In dental x ray machine How much the x ray exposure if the point is 4 inches from the target of the x ray machine?

Not nearly enough you need to worry about. You get more from a day at the beach or park.

Target Film Distance for a dental x-ray machine is generally 8"

• 16", with the optimal distance being 12". This is the distance

from the end of the PID (Position Indicating Device, the cone on the end of the machine) to the target INSIDE the machine, not the actual length of the PID. If your PID is 4", it is likely that the target inside the machine is a least 4 inches further, for a total TFD of 8". The amount of radiation intensity is inversely related to the distance - the less distance between the target and the film, the greater the amount of radiation; the greater distance, the less radiation. For the purpose of taking a dental radiograph, if you double your TFD, you need to increase the amount of time the film is exposed by the square of the original amount of time used. Time is measured in impulses, which equal 1/60 of a second. (A 15 impulse exposure is therefore only 1/4 of a second.) The amount of impulses used are kept really low so that the patient isn't exposed to much x-ray radiation at all.