Control of Lighting
a component has been properly processed, it is ready for inspection.
While automated vision inspection systems are sometimes used,
the focus here will be on inspections performed visually by a human
inspector, as this is the dominant method. Proper lighting is of
great importance when visually inspecting a surface for a penetrant
indication. Obviously, the lighting requirements are different
for an inspection conducted using a visible dye penetrant than
they are for an inspection conducted using a fluorescent dye penetrant.
The lighting requirements for each of these techniques, as well
as how light measurements are made, are discussed below.
Lighting for Visible Dye Penetrant Inspections
When using a visible penetrant, the intensity of
the white light is of principal importance. Inspections can be
conducted using natural lighting or artificial lighting. When
using natural lighting, it is important to keep in mind that daylight
varies from hour to hour, so inspectors must stay constantly aware
of the lighting conditions and make adjustments when needed. To
improve uniformity in lighting from one inspection to the next,
the use of artificial lighting is recommended. Artificial lighting
should be white whenever possible and white flood or halogen lamps
are most commonly used. The light intensity is required to be
100 foot-candles at the surface being inspected. It is advisable
to choose a white light wattage that will provide sufficient light,
but avoid excessive reflected light that could distract from the
Lighting for Fluorescent Penetrant Inspections
When a fluorescent penetrant is being employed,
the ultraviolet (UV) illumination and the visible light inside the
inspection booth is important. Penetrant dyes are excited by
UV light of 365nm wavelength and emit visible light somewhere in the
green-yellow range between 520 and 580nm. The source of ultraviolet
light is often a mercury arc lamp with a filter. The lamps
emit many wavelengths and a filter is used to remove all but the
UV and a small amount of visible light between 310 and 410nm.
Visible light of wavelengths above 410nm interferes with contrast,
and UV emissions below 310nm include some hazardous wavelengths.
Standards and procedures require verification of
lens condition and light intensity. Black lights should never
be used with a cracked filter as output of white light and harmful
black light will be increased. The cleanliness of the filter should
also be checked as a coating of solvent carrier, oils, or other
foreign materials can reduce the intensity by up to as much as
50%. The filter should be checked visually and cleaned as necessary
before warm-up of the light.
Since fluorescent brightness is linear with respect to ultraviolet
excitation, a change in the intensity of the light (from age or
damage) and a change in the distance of the light source from
the surface being inspected will have a direct impact on the inspection.
For UV lights used in component evaluations, the normally accepted
intensity is 1000 microwatt per square centimeter when measured
at 15 inches from the filter face (requirements can vary from
800 to 1200 µW/cm2). The required check should
be performed when a new bulb is installed, at startup of the inspection
cycle, if a change in intensity is noticed, or every eight hours
of continuous use. Regularly checking the intensity of UV lights
is very important because bulbs lose intensity over time. In
fact, a bulb that is near the end of its operating life will often
have an intensity of only 25% of its original output.
Black light intensity will also be affected by voltage variations.
A bulb that produces acceptable intensity at 120 volts will produce
significantly less at 110 volts. For this reason it is important
to provide constant voltage to the light. Also, most UV light
must be warmed up prior to use and should be on for at least 15
minutes before beginning an inspection.
When performing a fluorescent penetrant inspection,
it is important to keep white light to a minimum as it will significantly
reduce the inspectors ability to detect fluorescent indications.
Light levels of less than 2 fc are required by most procedures
with some procedures requiring less than 0.5 fc at the inspection
surface. Procedures require a check and documentation of ambient
white light in the inspection area. When checking black light
intensity at 15 inches a reading of the white light produced by
the black light may be required to verify white light is being
removed by the filter.
Light intensity measurements are made using a radiometer. A radiometer
is an instrument that translate light energy into an electrical
current. Light striking a silicon photodiode detector causes a
charge to build up between internal layers. When an external circuit
connected to the cell, an electrical current is produced. This
current is linear with respect to incident light. Some radiometers
have the ability to measure both black and white light, while
others require a separate sensor for each measurement. Whichever
type is used, the sensing area should be clean and free of any
materials that could reduce or obstruct light reaching the sensor.
Radiometers are relatively unstable instruments and readings often
change considerable over time. Therefore, they should be calibrated
at least every six months.
Ultraviolet light measurements should be taken using a fixture
to maintain a minimum distance of 15 inches from the filter face
to the sensor. The sensor should be centered in the light field
to obtain and record the highest reading. UV spot lights are often
focused, so intensity readings will vary considerable over a small
area. White lights are seldom focused and depending on the wattage,
will often produce in excess of the 100 fc at 15 inches. Many
specifications do not require the white light intensity check
to be conducted at a specific distance.