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i NBS TECHNICAL NOTE 1080

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Redefining the Scratch Standards

Matt Young

Eric G. Johnson, Jr.

100

.U5753

No. 1080

1935
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Redefining the Scratch Standards

Matt Young

Eric G. Johnson, Jr.

Electromagnetic Technology Division

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

f \£ \

VI

•**!<* •* N%S kdMMr ra V

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued February 1985

NATIONAL : ' OF STfif.'E LIBRARY
National Bureau of Standards Technical Note 1080

Natl. Bur. Stand. (U.S.), Tech Note 1080, 26 pages (Feb. 1985)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1984

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402
Contents

Page

1. Introduction and Objective 1

2. Elementary Diffraction Theory 3

3. Instrumentation 5 4. Measurements on Existing Standards 9

5. Proposed New Standards 12

6. Experimental Results 16

7. Conclusions and Recommendation for Future Work 19

8. References 20

m

Redefining the Scratch Standards

Matt Young and Eric G. Johnson, Jr.

Electromagnetic Technology Division

National Bureau of Standards

Boulder, Colorado 80303

The scratch standard (MIL-0-13830A) is a cosmetic standard

that is effected by a visual comparison with a set of submasters

that are in turn evaluated by comparison with a set of master standards. Both manufacture and certification of the submasters are somewhat unreliable. In this paper, we show that the submasters

can be classified according to the relative power scattered at a relatively small angle. We have designed etched gratings with

which to replace the submasters; these gratings have the appearance

of scratches but diffract light into a broad peak between 5 and 10

degrees off the axis of the incident beam. We have classified some prototypes both by comparison with the master standards and by a photoelectric measurement; agreement between the two methods is

good. We suggest that such gratings be used as the submasters and

possibly that they be classified by a photoelectric rather than

visual measurement.

Key words: cosmetic standard; diffraction; MIL standard; optical

quality; optical-surface quality; scattering; scratch and dig

standard; standards; surface quality; surfaces

1. Introduction and Objective

The Scratch and Dig Standard [1] is the most important descriptor of

optical -surface quality used in the United States. The dig standard is well

understood because the designation of a dig on a surface is related to the

diameter of the dig. The scratch standard is less well understood and less

well controlled; therefore, we take a few moments to discuss its current

status.

To begin, the scratch standard is in most applications purely a cosmetic

standard. Although scratches on a surface are very visible, in general they

contribute only a small fraction of the total light scattered by the surface.

Thus, unless a surface is located in or near an image or object plane, the

1
presence of a scratch is likely to be irrelevant to the performance of an instrument. (Scratches have, however, been implicated in damage to high-power

laser components, and the scratch width is suspected to be a factor [2], For

such components, possibly a standard of measured width needs to be developed;

the scratch standard as it exists today is unfortunately not appropriate.)

The designation (S-10, -20, -40, -60, or -80) of a scratch on a specimen

is determined by a visual comparison with a submaster. Typically, the submaster and the specimen are held at roughly the shortest distance of distinct

vision (about 25 cm) and viewed in transmission, with no intervening optics.

The inspector classifies scratches on the specimen by comparing the peak scattered intensity with that of the set of submasters. The numerical designation

of the submaster or of a scratch on the specimen has no direct relationship to

the width of the scratch. This is also true of a newly developed method of

measuring the relative visibility of a scratch [3].

The submasters themselves are classified by a visual comparison with a set of master scratches. This leads to significant error and, hence,

scratches with a relatively wide range of scattering parameters may be given

the same designation. In addition, trained inspectors often reject a high

proportion of the submasters they examine; part of the purpose of our investigation was therefore to find ways to improve manufacturing and thereby reduce

costs.

(In the past, it was believed that the designation of a scratch on a submaster was equal to about 10 times the scratch width in micrometers. We have

no way of verifying this contention; however, today the scratches are cer- tainly less than that width [4]. This has led some of our colleagues in the

industry to suggest that the primary standards may have healed with time and

that the current standard is therefore substantially more stringent than when

it was introduced. Again, we have no means to verify this claim.)

A careful literature search revealed few objective measurements besides

some integrated scatter scans that were reported several years ago [5], We

have therefore undertaken a comprehensive program to (1) provide quantitative

measurements of the light scattered by the scratches, (2) develop a body of

theory to relate the properties of the scratches to the pattern of scattered
light, (3) correlate the measurements with the assessment made by the eye

[6,7], and (4) develop a set of artifacts that can be manufactured reliably

and serve as the submasters [6-8].

One purpose of this program was to attack the measurement and characterization problem. To this end, we have developed photoelectric measurement

techniques and theory, and used them to characterize existing sets of submasters. These are scribed scratches as well as artifacts produced by other

methods (such as etching).

In the body of this Technical Note, we report the results of our investigations and further propose that the submasters be replaced by etched gratings

manufactured by high-resolution electron-beam lithography. The gratings are designed to scatter maximum intensity into the range between 5 and 10° off the

axis of the illuminating beam; we report preliminary measurements on such

gratings. If the gratings can be manufactured reliably, the artifact standards can be retired.

2. Elementary Diffraction Theory

In this section we apply the Fraunhofer-Kirchhoff diffraction theory to

diffraction by a one-dimensional phase object [6]. The results are useful for

design purposes and are accurate for grooves that are wider than a few wavelengths and shallow enough that we may assume single scattering. Likewise,

the scattering angles must be sufficiently small that single scattering

applies.

We begin by calculating the diffraction pattern of a groove whose depth

is so small that the groove affects only the phase of the beam. The width of

the groove is b and the width of the illuminating beam is B. The phase change

across the groove is A. The equation for the diffraction pattern in the far

field is

E(e) = / A(s) e'

i(k sine)s

ds, (1)

-B/2
where E is electric field strength, k is 2tt/x, and where multiplicative con- stants have been ignored [6], A(s) is the amplitude of the electric field at

any point s between -B/2 and B/2 and, for uniform and colli mated illumination,

is equal to A

Q e 1A at the location of the groove and A elsewhere. If the

index of refraction of the glass is n and the depth of the scratch is t,

A = k(n - l)t.

The integral, Eq. (1), may be evaluated by separating it into an integral

corresponding to the diffraction pattern of an aperture with width B and a second integral corresponding to the diffraction pattern of a slit with width

b but with a relative strength dependent on the phase shift A. The result is

the sum of two terms,

E(e) - sine $ + ^-5- e lA/2sin (a/2) sine <j>, (2)

D

where sine x = (sin x)/x, $ = (tt/x) B sine and <j> = (tt/x) b sine. The first

term describes the unscattered or zero-order light. The second term describes

the scattered light and is the diffraction pattern of the groove; it is weaker

by (2b/B) sin (a/2) than the first term. It also has angular spread B/b times

the first; therefore, the first term dominates around e = 0, whereas the

second term dominates elsewhere.

A system capable of measuring the entire scattered-radiation pattern must

be capable of measuring both the scattered and unscattered light. The relative intensity, just off specular transmission, is given by

I s (0)/I

Q

(0) = 4(b/B) 2 sin 2 (A/2), (3)

where subscripts stand for scattered and incident intensity. The scattered

intensity is a maximum when sin 2 (A/2) = 1, or when t = X/2(n - 1); for the case n = 1.5, the condition t = x (or some odd multiple of x) results in maxi- mum scattered intensity. Many scratches are much shallower than one wavelength. Thus, Eq. (3) shows that, if the detection system is limited to a range of six orders of magnitude or so, the width B of the illuminating beam

must be no greater than several hundred times the width b of the scratch.

This is a comparatively stringent requirement and requires the design of a unique optical system [6].
Microscope

Beamsplitter Objective

Reference

Detector

Relay Lent

Glare Stop virtual Aperture

Field Lena

Figure 1. Experimental apparatus. Dashed lines show how image of aperture

stop is projected into plane of sample. After reference 6.

3. Instrumentation

The optical system for measuring the scattered light as a function of

angle is shown in Fig. 1 [6]. A detector assembly is located on the circumference of a circle centered about the sample position. The transform lens

focuses the HeNe-laser beam to a point on that circle; the intensity at points

along the circle is the Fraunhofer or far-field diffraction pattern of the

sample.

The limiting aperture is a small rectangular opening located in a novel

position: to the left of a relay lens. The function of the relay lens is to

project an image of that aperture through the transform lens and into the

plane of the scratch. (The transform lens has to be located at least one

focal length to the left of the sample.) This is necessary because space

problems prohibit placing the aperture in contact with the scratch, and even a few centimeters' separation results in a Fresnel-diff raction pattern. The

intensity variation resulting from this diffraction pattern may be 20 percent

or so in the plane of the scratch. For studies of scattering by overall sur- face roughness, this may not be an important factor. For studies of scattering

by an isolated defect, however, an error will arise, depending upon the precise positioning of the defect in the peaks or valleys of the diffraction
Joc

tt

0.5 1.0

Position, mm

1.5

Figure 2. Intensity as function of position in plane of sample. Resolution

limit is 25 pm.

pattern. Using the relay lens to image the aperture stop in the plane of the

sample eliminates this error by locating a pupil, or a virtual aperture, precisely in that plane. The principle is identical to that of imaging pupils in

a telescope.

We determined the uniformity of the illumination by scanning the sample

plane with a 25-ym slit oriented perpendicularly to the plane of incidence.

Figure 2 is the result of one such scan; the beam is uniform to about 2 percent.

The impulse response of the system is the profile of the light diffracted

or scattered by the transform lens. At angles of 10 or 20° off axis, this may

amount to 10" 6 or so of the power measured at 0° and may be comparable to the

power scattered at those angles by a weak scratch. Hence, we inserted a glare

stop to the right of the transform lens to cut off a line of sight from the
lens to the detector, except very near the axis. The glare stop ensures that

the impulse response will be very nearly zero beyond a few degrees off axis.

A slit precedes the detector; it is 2.5 mm wide and 1 cm high. The width

of the slit determines the angular-resolution limit as well as the total scattered power that falls onto the detector at any angle; it is chosen so that

the resolution limit is about 0.3°. The height of the slit is sufficient to

capture light scattered out of the detector plane in case the scratch is not

precisely perpendicular to that plane.

The detector is a silicon photodiode with a built-in operational amplifier. The photodiode is a 2.5-mm disk. We therefore require a field lens to

project onto the detector all the light that passes through the centimeterhigh slit. This is accomplished by choosing the lens so that it images the

scratch onto the plane of the detector. The entire assembly is enclosed in a box with a red glass filter (not shown) to reduce stray light. A mechanical

chopper modulates the incident beam for detection of the scattered light with

a lock-in amplifier.

The entire experiment is computer controlled. The computer drives a stepping motor, which revolves the detector about the axis of the sample,

acquires data from the detector and the reference detector, and ultimately

processes and plots the data and records a paper copy of the graphics. In addition, the computer automatically changes the range of detection by switching a precision feedback resistor in the detector circuitry. There are six

such feedback resistors, each about ten times the resistance of the last;

counting an order of magnitude or so for the lock-in amplifier, this yields a range of seven or eight orders of magnitude in total.

We checked the linearity of the system by measuring the transmittance of

a filter on each of the ranges of the detector; the intensity was reduced each

time by attenuating the beam by approximately a factor of 10. The measured

transmittance of the filter was plotted against the nominal optical density of

the attenuating filters. The result should be a line with a slope equal to

zero, so we calculated the slope and standard deviation of the slope with a linear least-squares fit. The data shown in Fig. 3 yielded a slope of -1.0

x 10" 3 and a standard deviation of the slope of 0.8 x 10" 3

. The slope is
o

o

c

(0

E

M

C

(0

4)3

CO(0

4)

U.bUl 1 1 1 1 1

o

< » o -

- ®

o

o

o

o

o

o

o""

o

o

o -

o

o

o 0.57 ? ? 1 1 1

Nominal Density

6

Figure 3. Measured transmittance of a filter at various intensities. Hori

zontal scale is the nominal optical density of filters used to

attenuate the beam. Solid line is a line of best fit and shows

slope of ~0.1 percent per decade.

therefore practically zero but gives rise to an error of 0.1 to 0.2 percent

per decade, as the intensity is decreased. In addition, we estimate an error

of less than one percent per decade owing to uncertainty in the feedback

resistors, with an additional error of about three percent on the mostsensitive scale, where the precision resistor is not so well characterized.

After checking detector linearity we tested the system by measuring the

diffraction patterns of some precision slits. Figure 4 shows two such scans. The first graph is a plot of the measured diffraction pattern of a nominally

50-pm slit. The black dots are the calculated values of the maxima relative

to the intensity of the principal maximum; they are calculated using 51 pm,

which fit the data better than the nominal value. The minima are not zero
a>o

a.

0)>

DCo

-7

5 10

Scattering Angle, degrees

50-ym slit

15 5 10 15 20 25 30

Scattering Angle, degrees

1 50-pm slit

Figure 4. Diffraction patterns of bO-vim slit (left) and 150-ym slit

(right). Black circles are calculated values using 51 pm and

150-pm widths.

because of the finite size of the scanning aperture (which is only three or four times smaller than the width of the sidelobes). Beyond 10° or so, phase

errors resulting from slit irregularities cause slight deviations of theory

from experiment.

The second graph in Fig. 4 is the diffraction pattern of a 150-vim slit.

Here the system does not resolve the sidelobes because the detector width is

about equal to the sidelobe width. The four black dots are calculated values,

which include a correction factor to account for the detector width.

4. Measurements on Existing Standards

We have run polar scans of the intensity scattered by several sets of

scratches that had previously been classified by visual comparison with the

master standards housed at Picatinny Arsenal [6]. Scans of a set designated

number 1 are shown in Fig. 5. The structure seen in the 1-60 and -80 scans is

typical of some of the wider scratches and results from the use of coherent
o

Q.

J2

O)o

-7

No scratch

|

j I

|

o

Q.>

AC

O)o

-7

-30 30

Scattering Angle, Degrees

-30 30

Scattering Angle, Degrees

Figure 5. Power scattered as function of angle for set of scratches designated number 1. Curve labeled "No scratch" shows beam with no sample present. From reference 6.

10
• . ^* ? 100 S degrees 10 degree*

o •

^ •

Xw

>>•

C • / •

e P

• • •

« 10 /

• • *

%^* •

8

•> • /• •

• /

> " J

# * «••2 1 7 • " • ii i i I t i 10 20 40 60 80 10 20 40 60 80

Scratch Number Scratch Number

Figure 6. Power scattered at ±5 and ±10° as a function of scratch number.

Open circles are averages of five artifacts (ten points); closed

circles are extrema. From reference 6.

Table 1. Relative power detected at ±5° and ±10°.o ^

Scratch ±5° ±10°

designation

Lo Av ± 1 o Hi Lo Av ± 1 a Hi

10 0.89 1.8 ± 1.0 3.2 0.75 1.5 ± 0.9 3.2

20 3.2 4.9 ± 1.6 8.1 2.3 3.9 ± 1.2 6.1

40 5.7 14 ± 10 30 0.63 4.0 + 2.2 7.0

60 16 41 ± 19 66 2.2 6.3 ± 3.8 13

80 16 85 ± 76 220 6.4 29 + 26 84

*Units of 10" 6

. Average of four sets, or eight measurements per designation, a = standard deviation.

11
light; with a somewhat extended, broadband source such as those used for

visual observations, the curves would be substantially smoother.

Table 1 is a compilation showing the relative power scattered by different samples at angles of 5 and 10° off axis; it lists the lowest, highest, and

average values for four sets of scratches. When plotted on a semi -logarithmic

scale (Fig. 6), the average 5° data are nearly linear with designation.

Still, the data show a fairly large spread, only some of which results from

the accidental minima that result from the use of coherent light. The data

points overlap in such a way that a given designation could not always be distinguished from its nearest neighbor by a single photoelectric measurement.

5. Proposed New Standards

The data shown in Fig. 6 become very uncertain beyond 10° or so. Near

zero, the specular beam interferes with the observation. We therefore re- strict our consideration to the range between 5 and 10°.

We wish to design an artifact standard that can be manufactured by photolithography or electron-beam lithography. We suggest that the scattering pattern should approximately match our experimental 5° data and peak between 5 and 10°. An etched grating may be manufactured to meet these criteria.

This grating must be narrow enough that the eye cannot resolve it at the

shortest distance of distinct vision, d

v [9]. If the eye were diffraction

limited, this would imply that

w < Xd

v /D, (4)

where w is the overall width of the grating and D is the diameter of the pupil

of the eye. If we assume that d

v = 25 cm and D = 5 mm, we find that w must be

less than 27.5 ym for visible light whose average wavelength is about 550 nm.

(In reality, w can be somewhat larger than this value because the eye is not

diffraction limited when D = 5 mm.)

To be useful as an artifact standard, a grating must imitate the visual

appearance of a true scratch; a single sharp diffraction maximum will not suffice. Therefore, we, in essence, generate grating ghosts by specifying a

12
grating whose spatial frequency is not a constant but varies symmetrically

with distance from the center of the grating [10]. (If the grating is not

symmetrical about its center, the radiation pattern will be skewed to one side.) A grating designed in this way will display a scattering pattern that

is broad rather than sharp.

To design the gratings, we have developed a scalar diffraction theory; it

has been reported earlier and is too lengthy to discuss here [10]. We have

used the theory to calculate the intensity scattered by gratings composed of

rectangular grooves, with modulated spatial frequency as just described.

For simplicity, we make all the grooves the same width. The intensity

scattered (that is, diffracted) by a set of identical grooves is equal to the

appropriate interference pattern multiplied by the diffraction pattern of a

si

fi

wi

ti

ngle groove [9], Thus, the groove width determines the location of the

rst diffraction minimum, which we require to be beyond 15°, in agreement

th our decision to design for uniform scattering between 5 and 10°. Equaon (2) shows that the first minimum appears at the angle X/b, where

sine <f> = 0; each groove must therefore be less than 8 ym wide.

Equation (2) also shows that the scattered intensity varies with the

depth t of the groove. Because of the sin 2 variation of scattered intensity

with depth, the scattered intensity is least sensitive to depth error when the

grooves are one wavelength deep. If the grooves are less than this depth, the

scattered intensity is reduced by the factor sin 2 (irt/2x), but the overall

pattern is not changed. Therefore, we may calculate the scattered intensity

for any convenient depth and use this factor to scale to other depths.

By varying the depth and number of grooves in each grating, we are able

to simulate the scattering pattern of any scratch.

We performed calculations on frequency-modulated gratings shown in

Fig. 7. For simplicity, we impressed a linear modulation term onto the spatial

frequency of the grating. For the case of an odd number of grooves, the position xm

of the ±mth groove with respect to the grating center is given by

x m

= mx^l + r(m - 1)], (5a)

13
t_t

•* x **f

i_r

X3*f

Figure 7. Proposed frequency-modulated gratings. Sketch shows positions xm

of grooves with respect to center 0. Upper sketch: odd number of

grooves. Lower sketch: even number of grooves.

where m is an integer, < m < (N - l)/2; r is a number of the order of 0.1;

and xj is the location of the first groove from the center. If r were zero,

the grating would have uniform spacing xj. [In the terms of Ref. 10, x^

= X (a + 3) and r = 3/U + $).]

The case where the number of grooves is even is shown in the lower part

of Fig. 7; the position xm

may in this case be given by the expression,

x m

= (2m - l)x

1 Cl + r(m - 1)1 (5b)

where 1 < m < N/2. [In the terms of Ref. 10, x

x = (x /2)(a + 3/2) and

r = 3/(a + 3/2).] In this notation, the two centermost grooves are separated

by 2x^, center to center.

Figure 8 shows calculated scattering patterns of gratings with various

parameters. In all cases, the wavelength is 0.55 ym and the width of the

grooves is b = 1.1 ym. The latter value has been chosen to ensure that the

first minimum of the diffraction pattern of a single groove is well beyond

10°. By adjusting b, x^, and r, we may approximately match any data within a restricted range of angles. Scattering is small outside that range. Likewise, by adjusting the depth of the grooves, we can match any intensity.

Thus, we have a means for simulating the scratch standards by an etching technique.

14
After consulting with the sponsor and his consultant, we decided to

design the artifacts for peak intensity at 7.5°. The number of grooves, the

groove width, and the groove depth are to be chosen so that the peak scattered

intensity roughly follows the 5° data of Fig. 6; that is, a semi -logarithmic

plot of the peak scattered intensity should be approximately linear and have

an overall range on the vertical axis of just less than two orders of magnitude.

10 10

Angle, degrees Angle, degrees

Figure 8. Four calibrated curves whose peak intensities span about one order

of magnitude and are roughly linear when plotted logarithmically.

Intensities are normalized to intensity calculated for single

groove at 5°, actual values depend on depth.

Upper left:

Upper right:

Lower left:

Lower right:

In all cases

= 4.1 pm, t = 4.07 pm, r = 0.1, N = 2;

= 4.2 ym, t = 4.235 um, r = 0.09, N = 5;

x

x = 2.5 urn, x

x = 1.9 pm,

b = 1.1 ym.

t = 2.475 ym,

t = 1.925 ym,

r = 0.1,

r = 0.7,

N = 12;

N = 18;

15
Figure 8 shows theoretical curves for four gratings that could be used to

simulate the existing standards. All the grooves have the same width

b = 1.1 ym. The depth is arbitrary in that the vertical scale is in each case normalized to the power scattered at 5° by a single 1.1 -ym groove. The other

parameters are as noted in the caption.

The power scattered by these four gratings has a range of a factor of

7.5. Their peak powers are roughly linear on a semi -logarithmic plot like

Fig. 6. Unfortunately, the four gratings do not all have the same depth, so they could not be manufactured on a single substrate.

Figure 6 shows that the overall dynamic range must be a factor of 50, and more if an S-120 or -160 scratch is to be added. Equation (4) prohibits an arbitrarily large number of grooves. However, several sets of gratings such

as these four may be etched to different depths to extend the dynamic range to

several factors of 7.5.

6. Experimental Results

We have received several sets of artifacts etched into white crown glass.

One of these artifacts was a single groove, approximately 0.55 ym (one wavelength) deep and 1 ym wide. A scan of the intensity scattered by that groove

is shown in Fig. 9, where it is also compared with the prediction of Eq. (2).

The curves differ by a constant factor of 2; this may be attributed to the

fact that the actual cross section of the groove is not perfectly rectangular

and that the width is not precisely 1 ym. In particular, any rounding of the

groove will give rise to enhanced scattering; the experimental result is

therefore expected to be greater than the theoretical prediction.

Comparison with Fig. 6 shows that the single groove corresponds very

nearly to the average S-20 scratch. This piece of information, along with the

scaling rules and the theoretical curves such as those in Fig. 8, allows the

designer to specify a grating for virtually any scattered intensity.

We chose for further examination some single grooves and 10-groove gratings. The latter were tuned to exhibit a diffraction peak near 8°. They had

a "chirp factor" r of about 0.1; the spacing 2x 1 between the centermost

16
o r

-1

io

°- -3

•

>a

• "4

K

e>o _c

-30 -20 -10 10 20 30

Scattering Angle, Degrees

Figure 9. Power scattered by single, etched groove, nominally 1.0 ym wide and

0.55 urn deep. Lower curve is calculated for rectangular groove

with those dimensions.

30 -20 -10 10 20 30

Scattering Angle, Degrees

Figure 10. Power scattered as function of angle by 10-groove artifact that has

been classified S-40 by trained observers.

17
grooves was about 2.88 ym. The groove width was between 1 and 1.5 ym, depending on the etching process. The depth of the grooves depends on processing

and is unknown at the time of writing.

The artifacts had the appearance of a visual scratch. Some were sent to

Picatinny Arsenal for classification by their standard visual comparison with

the primary standards. They were then forwarded to NBS for our polar scan. Figure 10 shows a polar scan of one of the 10-line gratings. The peaks and

valleys are the result of using a coherent source, which has nearly zero beam

divergence. The peaks typically have angular subtense of about 1° (full angular width at half maximum). Visual observations are performed with an incoherent source that subtends several degrees; as a result, observers do not see the fine structure in the diffracted beam.

To compare the photoelectric measurements with the visual assessments, we averaged the highest peak with its nearest trough (in almost all cases this

was the same as dividing the peak value by 2). This procedure has nearly the

same effect as increasing the angular subtense of the source. For the single

grooves, we used the intensity scattered at 5°.

Figure 11 shows the results, plotted as in Fig. 6 [8]. The solid line

connects the average values, as in Fig. 6. The points are the new data: An x represents a single, chemically etched groove; a plus sign represents a 10-line, chemically etched grating; and a triangle represents a 10-line, ionbeam-etched grating. Each point is labeled according to its classification by

trained observers. H means the artifact was judged to be on the high side and

L, on the low side.

The data for the multiple-groove gratings at 5° fall agreeably (to us!)

close to the solid line, whereas the single-groove data show a variance com- parable to the ordinary submasters. We attribute this to the fact that the

gratings were designed to exhibit an off-specular peak, which forces the

observer to look at a specific angle. The way testing is performed today, we cannot know at what angle the observer views the single grooves, and this can affect the results. In addition, the isolated grooves simulated S-10 and -20

scratches; these are comparatively difficult to see, and we expect some uncertainty.

18
f 100 _ 5 degrees H + _ 10 degree* H +

o

K

>»

•*• A

/

A

+

c

•

c A

+

+

+

v 10

••

•

u

0)•

>

*•

/a Hx/

L7 T L

A /

xh ^y^

Hx' XL

Lx

1 1 "ii i i i "ii i i i 10 20 40 60 80 10 20 40 60 60

Scratch Number Scratch Number

Figure 11. Power scattered by etched artifacts at ±5° and ±10°, as function of

visual classification, as in figure 6. Solid line is the solid

line in Fig. 6. "H" and "L" stand for "High" and "Low" in appearance.

7. Conclusions and Recommendations for Future Work

Further analysis of the prototypes is still under way; standards can be

manufactured after perhaps one or two more experiments.

We recommend that, where possible, the artifacts be made of multiple-line

gratings, which we have found to be more accurately evaluated by eye. We have

further suggested that the sponsor consider manufacturing S-120 or -160

scratches as well as the existing set. Finally, we have noted that the S-20

scratch is quite close to the S-40 and should perhaps be dropped.

As long as the manufacturing process is sufficiently well controlled, a simple photoelectric device should be adequate to test the artifacts objectively. We recommend that (at least for testing the submasters) such a device

be designed and built to replace the present visual system.

19
Matt Young performed the experimental work, and Eric Johnson conceived

and carried out the chi rped-grating theory. We are indebted to John Salerno

for his support and encouragement, and for supplying the scratch samples; to

Richard Goldgraben for a yeomanly effort in providing the etched-grating

samples; and to Edie DeWeese for preparing the manuscript in her usual

excellent and cheerful fashion.

This work was supported by the U.S. Army Armament Research and Development Command, by the Department of Defense Calibration Coordination Group, and

by the U.S. National Bureau of Standards.

8. References

1. Military Specification MIL-0-13830A; 1963.

2. H. E. Bennett. Insensiti vity of the catastrophic damage of laser optics

to dust and other surface defects. Proc. Conf. Laser Induced Damage in

Optical Materials, 1982. Nat. Bur. Stand. (U.S.) Spec. Publ . 669:151-

155; 1982.

3. L. R. Baker. Microscope image comparator. Opt. Acta 31:611-614; 1984.

4. M. Young. Scratch standards should not be used to predict damage

threshold. Proc. Conf. Laser Induced Damage in Optical Materials, 1982.

Nat. Bur. Stand. (U.S.) Spec. Publ. 669; 1982. pp. 151-155.

5. J. M. Elson, H. E. Bennett, and J. M. Bennett. Scattering from optical

surfaces, chap. 7 of Applied Optics and Optical Engineering, Vol. VII,

R. R. Shannon and J. C. Wyant, eds., New York: Academic; 1979.

6. M. Young. Objective measurement and characterization of scratch standards, Proc. SPIE 362:94-100; 1983.

7. M. Young. Can you describe optical surface quality with one or two num- bers? Proc. SPIE 406, pp. 12-22; 1983. See also transcript of workshop

on optical specifications, pp. 119-131.

8. M. Young, E. G. Johnson, R. Goldgraben. Tunable scratch standards.

Proc. SPIE 525; 1985.

9. M. Young. Optics and lasers, including fibers and integrated optics, 2nd

revised edition. New York: Springer; 1984.

10. E. G. Johnson, Jr. Simulating the scratch standards for optical sur- faces: Theory. Appl . Opt. 22:4056-4068; 1983.

20
NBS-114A IREV. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions,)

1. PUBLICATION OR

REPORT NO.

NBS TN-1080

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

February 1985

4. TITLE AND SUBTITLE

Redefining the Scratch Standards

5. AUTHOR(S)

Matt Young and Eric G. Johnson, Jr,

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

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

U.S. Army ARRADCOM

Fire Control Division

Dover, NJ 07801

10. SUPPLEMENTARY NOTES

J

Document describes a computer program; SF-185, FIPS 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 scratch standard (MIL-0-13830A) is a cosmetic standard that is effected

by a visual comparison with a set of submasters that are in turn evaluated by

comparison with a set of master standards. Both manufacture and certification of

the submasters are somewhat unreliable. In this paper, we show that the submasters

can be classified according to the relative power scattered at a relatively small

angle. We have designed etched gratings with which to replace the submasters;

these gratings have the appearance of scratches but diffract a broad peak between

5 and 10 degrees off the axis of the incident beam. We have classified some prototypes both by comparison with the master standards and by a photoelectric measurement; agreement between the two methods is good. We suggest that such gratings be

used as the submasters and possibly that they be classified by a photoelectric

rather than visual measurement.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

cosmetic standard; diffraction; MIL standard; optical quality; optical-surface

quality; scattering; scratch and dig standard; standards; surface quality;

surfaces

13. AVAILABILITY

£X] Unlimited

| | For Official Distribution, Do Not Release to NTIS

)£><[] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

^] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

26

15. Price

•&U.S. GOVERNMENT PRINTING OFFICE: 1985-578 170/25151 USCOMM-DC 6043-P80



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