X-ray units are used to measure the amount of radiation emitted from a source. The three most common units for measuring X-rays are REMS (Roentgen Equivalent in Man), RADS (Radiation Absorbed Dose) and ROENTGENS (Roentgen). REMS measure the biological effect of radiation on humans, RADS measure the amount of energy absorbed by a material, and ROENTGENS measure the amount of radiation emitted from a source.