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Testing Visual Acuity with the Jaeger Eye Chart
by S. L. Weatherly*
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The basics need to be read or listened to over and over. This is because some of us forget and others of us never knew. This month's author gives us a short, easy to read article on understanding the use of the Jaeger chart for testing visual acuity. This may be a "Play It Again, Sam," scenario, but it is a good "Back to Basics" for those who have forgotten or those who never knew.
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Introduction
All personnel certifying in nondestructive testing (NDT) require an eye examination. Over the years, one of the standard reference charts has been the Jaeger eye chart (Figure 1). However, when purchased, this chart does not come with instructions. The following article explains some of my findings on the usage of the Jaeger eye chart.
Jaegar Eye Chart Specifications
NDT personnel who have had to use multiple specifications have realized, over the years, the degree of differences between specifications when it comes to the certification of personnel for visual acuity. NAS 410 (1996) recommends "Near Vision - Jaeger #1 test chart at not less than 12 inches or equivalent visual examination as determined by medical personnel." SNT-TC-1A recommends "Jaeger Number 2 or equivalent type and size letter at the distance designated on the chart but not less than 12 inches (30.5 cm) on a standard Jaeger test chart" (ASNT, 2001). After discussions with other NDT personnel, I realized that confusion exists with the interpretation of these recommendations. The following questions quickly became apparent.
The indication of the mere presence or absence of a crack is not enough.
Does the Jaeger eye test measure distance vision, near vision or both?
Does the Jaeger eye chart have to be held at exactly 305 mm (12 in.) or is 508 mm (20 in.) - an extended arms length - an acceptable distance?
Why use Jaeger number one or number two?
One would think that with multiple years of experience in NDT, these questions would be easy to answer, but even with research the answers are not all that apparent.
Jaeger and Snellen Eye Chart Differences and Specifications
The Snellen eye chart is the eye chart most of us have seen at the optometrist's office. This is the familiar chart with the big E 20/200 at the top and progressively smaller letters at the bottom of the chart down to 20/20 or better. This is a chart for measuring visual acuity at a distance - in other words, how well you can see things that are far away.The Jaeger eye chart is used for reading up close and for determining your near vision, which is an important issue if you are viewing X-ray film or looking for a crack in a part held in your hands. When reviewing the chart, you will see the notation J1 next to the paragraph with the smallest text and each progressive paragraph of larger text is noted with an increase in the J number. Also centered above each paragraph is a number. The number 15 is centered above the smallest text line noted as J1. This number represents 20/15 vision. J2 has the number 20, for 20/20 vision. As you progress to larger lettered paragraphs, the lettering size increases for lesser visual acuity.
When I took my eye exam, I could read the 20/20 line on the Snellen chart without glasses, but when I was handed the Jaeger eye chart and told to hold it approximately 305 mm (12 in.) from my eyes and read the J1 line, I could not read it. At about 406 mm (16 in.), it became clearer and I could read it. With reading glasses, it was readable at 305 mm (12 in.). This was like trying to read the menu at a restaurant when you keep moving the menu further and further away. For those of you who are younger technicians, if you don't know what I'm talking about, wait until you are about 40 or 50 years old and it will become apparent. The Jaeger eye chart measures near vision, not distance vision.
The second part of the question is, does the chart have to be held at exactly 305 mm (12 in.). Some specifications state that the chart can't be held less than 305 mm (12 in.) away while other specifications state that 356 to 406 mm (14 to 16 in.) is acceptable. This seems to suggest that it is acceptable to hold the chart 356 mm (14 in.) away. What if I have to move it out to arms' length? The standard accepted distance, although not stated on the chart but accepted by the optometry industry, is 305 to 356 mm (12 to 14 in.).
Addressing the third question, Jaeger number one is a smaller text than Jaeger number two and would be required depending on the criticality of the testing to be performed. As mentioned previously, the J1 line is 20/15 vision, which is a visual acuity that is better than the derived average of 20/20. It should be noted that this more restrictive requirement might unduly eliminate good inspectors. Therefore, I would advise caution when reviewing visual acuity requirements or generating new industry specifications.
Conclusion
Specifications that require J1 near vision and 20/30 distance vision almost seem contradictory. However, the inspector and the person performing the eye examination must understand that one part of the statement is asking for near vision acuity and the other part is used to determine distance acuity.As an NDT person, I currently have to apply and use the various specifications to the best of my ability, not as a medically trained person but as one who has researched the issue and has arrived at the best possible understanding of the specification to be applied. Yes, our industry may be better served if we standardized the type of chart to be used and were more specific regarding requirements such as the distance at which to read the chart. However, sometimes standardization leads to more restrictions and increased cost. I think that you can see the point from the discussion above. You may technically pass the test by holding the chart at some greater distance but as a professional inspector, it seems that you would want to provide the best possible test by providing yourself with the best possible vision.
References
ASNT, Recommended Practice No. SNT-TC-1A, The American Society for Nondestructive Testing, 2001.NAS 410, MIL-STD-410E NAS Certification & Qualification of Nondestructive Test Personnel, Aerospace Industries Association of America, Inc., Washington, DC, 1996.
* The Boeing Company, 2401 E. Wardlow Rd., Mail Code C1 054-0023, Long Beach, CA 90807; (562) 982-7073; fax (562) 593-9581;e-mail <stanley.l.weatherly@boeing.com>.
Copyright © 2002 by the American Society for Nondestructive Testing, Inc. All rights reserved.
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