NBSIR 76-1142

Considerations and Standards

for Visual Inspection

Techniques

Gary T. Yonemura

Sensory Environment Section

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

October 1976

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
Yonemura, G. T., Considerations and standards

for visual inspection techniques, (Proc.

Symp. on Nondestructive Testing

Standards, Gaithersburg, MD May 19-21,

1976), Am. Soc . Test. Mater. Spec. Tech.

Publ. 624. Nondestructive Testing Standards - A Review, H. Berger, Ed., pp. 220-250

(June 1977).

465 7

0U-hHX. 1
NBSIR 76-1142

CONSIDERATIONS AND STANDARDS

FOR VISUAL INSPECTION

TECHNIQUES

Gary T. Yonemura

Sensory Environment Section

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

October 1976

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
"• *
Considerations and Standards for Visual Inspection Techniques

Gary T. Yonemura

ABSTRACT: When we look at the capacity of the human visual system we

see that man can adjust to a wide variety of operating conditions. But,

unless we have detailed information on the conditions for which these

processes are to be standardized and quantitative descriptions of the

tasks to be performed, the advantages to be obtained by visual science

applications cannot be optimally utilized. The Modulation Transfer

Function would be an image evaluation technique applicable to NDT.

Standardized tests to assess day to day performance as well as initial

capacities should be developed. These tests should be derived from

visual capacities correlated with the tasks to be performed.

KEY WORDS: Nondestructive inspection, vision tests, visual standards,

visual performance.

In assessing the acceptability of mensurative techniques two

basic uncertainty (reliability) measures are involved: 1. The

repeatability of measurements with a given instrument, and 2. The

agreement between different instruments or installations. Similar

performance assessments leading to consistent performance should be

required of visual techniques in nondestructive testing. One

indication of this need may be the results of the Air Force round

robin involving eleven installations as reported by Gulley [1]. The

percentage of defects detected ran from a high of 93 to a low of

19 percent.

The performance of human observers in nondestructive testing

(involving visual inspection) can be separated into two broad

categories. The first involves detecting the inhomogeneity that may

or may not be a defect. The second involves the interpretation of the

inhomogeneity as being a fault or some artifact. To a large extent

detection can be said to be dependent upon the physiological attributes

of the observer and decision making, or interpretation, can be said

to be for the most part a function of the cognition and experience

of the observer. This dichotomy between physiological and cognitive

attributes is not clear cut, as there are contributions of both to

detection and interpretation.

In this presentation we will be discussing only the first problem:

detection. Furthermore, we will be primarily interested in the problem

of consistency of detection for the more difficult tasks. The

fundamental measure is the probability of detection; detecting an inhomogeneity that may or may not be a defect. I would like to

digress for a moment to describe the basic stimulus configuration

used in most of the experiments whose results will be used to

1
illustrate visual phenomena of interst to visual nondestructive

inspection (NDI) . In Fig. 1, (a) is the target to be detected seen against a background (b) , (c) being the area surrounding the task.

AL is the absolute value of the difference between the luminance of

the target and its background. Contrast is defined as AL divided by

the background luminance. In all of the experimental data shown,

no attempt will be made to precisely describe the stimulus parameters.

The purpose of these data is only to indicate the shape of the

function.

As in any instrumental measurement technique our first concern

is the consistency or repeatability of measurements with a given

instrument. This concern should also hold for the human eye. Will

the same inspector be able to detect targets of the same difficulty

equally often at different times? Fig. 2 shows the results from a highly experienced observer obtained on two different days. The

stimulus parameters were the same, the only observable difference

being that the two curves were obtained 24 hours apart. Note that on day one this subject detected the target 80 percent of the time when

there was a -0.18 log unit difference in the luminance between the

target and background. On day two a -0.18 log unit difference was detected only 35 percent of the time. Also note that the variability

in the response of this observer is about the same for day one and

two, as indicated by the similarity in the slope of the two ogives.

It has been the speaker’s experience that data taken 1 or 2 hours apart

are very similar, and do not manifest the changes that may occur over

a 24 hour period. This type of performance is typically obtained from

experienced observers in visual psychophysical experiments. An

inexperienced observer will display a larger separation between the

ogives, the separation decreasing with increases in experience. We see

that even experienced observers display variability in visual capacities

that may vary from day to day. This indicates the need for a test that

the inspector can use to calibrate himself. "Calibrate," in the sense

that he can determine whether he is performing at a prescribed performance

level or better at that time. That is, he will not be ready to perform

critical visual inspections, unless he can detect a target of

predetermined size, contrast, luminance and blur. We will later discuss

some variables that influence detection capacity and by following

prescribed procedures, the inspector may be able to bring his

performance to the required level. Of course, the above assumes the

inspector has displayed a minimum visual sensory capacity as tested

in NDI by physical examinations involving acuity tests.

No matter how consistent an inspector may be in repeating his

performance day after day, unless his performance meets some minimum

specified performance level his performance is unacceptable. An

analogy in instrumental measurement arises in round robins conducted

to assess consistency of measurements between laboratories and/or

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

repeatability time after time, but its measurements may be inconsistent

2
with those from other laboratories. This second source of

inconsistency in visual inspection techniques, is the variability

between observers. In Fig. 3 we see the results for two different

subjects obtained under identical conditions. One observer detects a -0.08 log unit difference about 70 percent of the time, whereas another

subject can only detect this same difference 40 percent of the time.

As stated earlier an appreciation of the need for standardizing this

performance is indicated in NDI by the physical examinations involving

acuity tests. An important question is: are we using the correct

physical correlate to assess this performance?

Within and between observers inconsistency can be minimized by

using more observers. Fig. 4 shows the results from an experiment

where the performance of the group as a whole is presented. That is to

say, at least one member of the group detected the target. For a single observer the probability of detection for a luminance level of

1.7 log luminance units is about 10 percent. When we double the number

of observers, where now the probability of detection is based on at

least one of two observers detecting the target, the percent detected

increases to about 30 percent. With five observers the percent

detected increases to 65 percent. We can bring percent detected to the

95 percent level by using ten observers. There is another important

item of information that this set of curves tells us. As the number of

observers is increased, the slope of the ogives becomes steeper.

Since the slope of the ogive is a measure of the standard deviation or the variability of the data, we can also state that as the number of

observers is increased, the consistency of the data increases.

The writer fully realizes the impracticality of using ten observers to

inspect the same sample. The data above were presented to give some indication of how consistency can be improved.

We saw that in standardizing the visual performance of

nondestructive inspectors, we must consider within observers, between

observers and between groups of observers' inconsistencies. The aim

is to obtain as consistent a performance as possible using the human

observer as the detector. The human visual system is a highly adaptive

one. In Fig. 5 we see that the eye can, under the most optimum conditions,

see a spot of light having a luminance less than 10

-

^ cd/m^^ The^upper

limit of visual tolerance or the pain threshold is about 10 cd/in . The

low to high range covers ten orders of magnitude or a ratio of 10 billion

to 1. This large sensitivity range of the eye should not be construed

as indicating that the eye is not sensitive to small changes. The eye

can detect luminance changes as small as 1 percent.

What are some of the physical variables that may lead to

inconsistencies in responding? I would like to state here that the

variables to be discussed do not necessarily apply equally to the

different nondestructive testing methods. A variable may be important

for one technique but have little effect on another. For example,

dark adaptation may be an important variable in x-radiography , but

3
may be of less importance to liquid penetrant techniques. Fig. 6 gives

the luminance level required to detect a spot of light against a' dark

background as a function of dark adaptation or time in the dark.

The parameter is light adaptation or the luminance level to which the

eye was adapted for 5 s previous to being dark adapted. It is obvious

that after 5 min in the dark, the sensitivity of the eye is still

differentially affected by the luminance level of the preadapting

light. Even after 10 min the luminance level required to see the spot

of light after preadapting to a photoflash is significantly greater

than that required for the other light levels. For critical or more

difficult tasks, the eye must be adapted for a longer period of time.

For example, if the inspector just stepped in from the outdoors on a sunny day, he would be significantly less sensitive to the inspection

task as opposed to having been in a dimly illuminated waiting room,

before performing the inspection.

We obtain similar results by varying light adaptation duration

rather than luminance levels of the adapting light. Fig. 7 is similar

to Fig. 6, but in this case the adapting-light luminance was kept

constant at 1060 cd/m^ and the light adaptation period varied. We

see that even after 10 min of dark adaptation, the luminance required

to see a spot of light is significantly affected by the length of time

the observer was light adapted prior to dark adaptation. For critical

tasks even leaving the radiography room for 10 min to go to the rest room may significantly affect the ability to detect a hairline crack

since the dark adapted state is quickly unadapted when the eye is

exposed to light. The purpose of presenting these graphs is not only

to describe the phenomena, but to indicate that in many cases we may

have the relevant quantitative data, the remaining need being to

determine the stimulus levels encountered in nondestructive testing.

Fig. 8 indicates that when working at threshold contrast levels,

target detection can be improved by increasing the luminance level.

For example, a target with log contrast of -0.5 cannot be detected at

-1.0 log cd/m^ but will be detected by increasing luminance level to

1.0 log cd/m^. These numbers apply only for a target of specific

size, in this case one subtending 40 min of arc, but the general form

of the function applies to targets of all sizes. A word of caution:

this sort of function applies to targets that can barely be detected.

For targets with high contrast, such that they are easily observable,

luminances above an optimum level may decrease the perceived contrast

or the "goodness" of the target.

Fig. 9 shows the obvious: as the size of the target is increased,

the luminance required to see a spot of light decreases. For NDI we will probably be more interested in the ability to detect targets

with different contrast levels. Fig. 10 indicates that as target

diameter decreases, contrast must be increased in order to detect the

target. Note that in ND testing we will be dealing with the smaller

sized targets, where there appears to be a linear relationship between

angular subtense and log contrast.

4
These will be some of the variables that must be considered in

standardizing the performance of ND inspectors, or at the least to

optimize consistency of performance for a given inspector, performance

between different inspectors and performance between groups of

inspectors

.

Several difficulties arise when we attempt to apply the data on the

capacity of the human visual system to NDI techniques. When we looked

at some examples of the capacity of the visual system we saw that it

had a large responding range depending on the circumstances under

which it was used. In fact, the data in any sensory field are data

that describe how a given capacity is dependent on any one of a large

number of variables. Any discussion on standardizing the sensory

capacity of the human eye must be based on the circumstances under

which this capacity is to be utilized. We must know that the eye is

expected to see and the conditions under which the discriminations are

to be made. We know considerably less of the demands made on the

visual system by nondestructive inspection than we do of the limitations

of the human eye. This deficiency is a serious one. We need

quantitative measures describing the physical correlates of what the

eye is expected to detect. For example: in radiography dimensional

descriptions of the defect measured on the material has limited value

in visual standards for NDI. The eye is asked to look at the radiograph,

consequently the physical measure of interest is the defect as displayed

on the film, regardless of how much it may differ from the actual defect.

What is required are microdensitometric scanning measures of the defect

taken directly from the radiograph. An analogous argument applies to

liquid penetrants and magnetic particle inspections. Microphotometric

scanning measures of the fluorescent indications will provide the

necessary physical correlates required to completely describe the

fluorescent indications as seen by the eye. Only then can we determine

the capacity demanded of the eye and formulation meaningful standards

leading to a more consistent defect detection probability within and

between observers as well as between installations.

There does not seem to be much doubt that the primary visual

parameters correlated with ND visual inspection tasks are contrast,

size, luminance and blur. The question that remains is the magnitude

of these parameters in NDI as discussed earlier. Time in almost all

instances can be treated as infinite, as far as task description is

concerned. The variables listed above are systematically treated by

the concept of Modulation Transfer Function (MTF) . We recommend that

this concept be used in formulating NDI standards. An advantage is

that the technique is currently being utilized in iptical evaluations,

and many of the techniques developed can be transferred directly to NDI.

MTF is already being used in medical radiography and is also being

utilized in NDT for image enhancement techniques. The net effect of

two variables can be treated as the product of the separate effects of

the two variables on modulation, that is, contrast. In the writer's

opinion, the ability of MTF to handle blur, an area which has been

5
neglected in most applied visual problems, is in itself sufficient

reason for using MTF.

In summarizing, we wish to suggest that the primary need is to

collect quantitative data describing the stimuli that the eye has to

detect. These data should preferably be in the form of microscanning

which can be translated into MTF. The MTF of critical faults can serve as the minimum acceptable limits of detection capacity required

from an observer and/or installation. These critical capacity

requirements will form the basis from which standardized tests should

be developed.

6
References

[1] Gulley, L. R. , Proceedings , Interdisciplinary Workshop for

Quantitative Flaw Definition, Air Force Materials Laboratory,

AFML-TR-74-238 , Nov. 1974, pp. 5-81.

[2] Mueller, C. G. , J. Gen. Physiol., Vol. 34, 1951, pp . 463-474.

[3] Riggs, L. A., in Vision and Visual Perception , C. H. Graham,

Ed. Wiley, New York, 1965, Chapter 2, pp. 1-38.

[4] Wald, George and Clark, A. B., J. Gen. Physiol., Vol. 21, 1937,

pp. 93-105.

[5] Graham, C. H., Brown, R. H. and Mote, F. A., J . Exp . Psychol

, Vol. 24, 1939, pp. 555-573.

[6] Graham, C. H. and Bartlett, N. R. , J. Exp. Psychol. , Vol. 27,

1940, pp. 149-159.

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J.s. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR R El PORT NO.

NBSIR 76-1142

2. Gov’t Accession

No.

4. TITLE AND SUBTITLE

Considerations and Standards for Visual Inspection

Techniques

7. AUTHOR(S)

Dr. Gary T. Yonemura

3. Recipient’s Accession No.

5. Publication Date October 1976

6. Performing Organization Code

8 . Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

NBS Nondestructive Evaluation Program

Institute for Materials Research

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. supplementary not ES Presented at Symposium on Nondestructive Testing Standards

sponsored by NBS, ASTM and ASNT. To be published by ASTM.

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

bibliography or literature survey, mention it here.)

When we look at the capacity of the human visual system we see that it has many

capacities depending on the circumstances under which it is used. The sensory data

show that man can adjust to a wide variety of operating conditions. But, unless we

have detailed information of the conditions for which these processes are to be

optimized and quantitative descriptions of the tasks to be performed, the advantages

to be obtained by visual science applications cannot be optimally utilized. Physical

correlates of the response should be quantified, followed by a systematic scaling

of the physical correlates for application to nondestructive testing. The Modulation

Transfer Function widely used in optical imaging assessments would be an evaluation

technique applicable to hardware, processing and image description.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Nondestructive testing; modulation transfer function; vision; visual acuity;

visual capacities

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