2. Common properties

The ranges of optical densities both of an original copy and resulting print:

2.3

are important for the reproduction setting. The relationship of brightness values on an image is also characterized by the contrast – ratio of reflectance factors. Lightness of a detail depends on the brightness of an adjacent field or on that average or ultimate for an image. That’s why along with the common contrast defined as 2.4

there are met its such interpretations as local contrast

2.5

or detail contrast:

2.6

when the reflectance of a pixel ρ_{х, у} or detail ρ_дет is divided on reflectance ρ_{x+Δх, y+Δу} of a proximal field or on a minimal, for all the image, value ρ_min.

According to (2.2 and 2.4) the density range comprises:

Δ D = lg K 2.7

All of these formal quantitative parameters don’t however sufficient for non ambiguous judgment of the so called apparent contrast as an impression arising at the image viewing. It’s known, for example, that the copy looks of higher contrast and colorfulness when such of its parameters as described below sharpness increases. Print of given formal contrast K and density range Δ D can, beside of that, look shiny or dull depending on its tonal content. The pixels with levels near extreme ones ρ_max or ρ_min will dominate for the first case in the computer program histogram of tone levels distribution over an image. For the second case, some intermediate tone values will alternatively prevail in spite of a presence in a picture of some small spots of ultimate brightness. This means that the described later procedures of tonal correction allow for apparent contrast changes by gradation redistribution within a density range with no effect on ultimate tone values, i.e. on a formal image contrast.

Print definition relates to dimensions of fine details and geometric accuracy of contours presented on a copy. In television this parameter is directly estimated by the number of sampled pixels, i.e. by the amount of minimal black and white details which can be placed in the frame in a chess board order [2.1]. In such interpretation it comprises

, 2.8

where k is the frame format index, z – number of strokes for the given TV standard and assumed that the size of a pixel is the same in both scan directions. With k = 4/3 and z = 625 the definition of b/w TV picture comprises about half million elements.

Being the informative parameter it also defines, for example, the size of a memory required for non compressed frame data:

,

2.9

where M is the number of a light parameter possible levels for each element.

Signal volume increases with the growth of number of samples and their quantization levels. In digital TV and reproduction the latter typically comprises 256 and, as far as such number encoding needs eight digits, the data volume of a TV frame comprises N bytes. It does not depend on size of a screen, while for the print copy this volume is, to the contrary, depended on the picture area and can be measured as

N_print = a b L², 2.10

where a and b are the horizontal and vertical image dimensions, while L is the screen ruling as an amount of halftone dots per its unit length. This way estimation for a monochrome A3 (12 x 16 inches) print of 150 Lpi gives the ten times greater, than for TV frame, definition of 4.3 million elements and data volume of about 4.3 Mbytes. Such fact points out the rightfulness of “graphic quality” term use underlining the higher, as compared to other imaging means, information capacity of print copies. Such informative nature of print image definition parameter is once again confirmed by the paying “per square inch” area of advertising in periodicals.

With definition related to the minimal size of a reproduced detail the edge quality of the latter is, not depending on its dimensions, characterized by sharpness. There is, however, the correlation of these parameters: the image of higher definition looks more acute.

The sharpness of photographs is stipulated by resolution of lenses, emulsions, CCD matrixes, light scattering in optical components, etc. It’s measured by the inverse value of an edge fringe width (Figure 2.5) as:

(1/mm)

2.11

Figure 2.5. Density distribution on the sharp black to white transition in a real image.

Greater fidelity of sharpness estimation is provided with the use of equations accounting the form of density dispersion or its gradient within a fringe which variation is indicated by a couple of dotted curves in figure 2.5. For images produced with the use of scanning by a finite size aperture, such as in TV or digital photography, this parameter can be also estimated by the relative value of a scan spot and fringe width ratio as:

,

2.12

It presumes that the sharpness is first of all limited in these applications by the size of scan spot or input sample. Such measure can be expanded for the halftone prints with taking for a unit area the screen mesh.