Nova Data Testing Winter Newsletter

Visual inspections are the oldest form of non destructive testing techniques. From pottery to arrowheads each craftsman was expected to produce the best products possible for the consumer. Today manufactures have a variety of methods to inspect their goods. But the eye is still the most important element in any inspector's tool box. Reflected light enters the eye through a tear film in the cornea where the majority of light refraction occurs. This light passes through the iris which controls the amount of light entering the eye. Past the iris is the lens which changes shape in order to focus the light on the retina in the back of the eyeball. The retina is covered with special cells called rod and cones that convert incoming light into nerve impulses. These nerve impulses travel to the brain through the optic nerve where they are decoded. Our sight is an elegant yet simple system.

Visual perception is the comparison of what the eyes see with what the mind sees. Although there is a tremendous amount of information available in any given image only a small percentage is used for detail recognition. For this reason the inspector may not notice all the information available in a particular field of view.

Visual acuity or the ability to discern detail is a product of the concentration of the cones at the center of the retina, also called the fovea centrallis. This acuity decreases rapidly as the image of the object is moved away from the fovea. Visual acuity is the measurement of resolution acuity, recognition acuity and temporal acuity. Resolution acuity measures the eye's ability to distinguish between fine lines or points. This is usually expressed as an angle and eye tests check this acuity using letters to measure the smallest target. Recognition acuity is the ability to differentiate between objects such a the letter "O" and the letter "Q". Temporal acuity is the measure of the response of the eye to changes in contrast over time. Temporal contrast is measured as the length of time and cycle time that a flash of light must last in order to be distinguished.

Stereoscopic vision provides the ability to distinguish the depth between objects which is due to the lateral distance between the two eyes. Visual and optical devices such as binoculars increase stereopsis, while monocular devices like microscopes, borescopes and metallographs remove all depth perception.

Visual acuity of inspectors may be assessed at both near and far distances. The 20/20 Snellen vision test is performed at 20 feet and is a common vision test. Near vision is usually checked using the Jaeger J1 or J2 vision tests at 16 inches. ASNT Recommended Practice No. SNT-TC-1A as well as ASME and other codes and specification require these visual tests for inspectors.

Color vision is detected by the cones concentrated at the center if the retina (fovea centrallis). It is dependent on the quality and quantity of light and the eye being properly adapted. In 1801 Thomas Young proposed only three types of color receptors with which all colors of the visible spectrum are detected. Hermann von Helmholtz expanded the theory fifty years later. Their theory says light is detected in an additive nature and that the three primary colors are responsible for the perception of all colors. Another theory by Ewald Hering proposed four primary colors, red, green, yellow and blue. Today both theories are blended to explain the physical nature of the eye and how the brain manipulates the information. Regardless of the theories most inspectors are required to take a color vision test using pseudoisochromatic plates such as the Ishihara plates. It has been found that approximately ten percent of males have some color deficiency while only one half of one percent of females have any color deficiencies. Our next newsletter will discuss the many types of lighting which is required for proper visual inspections.