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The Equivalency of Vision Acuity Exams for NDT Personnel

by Stan Weatherly*

 

Introduction

A recent study found that the mean far and near vision acuity of a sample of 150 NDT inspectors was at least 20/20 (Good and Nakagawara, 2005). This statement raised a question — which vision test method did the examiners use to make this determination? In this case, the vision testing was consistent, using the same vision test for both sample groups. Although this study was addressing guidelines specifically for visual testing, it did bring to light again the inconsistencies in current NDT specifications.

Many NDT documents continue to specify that near vision should be tested using the Jaeger eye card's J1 line "or its equivalent." While the research group in the above study wisely chose one set method to test the near vision acuity of the entire sample group, inconsistencies are created in vision tests of NDT personnel through the use of the word equivalent. Although the word was placed in NDT specifications to provide a variety of options for testing vision acuity, the word has prevented the NDT industry from having a consistent standard against which to determine an inspector's vision acuity. By using the word equivalent we have allowed different criteria from one test facility to the next by permitting variation in the eye exam method. The question is whether these different ways of testing vision acuity are truly equivalent.

Where We Currently Are

Even more recent nondestructive testing specifications (NAS 410 and ISO 9712, for example) assume that a variety of different methods and charts for determining vision acuity are equivalent. It does not require much research to see some obvious differences in the vision acuity charts and automated vision scopes used by medical facilities that provide eye exams for inspector certification.

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The question is whether these different ways
of testing vision acuity are truly equivalent.

Take, for example, the two lines of text shown in Figure 1: which line of text would you prefer to be tested on? Presumably, you chose the row of upper-case letters: the smaller size of lower-case letters makes them more difficult to read. This illustrates a clear difference between the Jaeger eye card and other acuity exam tools (such as the Snellen and LogMAR charts and the US Federal Aviation Administration [FAA] Near Vision Acuity Test Chart). For those of you who have not seen a Jaeger eye card, the text is in both upper- and lower-case letters, whereas the others use exclusively upper-case letters.

Figure 1 — While these two rows of letters are in the same font and at the same point size, their difference in case makes a considerable difference in legibility: (a) upper-case text; (b) lower-case text.

The next thing anyone reviewing these different eye charts would notice is the spacing between the letters. Look at the two examples in Figure 2. It is clear that the spacing between letters affects the ease with which one can read an eye chart. The Jaeger eye card has normal text spacing. The Snellen eye chart has a spacing of one capital letter between letters and the same spacing between all rows. The LogMAR eye chart has deliberately equalized spacing between letters, rows and a consistent number of characters per line. This does make the LogMAR chart less difficult to read when compared to the Jaeger card. LogMAR based vision exams are widely accepted as the "gold standard" for accurate and efficient measurement of vision acuity (Stewart, 2004).

Figure 2 — Differences in the spacing between letters also affects legibility: (a) widely spaced text; (b) compressed text.

The third area that can affect the ease with which one can read a given vision acuity exam card is the actual difference in the letters to be evaluated, for example the difference between c and o. On the Jaeger eye card, the text height of the J3 line is 1 mm (0.04 in.) for the lower-case letters. The gap in the c is approximately 0.4 mm (0.015 in.). This is the area you are attempting to distinguish. The lower-case text on the Jaeger J2 line is about 0.75 mm (0.03 in.) tall. A lower-case c has a gap of approximately 0.25 mm (0.01 in.). To distinguish the letter, an inspector must be able to determine if the letter is an e, o or c.

Finally, the direction of a gap in a letter such as C or E in a scope may be the determining factor for certifying acceptable vision. It should be noted that when using the tumbling/falling C or E charts, it is really the gap that is being read, not the letter. See the example in Figure 3.

Figure 3 — At times, it is not the letter that needs to be distinguished, but the gap within it, as in the difference between a lower-case c and an o: (a) the gap that distinguishes a c from an o; (b) a "tumbling" c chart.

So there are some obvious differences between these methods of gaging vision acuity. Most of these charts are based on different distances, from 6 m (20 ft) down to 305 mm (12 in.), and yet through interpretation they are supposed to be equivalent. This is mainly because specifications — not necessarily the manufacturers — imply that they are equivalent. However, the only real equivalency between them is that they are all vision measurement devices. Some are for distance vision and some are for near vision. Most of them have printed on them some form of a number based on the Snellen eye chart set at 6 m (20 ft) and then reduced from there. However, the distance the chart is to be evaluated at varies, the font varies and the text height varies.

Correcting the Problem

The problem, then, is that different charts and scopes use different fonts, that the text size varies with the required reading distance and that spacing varies between letters. It would be unreasonable to state that these charts should be treated as being equivalent. They are similar, but, without going into detail as to what each chart is for and how it is to be used, they should not be deemed equivalent.

The FAA has addressed the issue of multiple charts and issued their own near vision acuity chart (FAA Form 8500-1), but, once again, this has only added one more near vision chart to the roster.

Possible Solutions

There are possible solutions to this issue. The following solutions are proposed solely to standardize specification requirements for the purpose of having a uniform standard for verifying the vision acuity of nondestructive testing personnel.

As an industry, it would make sense to establish one set value for vision acuity. If an inspector is expected to be able to spot a 0.25 mm (0.01 in.) indication without the assistance of vision aids other than corrective lenses, then that should be set as the standard.

Any standard would require a set font and text size. For example, the readable text could be set at 0.75 mm (0.03 in.) in height, the font could be Arial and it could be deemed necessary to distinguish a 0.25 mm (0.01 in.) gap in a c. Distinguishing between c and o on a single line could determine if the actual vision acuity is met (for example, read the following line: o o c o c o o c).

Alternatively, the word equivalent could be dropped and a particular one of the near vision eye cards adopted as the standard. Any one of the charts and cards mentioned in this article can easily be obtained very inexpensively.

Standardization of the examination to determine vision acuity of NDT inspectors is the first step toward establishing a basis for all inspectors to be able to properly detect and interpret indications. Great strides have been made toward standardizing the requirements for qualification and certification of NDT personnel. Vision requirements are yet to be standardized. Through increased knowledge and the assistance of personnel in the optometry and ophthalomology fields, more finite standards can be established for testing the vision acuity of NDT inspectors.

References

Good, Gregory W. and Van B. Nagakarwara, "Vision Standards and Testing Requirements for Nondestructive Inspection (NDI) and Testing (NDT) Personnel and Visual Inspectors," Final Report for William K. Krebs, 15 August 2005, available at www.tc.faa.gov/logistics/grants/pdf/2002/02-G-031.pdf.

Stewart, Catherine Elizabeth, "LogMAR vs. Snellen Visual Acuity Measurements: Improved Accuracy for the Detection of Subnormal Vision and Change?," Eye News, Vol. 11, No. 2, August/September 2004, available at www.pinpointmedical.com/eye_news/article_archive/misc/AS04_logmar.pdf.

The Boeing Company, 2401 E. Wardlow Road, MC C054-0023, Long Beach, CA 90807; (562) 982-7073; fax (562) 593-9581; e-mail stanley.l.weatherly@boeing.com.

Copyright © 2007 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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