Density

Refers to the overall blackness (resulting from exposure) of a film. It is a combination of exposure and development that produces a particular density. Density is measured with a densitometer.

In sensitometric terms, Density is the common logarithm of the Reciprocal of Transmission . The Reciprocal of Transmittance ( 1/Transmittance ) is the ratio of ‘incident light’ to ‘transmitted light’ and is always greater than unity. The Reciprocal of Transmitance was formally known as ‘opacity’ and is now the preferred term. The reciprocal of transmittance increases as blackness increases, so it is a more convenient way of expressing blackness than transmittance, but its numerical value can range from 1 to 10,000. Such a large range of values would be difficult to plot on a graph. More meaningful characteristic curves can be produced by using the ‘logarithm of the reciprocal of transmittance’. This quantity, log ₁₀ ( 1/Transmittance ), is called ‘transmission density’

For instance: A film sample that transmits half of the Incident light, has a Transmittance of 0.50 or 50%. It also has a Density of 0.30. (Log ¹⁰ of 2.0=0.30)

The density of individual RGB colours can also be read with a densitometer that has the appropriate filters.

Transmission Density is measured in two ways:

• Totally Diffuse Density is determined by comparing all of the transmitted light with the incident light perpendicular to the film plane. This set up is analogous to the contact printer except that the receptor in the printer is film.
• Specular Density is determined by comparing only the transmitted light that is perpendicular to the film plane with the normal incident light, analogous to optical printing or projection.

Density is directly related to the appearance of the photographic image as our eyes response to light variation (luminance differences) is approximately logarithmic.