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Fluorescent Penetrant Testing and
Color Vision Deficiencies
by William H.
Bailey*
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Some time ago, I took a vision test for color blindness. I
found that I could not see several of the numbers in the little colored circles.
I have been concerned about this "defect" in my vision until this article by
Bailey explained it. Maybe there is something here for you too.
Frank Iddings
Tutorial Projects Editor
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Figure 1-3
Introduction
Inspectors of various materials and in different
industries have one thing in common. They must have the ability to see
adequately in order to conduct the proper testing of products, items or systems
being fabricated or installed. Based on many inherent factors, the inspector may
be requested or required to pass a normal or, in some cases, a special vision
test. Individuals working in the field of nondestructive testing (NDT) are
generally required to be certified to the quality of their vision capabilities
by specifications used in their specific area of employment.
Vision Testing
Vision requirements for NDT inspectors is a topic given
little attention over the years. It is usually assumed that if the inspector is
able to pass the normal vision test given by the average optometrist, then the
inspector would also be qualified to do fluorescent penetrant testing. Most
optometrists know very little about NDT requirements and do not attempt to
assist companies in providing vision tests sufficient to meet specifications.
For example, most are not familiar with the use of ultraviolet light for testing
parts or the limitations of color deficient personnel who may be using the
equipment. The test usually covers vision acuity, where the inspector must read
a specific print size at an acceptable distance, usually 305 to 381 mm (12 to 15
in.). The color vision portion of the examination normally consists of reading
at least 14 of 36 pseudo-isochromatic plates. The number of missed plates, 22 of
36, would be 61.1% of the total. This percentage, when considered as 22 missed
indications, cannot be considered acceptable when testing materials that may be
used in the manufacture of critical parts for aerospace or aircraft, pressure
vessels or vehicles.
Stringent color vision testing is necessary when safety is
a major concern.
In today's field of electronics, color coding is
essential. In the fabrication of highly stressed pressure vessels, the piping
and various assemblies are connected to each other and are usually assigned
particular color codes for what they convey and where they are going. Inspectors
with only a 38.9% acceptable color vision rating (having read 14 of 36 correct
plates) should not be considered acceptable. Stringent color vision testing is
necessary when safety is a major concern. In many cases, tests are carried out
at different stages of processing or fabrication in order to ensure that the
final product is safe for use.
Background
In 1966, a report was issued that described how the Navy
was attempting to design a method of testing personnel and placing them for
specific jobs (Paulson, 1966). Those who took the color vision tests and would
normally have been rejected by the pseudo-isochromatic plates could still be
deemed acceptable for some jobs. This routine testing was designed to pass those
with normal vision and fail all those who were color deficient. Ten percent of
the applicants failed. With the new tests described in the report, 30% of the
color deficient applicants were found to be acceptable for specific job
placements. These tests had been used since 1955.
The tests and equipment were developed for Navy use by
Commander Dean Farnsworth while he was stationed at the Naval Laboratory.
Subsequently, based on this work and the development of the "Farnsworth-Munsell
l00-Hue and Dichotomous Tests for Color Vision" (Farnsworth, 1943) and Deane B.
Judd's work (1943 and 1945), the Farnsworth Dichotomous Test for Color
Blindness, Panel D-15 (Farnsworth, 1947) was created. This test was designed to
distinguish the differences in visual ability between individuals who are
functionally colorblind or moderately color deficient and those having normal
color vision. Those individuals with color deficiencies can view similar hues
under variable conditions and see different colors. Red and green, blue and
green, blue and pink, yellow and blue, green and brown, and tan and amber are
some of the combinations that get mixed. The test was not designed to separate
the partially colorblind from those with normal vision nor to identify the color
aptitude of the persons with normal vision.
Likewise, selecting people for color matching, grading or
other work requiring special color differentiation would have to use
individualized special testing. The test can quickly separate individuals into
three categories: normal, colorblind and partially color deficient. The color
deficient group may further be subdivided into the color mix that they
misinterpret. Color discrimination begins to descend in desirability when using
northern daylight, daylight blue incandescent lamps, daylight fluorescents and
ordinary incandescent lamps. The testing of individuals with a moderate color
deficiency and the color range of indications all fall within the parameters of
their deficiencies. This alludes to a poor combination of color deficient
inspectors in an area where the colors are the most difficult for them to
identify. Whether the contrast ratio offsets the color deficiency of the
inhibited inspector has not yet been explored, but should not be forgotten.
Physiological Factors
There are two standard eye responses. At luminance levels
above 3 cd/m 2
(0.3 cd/ft2), the eye's spectral response is primarily determined
by the cone shaped receptors of the retina and is called the photopic response.
At luminance levels much lower, such as 30 µcd/m2
(3 µcd/ft2), the rod shaped
receptors respond and they are more sensitive in the blue color spectrum. This
is called scotopic response. The rod receptors are more sensitive to light than
the cone receptors (Ness, 1987). Luminance levels to which the eye responds with
a variable spectral response are called mesopic. They range from approximately
680 to 1746 lm/W. Observations made utilizing the cone receptors require
photopic conditions or the foveal area of the retina, with the rod receptors at
the near threshold illumination level surrounding the center. Scotopic vision is
not used for color recognition while photopic vision is used. The fovea
centralis is an area of tightly packed cone receptors located in the optic
center of the retina. The area is surrounded by both rod and cone receptors with
the ratio of cones to rods decreasing with the increasing distance from the
fovea. Faint indications are seen better when indirectly viewed. This is how a
person in a darkened room uses peripheral vision and can see to a better degree.
The fovea becomes a blind spot when low luminance and maximum sensitivity occur
at approximately ten angular degrees from the fovea center. The fovea itself has
about 1 to 2 degrees in the visual angular field (Figure
1).
The age of the inspector should also be considered. With
age, pupil size is reduced, as is the luminous flux at the retina. This does not
include contrast differences of the background reflectance and the indication
response. In Figures 2 and 3, the electromagnetic spectrum is shown, with the
visual spectrum of colors identified in Figure 2 and the ultraviolet region
shown in Figure 3. The near ultraviolet A region, used as the light source for
penetrant testing, is on the side of the ultraviolet region bordering the
visible spectrum. To a normal observer, the color spectrum appears as a series
of chromatic colors, starting with the shortest wavelength and proceeding
through the longest color wavelength in the visible range of the electromagnetic
radiation spectrum: violet, indigo, blue, green, yellow, orange and red.
Ultraviolet and Visible Spectra
Ultraviolet A is considered long wave and is not
considered harmful to the human eye. Ultraviolet B is considered middle wave and
can cause sunburn as well as be harmful to the eyes. Ultraviolet C is considered
short wave and is used for germicidal purposes and is harmful to the eyes and
body. Daylight viewing is considered to be a range of 540 through 570 nm (2.1 x 10 -5 through 2.24
x 10-5 in.), with the average being 555 nm (2.19
x 10-5
in.). Toward either end of the visual range, brightness diminishes. As the
colors appear to diminish to either side of the average wavelength, the
luminosity function located at 555 nm (2.19 x 10-5 in.) diminishes with the
color.
Colors in the visual range of any wavelength can be made
by mixing lights of the three basic colors (red, green and blue) in specific
proportions. These are the basic colors that account for normal vision being
classified as trichromatic. The normal observer may be classified to
discriminate light and dark, yellow/blue and red/green. An observer having
abnormal trichromatic vision requires a different mixture of the three colors to
see the same color as the normal observer. If the observer had mild trichromatic
abnormalities, his or her description of the created colors would be the same.
If the observer had extreme trichromatic abnormalities, the colors described
would be similar to those of a dichromatic, who would make the same
discriminations, but with one of the three lights at a lesser percentage,
usually red/green.
Testing for Color Vision Problems
There are two main types of trichromatic abnormalities,
called protanomaly and deuteranomaly. With either of these conditions,
inspectors could make color discrimination decisions with difficulty, while a
dichromatic could not. The inspector with protanomaly of all degrees is
deficient at the long wave end of the spectrum due to a loss of luminosity
function. Trichromatic and deutermatic inspectors have a luminosity function
within normal limits. Dichromatic observers require just two color wavelengths
to be mixed to form a wavelength color that may be discriminated. They can make
only two types of visual decisions. One is achromatic (light/dark) and the other
is chromatic (either yellow/blue or red/green, usually the former). Dichromatic
observers are limited in their visual range and they would not pass the color
vision requirements given by an optometrist.
Today, the method of testing individuals for color vision
capabilities relies on the Farnsworth-Munsell 100 Hue Test for the Examination
of Color Discrimination (Farnsworth, 1957). It encompasses all phases of color
vision. The Farnsworth Dichotomous Test for Color Blindness, Panel D-15
(Farnsworth, 1947) covers only a portion. Fifteen different color paper disks of
12.7 mm (0.5 in.) diameter are sunk 2.5 mm (0.1 in.) below the 2.5 mm (0.1 in.)
rim of plastic caps. The backs are numbered sequentially. A box with a terminal
color is fixed at one end of the box. The candidate is given the 15 colored caps
in a random mix and is required to match them in consecutive order to the
terminal color without looking at the numbers on the caps. About 80% of adult
males make fewer than 12 transpositions, most of which are simple two cap
exchanges. These errors are random, seldom reoccur on retests and are not
restricted to one region of the color scheme. The instruction booklet that
accompanies the box of caps has all the information required for a qualified
test of vision status. It can define the area of the colors that may be missed,
therefore limiting the type of test that could be performed. All the noted
testing is performed using white light of about 6500 K (11 200 ºF), average
daylight.
Individuals have variations in their spectral responses,
therefore setting up a single standard to which they would have to comply would
be quite difficult. Any standards being used must be able to encompass all of
the shortcomings of an individual inspector but be strict to the point that any
test will be capable of detecting vision problems that cause harmful conditions
in the use or operation of the article being tested.
Color Discrimination and Penetrant Testing
Fluorescent penetrant testing is carried out in a secluded
area where the ambient room lighting is controlled to less than 21.5 lx (2 ftc)
of white light. Total darkness is never achieved. The sensitivity of
discontinuity detection is controlled by the inspector's limitations, which may
include protective eyewear, reflective clothes, reflected near ultraviolet light
from the part surface, the angular reflections of the discontinuities and their
size and quantity of luminescence based on the type of penetrant used. The
nature of fluorescent penetrant testing raises the question as to whether an
individual with color deficiencies may make more errors when performing a test
of fluorescent indications as viewed under near ultraviolet illumination than
would an inspector with normal vision.
The viewing of fluorescent indications is done using a
mercury vapor lamp and a blue/purple filter. That allows only light at a
wavelength of 365 nm (1.4 ´ 10 -5 in.) to be used. The fluorescent dye that
makes up indications is yellow. There are other dyes that may be used for
special applications, such as red, orange, blue or green. These color groups are
the ones that confuse the moderate color deficient individuals. Fluorescence,
also know as "cold light," may occur in gases, liquids or solids. When the
material is radiated by the ultraviolet lamp, the energy excitement causes a
secondary emission of longer wavelengths to be emitted from the material. In
this case, the material is a liquid that has been applied to an object and
allowed sufficient time to seek out discontinuities that are open to the
surface. Capillary action carries the fluid into the discontinuity. When the
surface residue has been removed, the only fluorescent fluid left is that which
entered the discontinuity. With time, the fluid will again, by capillary action,
creep out to be exposed on the surface. In order to make the fluid easier to
detect, a blotting powder may be applied to the surface (it acts like a sponge
placed on a spill.) This coverage also acts as a contrasting background. The
ultraviolet testing lamp is held at approximately 305 to 381 mm (12 to 15 in.)
from the test surface and has a radiation value of 1200 µw/cm2. The contrast
ratio is much higher than it would be under normal daylight test conditions.
Conclusion
Based on the research performed, nothing was located to
show that any recent work has been done involving the testing of color vision
utilizing fluorescent materials and ultraviolet illumination. Quite by chance,
contact was made with the newly developed Eye Institute of the Cleveland Clinic
Foundation, Cleveland, Ohio. They were not aware of any new developments in this
direction, but they are doing some work in the color vision area. Perhaps they
may get new data regarding the use of color deficient persons in NDT.
REFERENCES
Farnsworth, Dean, "The Farnsworth-Munsell 100-Hue and
Dichotomous Tests for Color Vision," Journal of the Optical Society of
America,
Vol. 33, No. 10, October 1943, pp. 568-578.
Farnsworth, Dean, The Farnsworth Dichotomous Test for
Color Blindness, Panel D-I5, New York, Psychological Corporation, 1947.
Farnsworth, Dean, The Farnsworth-Munsell 100 Hue Test for
the Examination of Color Discrimination, Baltimore, Maryland, Munsell Color
Company, 1957.
Judd, Deane B., "Facts of Color Blindness," Journal of
the Optical Society of America, Vol. 33, No. 6, June 1943, pp. 294-306.
Judd, Deane B., "Standard Response Functions for
Protanopic and Deuteranopic Vision," Journal of the Optical Society of America,
Vol. 35, No. 3, 1945, pp. 199-219.
Ness, Stanley, "White Light/Visible in the I/II Dye
Penetrant and Fluorescent/Non-fluorescent Magnetic Particle NDT Inspection
Processes," Visual Committee Report, American Society for Nondestructive
Testing Fall Conference 1987, Columbus, Ohio, ASNT, pp. 1-8.
Paulson, Helen M., The Performance of the Farnsworth
Lantern at the Submarine Medical Research Laboratory and in the Field from 1955
to 1965, Report Number 466, US Naval Submarine Medical Center, Submarine Base,
Groton, Connecticut, 1966.
* 1162 Dover Center
Road, Westlake, OH 44145-1315; (440) 835-4017; e-mail <wbailey@quack.nacs.net>.
Copyright ©
2003 by the American Society for Nondestructive Testing, Inc. All
rights reserved.
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