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After completing this lesson you should be able to:
  • Identify how x-rays were discovered.
  • Define selected terms related to radiology, radiation hazards, and radiation protection.
  • Describe the parts of a dental x-ray unit and the three-step process required to produce x-rays.
  • List the protective measures used against the hazards of excessive exposure to radiation.



Radiography is a highly technical field, indispensable to the modern dental practice, but presenting many potential hazards. The dental radiographic specialist must be thoroughly familiar with the procedures necessary to produce radiographs of diagnostic quality. He must also have a thorough knowledge of the hazards associated with the use of radiation and how to protect himself and the patient against those hazards. This lesson deals with the production, characteristics, and effects of radiation and how it may be used safely in dentistry.


In 1895, Wilhelm Konrad Roentgen was searching for invisible light by experimenting with a Crookes vacuum discharge tube. This is a glass tube in which the vacuum is nearly complete, having a negative electrode (cathode) and a positive electrode (anode). Many investigators believed that invisible light rays were emitted from the negative electrode when a high voltage current was sent through the tube. With the room darkened and the tube covered with black paper, Roentgen passed a high voltage current through the Crookes tube and was surprised to observe that a fluorescent screen lying on a table at some distance was glowing brightly. He then noted that a shadow was produced when an object was placed between the tube and the screen. Further experimentation revealed that the rays that caused the fluorescent screen to glow also acted upon the emulsion on photographic plates in the same manner as light. Thus it was shown that the rays produced would pass through some substances through which light would not pass. Since Roentgen was unable to determine the exact nature of the rays produced, he referred to them as x-rays (x being commonly used to denote an unknown factor). In later years scientists have referred to them as Roentgen rays.


  1. Radiograph. An exposed and processed film (roentgenograph, roentgenogram). Also known as an x-ray negative.
  2. Roentgenology. The study and use of x-rays (radiology).
  3. Roentgen Ray. Electromagnetic radiation of pure energy and extremely short wavelength (X-ray), sometimes referred to as x-ray photons.
  4. X-ray Photon. Electromagnetic rays produced by the x-ray machine. (The x-ray photon will be dealt with in greater detail in Lesson 4 of this text.)


  1. General. There are two sources of radiation (natural background radiation and man- made) both of which are harmful to man.
  2. Natural Background Radiation. There are three sources of natural background radiation: cosmic, earth, and internal. Although natural background radiation may be harmful, man has lived in this environment without significant injurious effects since his appearance on earth.
  3. Man-Made Radiation. Man-made radiation has many sources. Some of them are from medical and dental radiographs, occupational exposure, fallout from weapons testing, television sets, and certain radioactive watch dials, clocks, and meters. Man-made radiation, used improperly, can be significantly more harmful to man than natural background.


  1. Particulate. Particulate or corpuscular radiation comes from radioactive decay or disintegration of radioactive materials. Alpha and beta particles are examples of this type radiation.
  2. Electromagnetic. Electromagnetic radiation covers a very wide spectrum ranging from electrical power to visible light to x and gamma rays. The portion of the electromagnetic spectrum most important to us in this particular study is the x-ray portion.

David L. Heiserman, Editor

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Revised: June 06, 2015