Magnetic Particle Inspection
particle inspection can be performed using particles that are
highly visible under white light conditions or particles that
are highly visible under ultraviolet light conditions. When
an inspection is being performed using the visible color contrast
particles, no special lighting is required as long as the area
of inspection is well lit. A light intensity of at least 1000
lux (100 fc) is recommended when visible particles are used,
but a variety of light sources can be used.
When fluorescent particles are used, special ultraviolet light
must be used. Fluorescence is defined as the property of emitting
radiation as a result of and during exposure to radiation. Particles
used in fluorescent magnetic particle inspections are coated with
a material that produces light in the visible spectrum when exposed
to near-ultraviolet light. This "particle glow"
provides high contrast indications on the component anywhere particles
collect. Particles that fluoresce yellow-green are most common
because this color matches the peak sensitivity of the human eye
under dark conditions. However, particles that fluoresce red,
blue, yellow, and green colors are available.
Ultraviolet light or "black
light" is light in the 1,000 to 4,000 Angstroms (100
to 400nm) wavelength range in the electromagnetic spectrum. It
is a very energetic form of light that is invisible to the human
eye. Wavelengths above 4,000A fall into the visible light
spectrum and are seen as the color violet. UV is separated according
to wavelength into three classes: A, B, and C. The shorter the
wavelength, the more energy that is carried in the light and the
more dangerous it is to the human cells.
The desired wavelength range for use in nondestructive
testing is between 3,500 and 3,800A with a peak wavelength
at about 3,650A. This wavelength range is used because it is
in the UV-A range, which is the safest to work with. UV-B will
do an effective job of causing substances to fluoresce, however,
it should not be used because harmful effects such as skin burns and eye damage can occur. This wavelength of radiation is found
in the arc created during the welding process. UV-C (1,000 to
2,800A) is even more dangerous to living cells and is used to kill
bacteria in industrial and medical settings.
The desired wavelength range for use in NDT is obtained
by filtering the ultraviolet light generated by the light bulb.
The output of a UV bulb spans a wide range of wavelengths. The
short wavelengths of 3,120 to 3,340A are produced in low levels.
A peak wavelength of 3650A is produced at a very high intensity.
Wavelengths in the visible violet range (4050A to 4350A), green-yellow
(5460A), yellow (6220A) and orange (6770A) are also usually
produced. The filter allows only radiation in the range of 3200
to 4000A and a little visible dark purple to pass.
Basic Ultraviolet Lights
UV bulbs come in a variety of shapes and sizes. The more common
types are the low pressure tube, high pressure spot, the high
pressure flood types. The tubular black light is similar in construction
to the tubular fluorescent lights used for office or home illumination.
These lights use a low pressure mercury vapor arc. Tube lengths
of 6 to 48 inches are common. The low pressure bulbs are most
often used to provide general illumination to large areas rather
than for illumination of components to be inspected. These bulbs
generate a relatively large amount of white light, which is concerning since inspection specifications require less than two foot-candles
of white light at the inspection surface.
lights are also used to illuminate the inspection area, since they
provide even illumination over a large area. Intensity levels
for flood lamps are relatively low because the energy is spread
over a large area. They generally do not generate the required
UV light intensity at the given distance that specifications require.
Spot lights, on the other hand, provide concentrated energy that
can be directed to the area of inspection. A spot light will generate
a six inch diameter circle of high intensity light when held fifteen
inches from the inspection surface. One hundred watt mercury vapor lights
are most commonly used, but higher wattages are available.
In the high pressure mercury vapor spot or flood lamps, UV light
is generated by a quartz tube inside the bulb. This tube contains
two electrodes that establish an arc. The distance between electrodes
is such that a starting electrode must be used. A resister limits
the current to the starting electrode that establishes the initial
arc that vaporizes the mercury in the tube. Once this low level
arc is established and the mercury is vaporized, the arc between
the main electrodes is established. It takes approximately five
minutes to "warm up" and establish the arc between the
main electrodes. This is why specifications require a "warm
up time" before using the high pressure mercury vapor lights.
Flood and spot black lights produce large amounts of heat and
should be handled with caution to prevent burns. This condition
has been eliminated by newer designs that include cooling fans.
The arc in the bulb can be upset when exposed to an external magnetic
field, such as that generated by a coil. Care should be taken
not to bring the lamp close to strong magnetic fields, but if
the arc is upset and extinguished, it must be allowed to cool
before it can be safely restarted.
Intensity Ultraviolet Lights
The 400 watt metal halide bulbs or "super lights" can
be found in some facilities. This super bright light will provide
adequate lighting over an area of up to ten times that covered
by the 100 watt bulb. Due to their high intensity, excessive light
reflecting from the surface of a component is a concern. Moving
the light a greater distance from the inspection area will generally
reduce this glare. Another type of high intensity light available
is the micro-discharge light. This particular light produces up
to ten times the amount of UV light conventional lights produce.
Readings of up to 60,000 uW/cm2
at 15 inches can be achieved.