OPTICAL DENSITY

Optical Density

The degree of blackening in the finished radiograph

Primarily controlled by mAs

Directly proportional to the mAs

Determines the amount of light transmitted through a radiograph

A logarithmic function

The Log 10 of opacity (Io /It )

Radiographic film

0 -4 OD (clear to black)

Opacity

the relationship of incident light to transmitted light

Black

3 OD or greater

No light is transmitted through the radiograph

High OD

Clear

<0 .2 OD

All light is transmitted through the radiograph

Low OD

0.7 OD

20 % light transmission (5 opacity)

1 OD

10% light transmission (10 opacity)

1.3 OD

5% light transmission (20 opacity)

0.3 OD increment

doubling or halving of opacity

High quality glass

zero OD (100 % transmission)

Unexposed radiograph film

0 .1 OD (80 % transmission)

Processed radiographic film

0 .3 OD (50 % transmission)

Characteristic Curve

Sensitometric or H&D curve

The graphic relationship between OD and exposure

Described by Hurter and Driffield

Shoulder Portion

High radiation exposure levels

Straight-line Portion

Intermediate radiation exposure levels

Toe Portion

Low radiation exposure levels

Base plus fog

ODs of unexposed film

Base Density (0.1 OD)

The OD that is inherent in the base of the film

Due to base composition (tinted with blue dye)

Fog Density (< 0 .1 OD)

The development of silver grains that contain no useful information

Due to :

Exposure of film during storage

Undesirable chemical contamination

Improper processing

Overexposure

Produces a radiograph that is too dark

Rationale : because too much radiation reaches the IR

Underexposure

Produces a radiograph that is too light

Rationale : because too few radiation reaches the IR

30% changed in mAs

required to produce a perceptible change in OD

4% changed in kVp

required to produce a perceptible change in OD

Hot Light

required to view an overexposed radiograph

0.25-2.50 OD

The OD within the range of human visibility; the useful OD range with gross feature

0.1-0.3 OD

The OD range of base plus fog

The OD range of most unexposed and processed radiographic film

0.5-1.25 OD

The OD range of most radiographs

3.5 OD

The OD with fine features

Factors Affecting Density

mAs

kVp

Screen Speed

Film Speed

Development time, temperature and Concentration

SID

OID

Patient thickness

Atomic number

Mass density

Pathology

Grid ratio

Voltage ripple

Collimation

Filtration