Light is the first and perhaps the last physical energy that we as humans can perceive. Sir Isaac Newton described light as a flight of small particles moving in a straight line to stimulate the human retina. Later James Maxwell developed the theory of electromagnetism. This is one of two theories that describe light as it applies to the science of nondestructive testing. The theory is based on three principles. The first is that luminous bodies emit light in the form of radiant energy. The second is that this radiant energy is propagated in the form of electromagnetic waves. The third is that these electromagnetic waves act on the retina of the human eye.
Maxwell's theory was published in 1873. Twenty seven years later in 1900 Max Planck proposed that a radiating body contained a large number of small oscillators that emitted energy with all the possible frequencies represented. As the temperature of the radiating body increased the emitted wavelength became shorter. As the wavelength became shorter the energy of the most energetic of the particles would be increased. The mathematical formula he used to define his quantum theory was E = hv. It is based on the premise that energy is emitted and absorbed in photons (quanta) and that the energy is each quantum is hv. The "v" term is frequency and the "h" term is the always confusing yet memorable Planck's constant.
There are basically four types of lighting that can be used in nondestructive testing. Incandescent lighting is produced by passing a current through a tungsten filament that is heated to incandescence. As a current is passed through a wire is heats and glows. The halogen lamp is a simple refinement of the tungsten lamp and uses an inert gas with active halogen within the envelope. Luminescent light results from the excitation of a single valence electron and it is more monochromatic in nature than is the incandescent light source. Examples of this type of light are lasers, light emitting diodes (LED) and fluorescent lamps.
Two other light sources are polarized light and coherent light. The vibrations of polarized light have been orientated to show a preference. That preference can either be linear or circular. Linear polarization looks like the common wave form while circular polarization resembles a helix. Polarized light can be produced directly but it is usually produced using a conventional light source and a polarizing filter. These filters are used to control the intensity, color and glare of the light. Many optical techniques use polarized light because of its ability to produce uniform patterns of constructive and destructive interference of the light waves. The characteristics of many products can be evaluated by measuring the interference patterns when polarized light is either transmitted through or reflected from an object. Coherent light such as that produced by a laser is visible light or radiant light with a high degree of phase coherence. This phase coherence requires the waves of light to be of the same frequency and wavelength so they can be aligned. This allows the electrons to pause momentarily in an orbit and gain more energy before they are released.
The electromagnet wavelength spectrum covers the spectrum of 10-16 to 105 m. Visible light is considered to be in the range from 380 10-9 to 770 10-9 m. All forms of electromagnetic radiation travel at the speed of light (186,282 mi/s.) However when light travels through any medium besides a vacuum its velocity is altered but its frequency remains fixed and is independent of the medium. This difference in the time it takes light to travel through different mediums is responsible for many of the principles used in optical instruments.
When light is reflected from a smooth surface the angle of incidence is equal to the angle of reflection. But diffuse reflection is generated when light strikes an uneven surface. The rough surface has many different plane surfaces. Each incident light beam strikes at a different reflecting plane. The difference from a perfect normal reflection forms the basis of many optical techniques.
The primary purpose of lighting for a visual inspection is to provide adequate contrast so that the detection of relevant objects or discontinuities is accomplished with a high degree of success. Contrast detection is the most basic of visual tasks. It is the property of the difference between an object and its background of either luminance or color. This luminance contrast is the difference in reflected light between the discontinuity and its background. As can be expected the probability of detection increases as the relative contrast values increase.
Chromatic contrast is the difference in hue and saturation between an object and its background. Chromatic contrast produces visibilities that are less than 20 percent of the detect ability of luminance contrast. To avoid inspector eye fatigue and to enhance the probability of detection due to size, the luminance ratios of the inspection area should be controlled.
Disability glare reduces visibility and visual performance. Glare is caused by light sources or reflections from light sources in the field of view. Glare can cause actual visual discomfort. To reduce glare it may be necessary to decrease the intensity of the light source or reduce the area of the light. It is also possible to increase the angle between the light source and the field of view as well as decrease reflections by using a light source with a larger area but with lower luminance.
Shadows in the inspection area can also be controlled by the use of horizontal illumination produced by a light source with a large surface area or by reflection from luminant walls. Harsh shadows generally cause eye strain, but some shadow effect can accentuate contrast and aid in the detection of defects. The inspection area lighting can be direct, semi-direct or general diffuse lighting. Direct lighting is where 90 to 100 percent of the illumination is directed downward. Semi-direct lighting directs 60 to 90 percent downward with the rest light upwards. Finally general diffuse lighting directs equal amounts of light upward and downward. Glare is usually minimal with this type of lighting.
As can be noted the importance of choosing the right light for the proper inspection cannot be over emphasized. The next time someone requests a simple visual check on a piece of equipment or one of its many components, remember to grab the right light source for the job.
