Curve (tonality)

The term tone refers to the different levels between light and darkness of a color or black- and-white image, be it in a digital record on a transparent support (film ) or on a solid image, photographically or printed. He describes in a pixel (dot) a color or gray value within a predetermined color or grayscale spectrum, given in 0 - 100%. 100% means maximum darkness or color coverage ( full tone ) of the imaging medium. According to 0 % represents complete transparency of the film or blank paper with screen printing. The tonal value of the optical density and the reflectance measurements (formerly remission) and calculated according to the Murray-Davies formula from these measured values ​​. It is characterized by the process standard offset by the symbol.

Shade to copy film

The tone here is a name for the gray value in halftone. All Levels of highlights ( bright areas ) and lows ( dark areas ) correspond to different degrees of blackening. The tone is a relative measure, knows the values ​​between full transparency of a film material used (0%) and full of darkness under the selected operating conditions ( 100%). Important in this context is the optical density, a perceptually fair measure of the intensity of blackening. With a positive film, the apparent proportion of the covered area is given in percent. In contrast, a negative film is given as percentage missing to 100 % tonal value.

Tone in digital imaging technology

The tonal range of an RGB color channel of digital image files, including a region which is determined by the bit depth. In 8-bit to 28 = 256 discrete values ​​for each color result. The lowest value does not correspond to color available, the highest maximum color available. From the combination of additive color mixing in the three color channels results in the final color. So corresponds to RGB = ( 0,0,0 ) Black and RGB = ( 255,255,255 ) white.

For some applications, the gradation is no longer sufficient in 256 color values. Digital scanners offer a tonal range of 16 bits (corresponding to 216 = 65536 increments ) and more per color channel. High quality DSLR cameras of 2009 have a theoretical tonal range of 14 bits, the area effectively is even smaller. It is limited, inter alia, by the processes in the AD converter. Also for HDR images, a higher tonal range can be useful. While JPEG images are usually limited to 8 bits per color channel, 16 -bit data can be stored, for example in the TIFF file format.

Levels

Levels in this context is a mathematical function f (x ) that changes the distribution of brightness of individual color channels or of the whole image, for example:

• F ( x ) = x is a linear function ( "Straight" ) leaves the tonal values ​​unchanged.
• A correction function, with respect to the straight line has a slight S-shape, increases the image contrast. Reflects one curve to the diagonal f ( x ) = x, the contrast is attenuated to the same extent.
• A linear correction function ( "Straight" ) f ( x ) = 1 - x "inverted" the brightness values ​​toward a negative image.
• A simple power function f ( x ) = x (so-called gamma correction ) changed light and dark values ​​non-linear.
• The Levels may eventually also be discontinuous on a table ( lookup table).

Tone in printing technology

The tone is a measure of how covered a solid color to a normal observer appears. It is the idea that a solid color from a share fully covered color ( full tone ) and color- free sites is and the eye can not resolve these different shares. The effect on the eye (and optical instruments ), there is in addition a physical phenomenon, the light -catching. Both together form the tone. He is a visual and not a geometric measure. Therefore, it was formerly called " optically effective surface coverage ". It is calculated using the Murray-Davies formula and can be measured with the help of densitometers. In some instruments it is still incorrectly referred to as " coverage ".

The tone is true, strictly speaking, always for one color, black, cyan, magenta or yellow. A unprinted area has the level 0 %, a solid area 100 %. If the levels of all the colors of an image point added together, the result is the amount of color. The amount of color is also known as ink coverage. So it may be that a pressure surface with, for example, 300 % total ink coverage is characterized. In the printer language where a 25% tone value is referred to as quarter tones. Accordingly, ie a 50% tone semitone or half- tone and a 75% tone Three -quarter tone. In the photographic technique is referred to, however, all non-raster, homogeneously colored patches as a semitone. Therefore, a halftone is called a halftone simulation.

Dot gain is also a term used in printing technology. It describes the effect of grid points on the printed sheet due to the process are larger than on the print template, so the printed image is darker. Dot gain is specified as a percentage of the tone. A grid area, which is applied with a surface coverage of 80% in the pressure generated by a cover, for example, 85%. In addition, also comes the light capture, for example, with a contribution of 5%. The dot gain would be in this case 10 %.

Measure and calculate Levels

In a movie

For black - and-white films is measured with a see-through densitometer, the optical densities. We calibrated the device to a transparent and a blackened surface point and then measure the height or parts of the image that you want to evaluate.

At a pressure

With a [ spectrophotometer ] to measure the reflection coefficients an unprinted surface of the printing material ( blank ), a fully printed ( full tone ) and the intended measuring points ( height) in an image or printing.

From this one can calculate the reflectances for the grid area and for the full tone. The reflectance, we refer to the blank paper and compare the measurements so that only over grid and Concentrated:

The optical density is a perceptually accessible, logarithmic measure:

= Optical density in the grid = optical density in full tone.

From these values ​​we calculated the [ Murray - Davies formula ] the tone, expressed in%:

One can measure the optical density with a densitometer or modern Spectrodensitometer and put them directly into the Murray - Davies formula. Modern measuring instruments give any readings already finished out, even the tone.