A densitometer (also called absorptiometer) is used to measure the density of a film by the amount of light which shines through; or tissue density (e.g., bone, lung) by the amount of radiation transmission. The logarithm of the reciprocal of the transmittance is called the absorbance or density.

Bone densitometry measures the strength and density of bones. Changes in trabecular bone mineral density (BMD) is an early indicator of change in metabolic function. Bone densitometry measures the amount of calcium in regions of the bones. A bone densitometer is used to determine the risk of developing osteoporosis and can also be used to estimate a patient's risk of fracture.
Bone densitometry methods involve:
Dual energy x-rays (DEXA) or CT scans (Osteo CT or QCT) compare the numerical density of the bone (calculated from the image), with empirical data bases of bone density. DEXA is widely available and has an accuracy between those of QCT and ultrasound.

Densitometry is a method to image the density of for example, the bones to measure the mineral content or lung tissue to quantify emphysema.
Densitometry is also used to control the optical density of x-ray films by shining light on the film and measuring the transmission with a densitometer.

See also Bone Densitometry and Osteo CT.