARDS

DEPARTMENT OF COMMERCE

V.'ASIUNGTON, D.C. 20234

11. Contiact/Grmt No.

12. Sponsoring Organization Name and Address

SAME

13. Type of Report & Period

Cove redFinal

14. Sponsoring Agency Code 1

i

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factu;il summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

. Some optical flat owners are concerned about the long term .<^tability

of their optical flats. To examine this problem, a survey was made

of several optical flats which were calibrated by the National Bvn-eau

of Standards at least three times during the period from 1959 to 1972.

The m.easured profiles of these flats are presented graphically so that

the different calibrations can be compared. The conclusion of this

survey is that optical flats are quite stable over a period of several

years, but since the individual requirements for precision may vary

greatly, it should be left up to the owner to weigh the appropriate

factors in determining how frequently he needs his optical flat calibrated,

17. KEY U'ORDS ( Alph.->betical or der, separated b;- s'?micolons)

Calibration; optical flat, profile; stability; standard deviation.

18. AVAlLABILliY STATEMENT

UNLIMITED.

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECUKl TY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCi.ASSiFii-;n

21. NO. 01 PAGES

25

22. Price

ij r. r. oi.'.M-oc CDS'! '.-r-? !


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