What is Visual Acuity and Why Do NDT Technicians Need It?

By: Publisher Team | Jan 29, 2020 12:43 PM
What is Visual Acuity and Why Do NDT Technicians Need It?

Introduction: What is Visual Acuity?

Visual acuity is defined as the sharpness of vision, measured by the ability to discern letters or numbers at a given distance. You have probably performed a visual acuity test with an eye chart at the doctor's office.  20/20 vision is the baseline standard for visual acuity. If you have 20/20 vision, then you can process a normal amount of visual information at a distance of 20 feet. If you have 20/80 vision, then your vision at 20 feet is equal to a person standing 80 feet away with standard vision. Many people do not meet the 20/20 vision standard, so corrective lenses such as glasses or contacts are readily available.

How does vision work? Light enters the eyes through the pupil. It passes through the lens and is projected on the retina at the back of the eye. The retina is made up of photoreceptors called rods and cones. Rods are primarily used in dark conditions, because they are sensitive to blue and green light with a maximum sensitivity of 498 nm wavelength. Cones give us our basic color vision in the daylight, and they are comprised of L-cones, M-cones, and S-cones.

  • L-Cones: Detect red light with a peak sensitivity of 564 nm
  • M-cones: Detect green light with a peak sensitivity of 533 nm
  • S-cones: Detect blue light with a peak sensitivity of 437 nm

NDT Visual Acuity Requirements

According to ASNT, visual certification for an NDT technician requires visual acuity in at least one eye. Visual acuity is tested by a person properly reading the Jaeger J1 eye test chart, or equivalent, at a distance of 30.5 cm or 12 in. Additionally, an NDT technician must be able to differentiate between the colors used in NDT methods. Eyes tests can be performed by a medical practitioner or a person that is certified to give the test.

Why is Visual Acuity Important for NDT Technicians

Whether it's ultrasonic testing, eddy current testing, or liquid penetrant testing, all NDT methods involve some form of visual examination. For instance, radiographic testing uses X-rays to generate an image on film, but that film must be carefully examined by the technician in order to discover flaws. Additionally, most nondestructive testing starts with the naked eye through visual inspection. Flaws rarely stand out like a waving red flag; scrutiny of a component is required. That's why NDT technicians must be trained and certified. Otherwise, anyone could perform this work.


Without quality vision, defects can be overlooked, and this defeats the entire purpose of nondestructive testing. If the eyes are deficient, corrective lenses can be used, so there is no excuse for anything less than 20/20 vision in the workplace. You wouldn't run equipment that's only performing at 60% effectiveness, so why would be it acceptable for a technician to perform his job with less than 20/20 standard vision? Yes, the eyes are just another type of equipment that a technician uses in order to perform quality inspections.


Visual Acuity of the Human Eye. (n.d.). Retrieved January 28, 2020, from https://www.nde-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/visualacuity.htm

ASNT Visual Certification Requirements. (n.d.). Retrieved January 28, 2020, from https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=2ahUKEwiExsS86abnAhWNCuwKHbgTA0UQFjABegQIChAF&url=https://www.asnt.org/~/media/Files/Certification/ACCP/Level-II/visualacuityform.ashx&usg=AOvVaw3a_mE7fgaWE1uQPho0HBDv

Blog Source: https://www.magnumndt.com/what-is-visual-acuity-and-why-do-ndt-technicians-need-it-blog