Density

Optical density is often used as a means of evaluating the tonal depth in different parts of an image. It correlates well with the way the eye perceives tonal depth and with the thickness of image forming films (e.g. photographic emulsions and printing inks).

• Density is a measurement of the amount of light reflected or transmitted through a surface. It can be measured with a densitometer (transmission of reflecting).

The optical density D is defined mathematically by

D = -log₁₀R

where R is the reflectance in reflection and

D = -log₁₀T

where T is the transmittance in transmission.

In order to obtain color information, a filter complimentary to the color being measured is selected and the amount of light transmitted or reflected is detected by a photocell.

A logarithmic scale is used because it

1. correlates closely with the human visual mechanism since the response of the rods and cones is logarithmic.

2. has a linear relationship with the thickness of photographic emulsions and printed inks. (In spectroscopy this relationship is known as Beer’s law.)

3. compresses the scale.

Typical density ranges for graphic arts materials are shown in Table 4-2.

• Since the density ranges of some originals may be greater than can be reproduced, it is necessary to compress the density range of the original to avoid loss of shadow detail. (Fig 4-4)

Contrast

The difference in tone between the lightest and darkest areas. The lightest is usually the “white” of the paper and the darkest is where the density is highest.

• Thus, the amount of contrast depends on both the brightness of the paper and the maximum density.

• The degree of contrast is used to define the distribution of tones within an image. A high-contrast image has little midtone detail with heavy shadows and bright highlights. A low-contrast image has lots of midtone detail and little in the highlights and shadows.

• For display color, contrast is controlled by adjusting the electron beam intensities and the room light.

• For printed color, contrast is controlled by making the paper brighter and increasing the range of ink densities

. • In the HSL color model, both a luminance and color contrast can be defined.