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CO NBS TECHNICAL NOTE 1143

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

Visual Acuity Testing

of Radiographic Inspectors

in Nondestructive Inspection

QC

100

U5753

no. 1143

1981

c 2
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities2 — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

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Engineering and Process Technology2 — Building Technology — Fire Research —

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THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington. DC 20234.

2 Some divisions within the center are located at Boulder, CO 80303.
NATIONAL BURBAO

Or tftANDARDS LIBRARY Visual Acuity Testing

^

«

of Radiographic Inspectors

in Nondestructive Inspection

1981

Gary T. Yonemura

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

Prepared for:

Office of Nondestructive Evaluation National Measurement Laboratory

National Bureau of Standards Washington, DC 20234

<**T 0F o0t

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued June 1981
National Bureau of Standards Technical Note 1 143

Nat. Bur. Stand. (U.S.), Tech. Note 1 143, 29pages(June 1981) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1981

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

Price $2.50

(Add 25 percent for other than U.S. mailing)
Visual Acuity Testing of Radiographic Inspectors in

Nondestructive Inspection

Gary T. Yonemura

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

Visual acuity tests for radiographic inspectors

should be correlated with the type of tasks

encountered in real world radiography. The

testing procedures should be capable of

assessing differences in day to day performance

of a given inspector as well as the performance

of one inspector relative to other inspectors.

Single line targets with specific parametric

values for contrast, width, and blur are recommended to provide a means for testing a radiographic inspector for visual acuity.

These targets may be used for periodic tests

by the employing organization or for more

frequent self testing by the inspector.

Statistics from the National Health Survey,

procedures recommended by the NAS-NRC

Committee on Vision and real world radiographs

have been utilized in arriving at recommended

test configurations

.

Key Words: Acuity tests; nondestructive testing; quality testing;

radiograph evaluation; visual inspection; visual testing.

Disclaimer:

Certain trade names and company products are identified in order to

adequately specify the experimental procedure. In no case does such

identification imply recommendation or endorsement by the National

Bureau of Standards, nor does it imply that the products are necessarily the best available for the purpose.

111
Table of Contents

Page

1 . Introduction 1

1.

1

Visual Inspection 1

1.1.1 Detection 1

1.1.2 Interpretation 1

1.1.3 Evaluation 1

1.2 Scope 2

2 . Background 2

2 . 1 Variability 2

2.1.1 Intra (within) Observer Variability 2

2.1.2 Inter (between) Observer Variability 3

2

.

2 National Norm 3

3 . Reference Target Parameters 6

3.1 Size 6

3.1.1 Width of Target 8

3.1.2 Length of Target 8

3.2 Contrast 8

3

.

3 Blur 12

3 . 4 Form 12

3

.

5 Luminance - Density 15

3.6 Figure - Background Brightness Relationship 15

4 . Other Considerations 15

4. 1 Viewing Distance 15

4

.

2 Light Source 15

4.3 Test Room 16

4 . 4 Scoring 16

iv
5 . Recommendations 16

6 . References 22

v

1. Introduction

In assessing the acceptability of mensurative techniques, two basic

uncertainty (reliability) measures are involved: the repeatability of

measurements with a given instrument and the agreement between

different instruments or installations. Similar performance assessments leading to consistent performance should be required of visual

inspections in nondestructive evaluation.

1.

1

Visual Inspection

The lack of precision in radiographic inspections has been discussed

and documented in several studies. -

^ The sources of unreliability

in radiographic inspections may be due to any of several causes,

among them being differences in the components and procedures for

taking of the radiographs, and differences in visual capacity among

the inspectors reading the radiographs. This paper shall be concerned

with the latter. By reliability, we mean the precision of inspections,

that is, agreement between different observers within a unit, between

units within an installation, and between installations within an industry as well as the repeatability (consistency) in the performance

of a given inspector. To achieve this end, procedures, apparatus,

and reference materials should be standardized to minimize sources of

variability.

In visual inspections, the human eye is the key component. Visual

capacity must therefore be calibrated and reference materials necessary

for calibration must be available. The complete evaluation of the

inspector involves the calibration of the inspector's sensory capacity

and the level of experience and knowledge that person brings to the

task.

1.1.1 Detection - The inspector must first detect the possible

presence of a defect, generally indications of a discontinuity in

the radiograph. This resolution of a nonhomogeneity in the radiograph is primarily dependent on the visual sensory capacity of the

inspector, i.e., the psychophysiological capacity of the inspector.

1.1.2 Interpretation - Having detected the presence of a discontinuity, it is necessary to interpret this discontinuity to determine

whether it is a defect and, if so, to classify it as to the type of

defect.

1.1.3 Evaluation - The final inspection involves the determination of

the magnitude and importance of the defect. Should the object be

rejected, repaired, or passed?
1. 2 Scope

All three of the above inspector performances involve visual sensory

discrimination ability, the proportion of sensory involvement being

maximal for detection but decreasing as we go from interpretation to

evaluation. As the role of sensory discrimination decreases, the

role of information processing increases, i.e., the role of

experience and knowledge in interpreting the visual cues become

importance sources of differences in performance between observers.

Inspectors can improve their knowledge by training, but visual

capacity, which includes corrective devices the inspector brings to

the job, is fixed. This report deals with visual acuity tests for

radiographic inspectors that are simple to administer and uses standardized procedures and reference tasks. It does not treat the

cognitive (experience) factors in visual inspection of radiographs,

although a significant source of variability between inspectors may be

the lack of good training criteria.

2.0 Background

2.

1

Variability

Measurement variability may exist within an instrument and/or between

different instruments. Within instrument variability is generally

called precision or repeatability. Between instruments variability is

the agreement between different instruments, generally assessed by

laboratory intercomparisons of a reference material. We have these same sources of variability with visual inspection of radiographs.

2.1.1 Intra (within) Observer Variability -

Consistency or repeatability of measurements is of primary concern in

any mensurative technique, including measures relying on the performance of the human eye. Will the same inspector be able to detect

targets of the same difficulty equally often on different days?

Psychophysical experiments indicate that even experienced observers

display variability in visual capacity that may vary from day to day.

These changes can be due to physiological or psychological factors. In some cases performance at the prescribed level can be achieved by following recommended procedures, for example, spending more time adapting

to the darkened environment of the inspection room.

For critical inspections or for the purposes of minimizing variability,

a test is needed that inspectors can use for self calibration. They

can then determine whether they are performing at a prescribed sensitivity level (or better) at the time that the test is taken.
2.1.2 Inter (between) Observer Variability -

No matter how consistent an inspector may be in performing at the same level day after day, unless the performance meets some specified level,

that performance is unacceptable. An analogy in instrumental measurements is the agreement between different instruments and/or different

laboratories. A given instrument in a given laboratory may give high

repeatability, but the measurements may be inconsistent with those from

other instruments or laboratories.

This second source of inconsistency, the variability between observers,

is commonly observed in visual psychophysical experiments. The

observer must be calibrated so that the performance of one person

approximates that of other observers in the same unit. But it is just

as important that the performance of those in one unit be comparable to

that of inspectors in other units or installations. These and

other physiological considerations have been discussed by Yonemura.

The visual capacity of radiographic inspectors can be evaluated in terms

of an "absolute" score (meeting minimum requirements) or a relative

ranking procedure (ranking the inspector's performance relative to other

inspectors) . The author believes that specifying minimum requirements

based on radiographic inspection needs with consideration of national

norms is the preferred criteria.

2. 2 National Norm

The National Center for Health Statistics conducted a survey of binocular visual acuity during the years 1960-1962. The Center examined

a total of 6,672 individuals, ages 18-79 years. Figures 1 and 2 present the number of examinees who were able to identify letters whose

stroke widths were equal or larger than the angular subtense given in

the abscissa. There appears to be a separation between those over and

under 45 years of age. This separation is more clearly demonstrated

in figure 1, the results for persons with uncorrected vision, but is

also clearly evident in figure 2 for persons with corrected near vision

(14 in). In the National Health Survey, the examinees were not

optimally refracted, but tested with whatever correction they were

using. Since acuity tests for nondestructive inspectors should be

performed with whatever corrections they are using, the results shown

in figure 2 are those most applicable to this study. These data will

be considered in the determination of visual acuity requirements for

radiographic inspectors. Differences between the National Health

Survey methodology and optotype (characters used as test objects for

measuring visual acuity) from those adopted in this study will be

discussed under the pertinent variable headings. In general, the

inclusion of other variables (different levels of contrast and blur)

has a net effect of making the nondestructive inspection acuity

requirements stricter. The National Health Survey used high contrast

details with sharp edges.
CO

VISUAL ANGLE (MINUTES)

Figure 1. Percent of testees identifying letters as a function of

stroke width for uncorrected near vision. The age

groupings are given for each curve. Data from the National

Center for Health Statistics.
100 -

90 -

80 -

70 -

60 -

S 50 -

oo

o

40 ~

30 -

20 ~

10 -

VISUAL ANGLE (MINUTES)

Figure 2. Percent of testees identifying letters as a function of

stroke width for corrected near vision. The age groupings

are given for each curve. Data from the National Center for

Health Statistics. 7
The Committee on Vision, National Academy of Sciences - National

Research Council (NAS-NRC) has recently published "Recommended

Standard Procedures for the Clinical Measurement and Specification of

Visual Acuity". Although their recommendations are for clinical

applications, they will be utilized in our study where applicable,

since the committee report will serve as the basis for nationally

standardized procedures for testing visual acuity.

3. Reference Target Parameters

A valid basis for standardizing the sensory capacity of the human eye

should be based on the circumstances under which this capacity is to be

utilized. We must know what the eye is expected to see and the conditions under which the discriminations are to be made. Quantitative

measures describing the physical correlates of what the eye is expected

to detect should be the basis for designing visual acuity test targets.

In radiographic inspections, dimensional descriptions of the defect

measured on the material have limited value. The individual is asked

to look at the radiograph, consequently the physical measure of interest

for visual acuity testing is the defect as displayed on the film,

regardless of how much it may differ from the actual defect. Microdensitometric scans of the defect taken directly from the radiograph

are required. Table I describes fatigue cracks obtained by microdensitometric scans of radiographs. The radiographs show C-130 wing

samples used in the Lockheed study for the Air Force.^ "Reliability

of Nondestructive Inspection on Aircraft Structure".

? Although they

represent only a fraction of possible defects, they do serve as indicators of the levels of difficulty for the various defect parameters

that may be expected in radiograph inspections. The data in table I will be referred to during the discussions of task variables that must

be considered in standardized tests for visual acuity testing.

3.1 Size

The dimensions of visual targets are generally expressed in angular

terms, more specifically, in minutes of arc. Visual acuity is the

reciprocal of the minimum resolvable line, or separation between two

lines, expressed in minutes of arc. For purposes of this paper,

where we are interested in specifying target dimension, the measure

should be an angular one. For example, in figures 1 and 2, the

frequency of subjects identifying letters with visual angles subtending X minutes of arc are presented, instead of the Snellen or decimal acuities given in the original source.

*The radiographs were supplied by W.H. Sproat of Lockheed-Georgia Co,

and B. Boisvert of the San Antonio Air Force Logistic Center.
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3.1.1 Width of Target - In figure 2, about half of the subjects under

44 years old will be able to identify a letter with a stroke width of

0.75 1 and about 85 percent can identify a stroke width of 1.0'. In

acuity testing a 1.0' stroke width is taken as normal acuity.' We

should compare these values with typical acuity required in inspection

of radiographs.

The visual angles given in table I (column H) were obtained from

microdensitometry scans of defects. The width of the defect was taken

as the distance between those two edges, where the rate of change was

less than 0.005 in density per micrometer, as recommended by Higgins

and Jones. 9 The microdensitometry scans are also presented in figures

3-5. These defects are critical cracks that are expected to be detected.

Since the purpose of this study is to recommend minimum acuity requirements, the more difficult images (subtending smaller visual angles) are

of greater significance. Target no. 102A subtends the smallest visual

angle (0.47') and target no. 101D subtends (0.73'). Although the rest

of the targets are larger than one minute of visual angle, the

blurriness and low contrast of the targets render them more difficult

to detect than similar sized letters used in the Health Statistics

Survey. Unless the validation of the proposed reference acuity tests

indicates otherwise, the 1 minute angular subtense currently being

used appears to be a reasonable value.

3.1.2 Length of Target - In the stimulus used in the National Health

Survey, the height of the letters was 5 times the stroke width of the

letters. Since the stroke width was the critical dimension from which

acuity measures were obtained, the target length varied, but was always

5 times the critical dimension. In the Landolt C, the gap in the C

determines acuity. The gap is always equal to the stroke width of the

C, therefore for the Landolt C, the length of the detail is the same

as the width. In both cases target length is a multiple of the threshold

angular measure of acuity.

Hecht et al, " report that for detecting a single line under high

luminances (6,300 cd/m^) the best estimate of the critical length is

about 1 degree i.e., the length beyond which increases in length do

not improve acuity. For a 40 cm (16 in) viewing distance, a line

length of about 0.7 cm (0.28 in) subtends 1 degree. Therefore, for a single line target, the length should be equal to or greater than 0.7 cm in order to eliminate target length as a variable.

3. 2 Contrast

Luminance (L) contrast is defined as:

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b

" L d)/Lb

' where the subscripts b and d refer to brighter and darker, respectively.

In visual acuity tests, the contrasts of the targets are large, 0.9 or larger. But the contrasts encountered in radiographs are signficantly

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of aircraft wing samples.

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11
less as can be seen in table I, column F. The contrast of a target

importantly affecus the visual acuity.

Graham & Bartlett11 have shown that the diameter of a circular detail

has to be increased as contrast is decreased for the disc to be detected,

(See figure 6.) High contrast targets are the exception rather than the

rule in radiographs, therefore the reference acuity tests should include

this real world variable (contrast) . Although there is no definitive

basis for recommending specific contrast values, the sample radiographs that were measured indicate that contrasts of 0.1, 0.3, and 0.85

may be reasonable estimates. (See table I.) The highest contrast

complies with the NAS-NRC Committee on Vision's recommendation that

the contrast of the test targets should not be allowed to fall below

0.85. The use of lower contrast levels has the effect of setting a stricter criterion for radiographic inspection acuity tests if the size

dimensions are retained.

3.3 Blur

In standard acuity tests the optotypes have sharp edges, whereas in most

real world radiographs, the details have fuzzy or blurred edges.

Figures 3, 4, and 5 indicate the degree of blurriness to be expected in

radiographs. Blur here applies only to the edge density distribution of

the detail measured on the radiograph, and does not include the blur

resulting from the spread function of the eye. As figures 3 and 4

indicate, the rate of density changes can range from a highly blurred

image (sample 111) to defects that have a steep gradient (specimen 101)

.

These variations in the sharpness of the images should be considered in

reference visual acuity tests for radiographic inspections. Following

standard practices in acuity testing, one level of sharpness can be

similar to that of sample 101A i.e., a steep edge gradient. Since real

world radiography involves less sharp defects, a test target with a flatter slope should be included, one approaching specimen 101C. This

specification can be given in the form of discrete empirical values for

the edge gradient. If radiographic image evaluation is expressed in

terms of modulation transfer functions (MTF) , it may be desirable to

express the blur as line spread functions (also as discrete values)

.

3.4 Form

The optotype used in the National Health Survey was letters. The

optotype considered in this paper (and probably the acuity requirement most frequently encountered in radiograph inspections) is thin

lines. The form of the optotype can affect acuity. Figure 7 presents

the results for a Landolt C (open circles) and for a square wave grating

pattern (filled circles). -* For clinical purposes, the Committee on Vision, NAS-NRC recommends that equivalence in acuity scores with the

Landolt C as the reference standard be demonstrated for other optotypes.

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The standard acuity tests considered in this paper are not for

diagnostic purposes, but it is recommended that an equivalency between

single thin lines and Landolt C be established and/or the line targets

be tested with a random selection of inspectors.

3 .

5

Luminance-Density

Figure 7 indicates that visual acuity increases as luminance is

increased. This finding suggests the need to recommend a standard

luminance level to be used in testing. The Committee on Vision, NAS-NRC

recommends a background luminance of 85 + 5 nits (25 +1.5 fL) . With a background transmittance of 2 percent (density = 1.7) the desired 85

cd/m should be within the range of most radiographic viewers.

3.

6

Figure-Background Brightness Relationship

The National Health Survey optotypes were black letters on white background of high contrast. ^ The Committee on Vision, NAS-NRC also assumes

that all optotypes are black figures on a white background. In radiographs, dark lines on a bright background and bright lines on a darker

background are encountered but with the former occurring more frequently.

The above conisderations indicate that the reference acuity standard

should be one with dark lines on a brighter background.

4. Other Considerations

4 .

1

Viewing Distance

In acuity testing of radiographic inspectors we are interested in near

acuity. The exact viewing distance is not critical, but once a viewing

distance is specified, it is important that all tests be conducted at

the specified distance, since the angular subtense of the target is

calculated for a given viewing distance. The National Health Survey

used a viewing distance of 35.6 cm (14 in). The Committee on Vision,

NAS-NRC recommends a near acuity test viewing distance of 40 cm (16 in)

primarily to be compatible (simple multiple) with the 4 m distance

recommended for distance acuity. The author recommends the 40 cm viewing distance in order to minimize differences from standard practice.

4.

2

Light Source

14 The wavelength composition of the light sources can affect acuity.

The primary consideration of the light source is the spectral centroid

(the wavelength for which the eye must be focused) . The spectral

centroids of the commonly used lamps in viewers, incandescent and

fluorescent lamps, when operated at designated voltages, are satisfactory.

15
4.3 Test Room

The only visual variable of significance in the test room is the ambient

light level. We must avoid a high luminance space. Practical

considerations would dictate a space with low ambient light levels.

The luminances as seen by the testee should not be higher than the

background luminance of the test target. The luminance of the surround

affects pupil size and the amount of stray light in the eye. We

recommend that the standard condition be one in which the space is

darkened, with a portable light source for the testing examiner's

use, making certain that it is placed so as to minimize the light being

directed toward the testee.

4.4 Scoring

The performance requirement should be closely related to the performance

of the average human observer. But for tasks in which the visual

capacity is an important component of job performance, the criterion

level may be set higher than that required for tasks where visual

capacity is not as important. This is a policy decision that must be

made by the users of the test.

It is estimated that the acuity test to be recommended in this paper,

calling for a 70 percent performance score, will be slightly stricter

than that for the general population, where the criterion for average

vision is 1 minute of arc for a high contrast target with minimal blur.

It is recommended that whatever value is decided on, a validation test

with radiographic inspectors be performed, since the base level (average

acuity) should reflect the performance of active radiographic inspectors.

5. Recommendations

Based on the preceding discussions, the recommended parameters are summarized in table II. The optotype should be a thin line darker

than the background. The background luminance of the test target is

kept constant at 85 + 5 nits (25 + 1.5 fL) . Three contrast levels are recommended. The highest level, 0.85, is recommended by the

Committee on Vision, NAS-NRC. The transmittance-density values for

obtaining these contrasts are given in table III. The line widths

recommended are 0.75, 1.0, and 1.5 minutes of arc. The length of the

lines is kept constant at 107 minutes of arc. These angular measures

are based on a viewing distance of 40 cm (16 in) . The corresponding

dimensions in inches and centimeters are given in table IV. Two levels

of line sharpness should be considered—one with a sharp edge that

will be similar to most optotypes used in visual acuity testing, and one with a blurred edge conforming to real world radiographs. The blurred

line will be described by discrete empirical values, the one recommended

being that which describes the edge gradient of sample 101C, figure 4.

16
Table II. Visual Acuity Test Parameters

Variable

Figure-Ground

Background Luminance (nit/ftL)

Contrast

Line Width (minutes)

Line Length (minutes)

Viewing Distance (cm/in)

Blur

Line Orientation

Light Source (viewer)

Number of Condi

Conditions

1 Dark on light

1 85 ± 5/25 ± 1.5

3 0.1, 0.3, 0.85

3 0.75, 1.0, 1.5

1 107

1 40/16

2 Sharp, blurred

4 Perpend icular/h

oblique right/left

Incandescent, fluorescent

Total (combinations) 72

17
Table III. Transmittance-Density of Visual Acuity Test Targets

Line

Contrast Transittance Density

0.1 0.018 1.74

0.3 0.014 1.85

0.85 0.003 2.52

Background

Transmit tance Density

0.02 1.70

0.02 1.70

0.02 1.70

18
Table IV. Dimensions of Line Target

Width

Minutes* Inches Centimeter

0.75 0.0035 0.0087

1.0 0.0047 0.0116

1.5 0.0070 0.0175

Length

107 0.5 1.24

*Viewing distance = 40 cm (16 in)

19
Samples of the reference test targets are presented in figure 8. The

relevant parameters of the reference test have been described, except

for line orientation. Line orientation as a parameter serves two important functions: (1) it increases the number of possible responses

(two with no orientation to four with the orientations recommended in

this report), and (2) it includes astigmatic effects. The four

orientations are horizontal (H) , vertical (V), oblique-right (R) , and

oblique-left (L) . Contribution by guessing can be decreased by

dividing the targets into four quadrants with the subject being required

to denote which quadrant the line is in, in addition to the orientation

of the line. This additional precaution should not be necessary, but

serves the purpose of decreasing the probable contribution by guessing

from 1/4 to (1/4) (1/4) = 1/16.

These targets will be used for self testing (intra-inspector assessment) as well as testing by designated examiners (inter-inspector

differences) . The front of the slide (see figure 8) will be blank

with all necessary information given on the reverse side. This

procedure assumes that for self testing the examinee will not cheat,

and will look at the side giving the correct response (H, V, R, or L)

only after evaluating the target orientation. When used by professional examiners, they will be looking at the side with the target

information.

These slides are to be used with a standard dimmable radiographic

viewer, masked so that only a 5 cm (2 in) square is exposed. The light source for the viewer would be incandescent or fluorescent lamps.

The test room should be darkened with a portable light source for the

examiner but in no case should the luminance be higher than 85 nits

(25 fL) within the testee's field of view.

The actual performance by experienced radiographic inspectors cannot be

predicted at present. It is recommended that a validation test with

radiographic inspectors be performed, the passing score based on the

performance for the above test, and the actual scoring including

policy considerations. For example, since visual acuity is an important job-related requirement, how severe should we make the

acuity requirements?

20
FRONT BACK

Vertical

Contrast 0.85

Sharp

Rotated 90°

Horizontal

Contrast 0.85

Sharp

Oblique/Left

Contrast QJ.

Blurred

Rotated 90°

Oblique/Right

Contrast 01

Blurred

i

0.5"

T

s

0.85 V

oo

1

in

oo•

s

0.85 H —

09

B

0.1 L

30

/

B

0.1 R

CO

\

Figure 8. Examples of reference acuity tests showing line orientations

and dimensions. See text for complete description.

21
6.0 References

1. Brock, J. F.; Wells, R. G.; Abrams, M. L. Developments and

Validation of an Experimental Radiograph Reading Training

Program. Report TR 74-33, Navy Personnel Research and Development

Confer, San Diego, California, NTIS AD782 332, June, 1974.

2. Megling, R. C; Abrams, M. L. Relative Roles of Experience/

Learning and Visual Factors on Radiographic Inspector Performance.

Research Report SRR 73-22, Naval Personnel and Training Research

Laboratory, San Diego, California, June, 1973.

3. Gulley, L. R. Proceedings, Interdisciplinary Workshop for

Quantitative Flaw Definition, AFML-TR-74-738, Air Force Materials

Laboratory, Nov. 1974, pp. 5-81.

4. Lewis, W. H. ; Sproat, W. H. ; Dodd, B. D.; Hamilton, J. M.

Reliability of Nondestructive Inspections - Final Report, SA-ALC/

MME 76-6-38-1, San Antonio Air Logistics Center, Kelly Air Force

Base, San Antonio, Texas, Dec. 1978.

5. Mueller, C. G. Frequency of Seeing Functions for Intensity

Discrimination at Various Levels of Adapting Intensity. J. Gen.

Physiol. (34): 1951, 463-474.

6. Yonemura, G. T. Considerations and Standards for Visual Inspection

Techniques. In ASTM STP No. 624 Nondestructive Testing Standards

- A Review. H. Berger (ed.) 1977, 220-230.

7. National Center for Health Statistics, Binocular Visual Acuity of

Adults, United States 1960-1962. Public Health Service Publication

No. 1000-Series 11-No. 3, June 1964.

8. Committee on Vision, NAS-NRC. Recommended Standard Procedures

for the Clinical Measurement and Specification of Visual Acuity.

Adv. Ophthal. (41): 1980, 103-148.

9. Higgins, G. C; Jones, L. A. The Nature and Evaluation of the

Sharpness of Photographic Images. J. Soc. Mot. Pic. Telev. Eng

.

(58): 1952, 277-290.

10. Hecht, S.; Ross, S.; Mueller, C. G. The Visibility of Lines and

Squares at High Brightness. J. Opt. Soc. Amer. (37): 1947,

500-507.

11. Graham, C. H. ; Bartlett, N. R. The Relation of Size of Stimulus

and Intensity in the Human Eye: III The Influence of Area on Foveal Intensity Discrimination. J. of Exp. Psychol. (27): 1940,

149-159.

22
12. Perrin, F. H. Methods of Appraising Photographic Systems Part 1. Historical Review. J. Opt. Soc. Amer. (69): 1960, 151-156.

13. Shlaer, S. The Relation Between Visual Acuity and Illumination.

J. Gen. Physiol. (25): 1942, 553-569.

14. Shlaer, S.; Smith, E. L. ; Chase, A. M. Visual Acuity and

Illumination in Different Spectral Regions. J. Gen. Physiol.

(25): 1942, 553-569.

23
NBS-114A (REV. 2-8C) U.S. DEPT. OF COMM. 2. Performing Organ. Report NoJ 3. Publication Date BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS TN 1143 June 1981

4. TITLE AND SUBTITLE

Visual Acuity Testing of Radiographic Inspectors in Nondestructive Inspection

5. AUTHOR(S)

Gary T. Yonemura

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 Final

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

Office of Nondestructive Evaluation

National Measurement Laboratory

National Bureau of Standards

Washington, DC 20234

10. SUPPLEMENTARY NOTES

_J 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)

Visual acuity tests for radiographic inspectors should be correlated with the

type of tasks encountered in real world radiography. The testing procedures

should be capable of assessing differences in day to day performance of a given inspector as well as the performance of one inspector relative to other

inspectors. Single line targets with specific parametric values for contrast,

width, and blur are recommended to provide a means for testing a radiographic

inspector for visual acuity. These targets may be used for periodic tests by

the employing organization or for more frequent self testing by the inspector.

Statistics from the National Health Survey, procedures recommended by the

NAS-NRC Committee on Vision and real world radiographs have been utilized in

arriving at recommended test configurations.

t

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

Acuity tests; nondestructive testing; quality testing; radiograph evaluation;

visual inspection; visual testing

13. AVAILABILITY

Qx) Unlimited Q For Official Distribution. Do Not Release to NTIS

Hvl Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

~3] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

29

15. Price

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