the lumen. The units of measurements for visible light intensity are foot-candles where one foot-candle equals one
lumen per square foot. Another term often used is lux, which equals one lumen per square meter. The conversion
between the two terms is 1 foot-candle equals approximately 10 lux.
The human eye becomes much more sensitive to light under dark conditions. This increased sensitivity gradually
occurs when the light conditions change from light to dark. When entering a darkened area from a lighted area, little
or nothing can be seen at first. The pupil of the eye must widen to admit more light. The time required for the eye to
adjust to darken condition depends upon the overall health and age of the individual. Full sensitivity or dark adaptation
requires about 20 minutes. A dark adaptation time of 5 minutes is usually sufficient for magnetic particle inspection
with black light. An inspector entering a darkened area SHALL allow at least 5 minutes for dark adaptation before
examining parts. Once dark-adapted, the pupil of the eye responds very rapidly to bright light. A very short bright
light exposure cancels the slowly acquired dark adaptation. Time for dark adaptation must be allowed whenever an
inspector enters the darkened booth or is exposed to a bright light. A timer capable of measuring this time period
should be available within the darkened area.
The inspection area as well as the hands and clothing of the inspector should be clean and free of extraneous
fluorescent materials. Nonrelevant indications may be formed when parts contact extraneous fluorescent materials. In
addition, the fluorescence from this material will raise the ambient light level, thus increasing the amount of blacklight
necessary to produce a visible indication of a small defect.
a. Any ferromagnetic material subjected to magnetic particle inspection requires demagnetization. When
performing magnetic particle inspection of aircraft parts, it is essential to demagnetize them. The
inspector should understand the reasons for this step, as well as the problems involved and the available
means for solving them.
b. The earth's magnetic field can contribute to the difficulty of demagnetizing parts. A long part to be
demagnetized should be placed so that its principal axis is in an east-west direction. A long part lying
in a north and south direction can never be demagnetized below the level of the earth's field. Rotating
the part or structure on its east-west axis while demagnetizing often helps reduce the field in transverse
members that are not lying east and west. Vibration of the structure during the demagnetization
process is also helpful under these circumstances. Complete removal of all magnetic fields is virtually
Ferromagnetic materials retain a certain amount of residual magnetism (or remnant field) after application of a
magnetizing force. This does not affect the mechanical properties of the part. However, a residual field can impede the
operation of some parts as well as affect the operation of adjacent equipment that are sensitive to low level stray
Principles Of operation.
Demagnetization may be accomplished in a number of different ways. The method used depends upon the electrical
power and equipment available, the degree of demagnetization required and the skill of the inspector.