b. Filter. The aluminum filter or disk is placed in the path of the x-ray beam. It
is located at the base of the cone or position indicating device (PID) just inside the metal
housing. Figure 1-8 shows the location of the PID. The filter completely covers the
opening where the x-ray beam emerges from the x-ray tube. The reason for the
aluminum filter is to absorb the low energy, long wavelength x-rays (photons) and allow
the high energy, short wavelength x-rays (photons) to pass through the filter. Filters on
dental x-ray machines with over 70 kVp have a minimum thickness of 2.5 mm of
aluminum. Those machines below 70 kVp have a safety standard minimum of 1.5 mm
The terms cone, PID, or tube are used interchangeably throughout this text.
See figure 1-8.
c. Collimator. The lead diaphragm is collocated with the aluminum filter. It
restricts the x-ray beam to the desired size. The diaphragm or collimator is constructed
of 1/16-inch lead. Without this collimator, x-ray photons would cover a wide area of the
patient's head. With the lead diaphragm or collimator in place, only the area necessary
for exposure receives the primary beam. This is depicted in figure 1-7. The diagram in
figure 1-8 represents an x-ray tube, cone, or PID removed to show the location of the
lead diaphragm or collimator and the aluminum filter.
1-10. PROTECTIVE MEASURES AND STANDARDS
a. General. Every possible safety precaution must be utilized when exposing
radiographs. Collimation and filtration are only two of the several measures used to
protect the patient and the technician from ionizing radiation. If all safety rules are
strictly adhered to, the technician should receive no radiation and the patient exposure
will be minimal. Even with the numerous safety precautions, accidental exposure is still
b. Technician Protection and Standards.
(1) Protective booth and shields. Standards for dental x-ray booths or
rooms require a shielding thickness of 1/16-inch lead or equivalent. The timer switch
used to activate the machine for exposures is permanently affixed to the outside wall.
The timer switch is mounted outside the protective shielding to prevent the operator
from standing inside the booth during exposures. The shield is so designed that the
radiation must scatter at least twice before reaching the x-ray technician. Leaded glass
on the booth or shield provides a continuous view of the patient during the exposure.
Consequently, any holding of the film or tube head by the x-ray technician is strictly