Tone reproduction

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In the theory of photography, tone reproduction is the mapping of scene luminance and color to print reflectance or display luminance, [1] with the aim of subjectively "properly" reproducing brightness and "brightness differences". [2]

Photography Art, science and practice of creating durable images by recording light or other electromagnetic radiation

Photography is the art, application and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employed in many fields of science, manufacturing, and business, as well as its more direct uses for art, film and video production, recreational purposes, hobby, and mass communication.

Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted or reflected from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square metre (cd/m2). A non-SI term for the same unit is the nit. The CGS unit of luminance is the stilb, which is equal to one candela per square centimetre or 10 kcd/m2.

Color Characteristic of human visual perception

Color, or colour, is the characteristic of human visual perception described through color categories, with names such as red, orange, yellow, green, blue, or purple. This perception of color derives from the stimulation of cone cells in the human eye by electromagnetic radiation in the visible spectrum. Color categories and physical specifications of color are associated with objects through the wavelength of the light that is reflected from them. This reflection is governed by the object's physical properties such as light absorption, emission spectra, etc.

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The reproduction of color scenes in black-and-white tones is one of the long-time concerns of photographers. [3]

A tone reproduction curve is often referred to by its initials, TRC, and the 'R' is sometimes said to stand for response, as in tone response curve.

In photography

In photography, the differences between an "objective" and "subjective" tone reproduction, and between "accurate" and "preferred" tone reproduction, have long been recognized. Many steps in the process of photography are recognized as having their own nonlinear curves, which in combination form the overall tone reproduction curve; the Jones diagram was developed as a way to illustrate and combine curves, to study and explain the photographic process. [4] [5]

A Jones diagram is a type of Cartesian graph developed by Loyd A. Jones in the 1940s, where each axis represents a different variable. In a Jones diagram opposite directions of an axis represent different quantities, unlike in a Cartesian graph where they represent positive or negative signs of the same quantity. The Jones diagram therefore represents four variables. Each quadrant shares the vertical axis with its horizontal neighbor, and the horizontal axis with the vertical neighbor. For example, the top left quadrant shares its vertical axis with the top right quadrant, and the horizontal axis with the bottom left quadrant. The overall system response is in quadrant I; the variables that contribute to it are in quadrants II through IV.

The luminance range of a scene maps to the focal-plane illuminance and exposure in a camera, not necessarily directly proportionally, as when a graduated neutral density filter is used to reduce the exposure range to less than the scene luminance range. The film responds nonlinearly to the exposure, as characterized by the film's characteristic curve, or Hurter–Driffield curve; this plot of optical density of the developed negative versus the logarithm of the exposure (also called a D–logE curve) has central straight section whose slope is called the gamma of the film. The gamma can be controlled by choosing different films, or by varying the development time or temperature. Similarly, the light transmitted by the negative exposed a photographic paper and interacts with the characteristic curve of the paper to give an overall tone reproduction curve. The exposure of the paper is sometimes modified in the darkroom by dodging and/or burning-in, further complicating the overall tone reproduction, usually helping to map a wider dynamic range from a negative onto a narrower print reflectance range.

Illuminance total luminous flux incident on a surface, per unit area

In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.

Exposure (photography) amount of light captured by a camera

In photography, exposure is the amount of light per unit area reaching a photographic film or electronic image sensor, as determined by shutter speed, lens aperture and scene luminance. Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance in a specified region.

Photographic film sheet of plastic coated with light-sensitive chemicals

Photographic film is a strip or sheet of transparent plastic film base coated on one side with a gelatin emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of the crystals determine the sensitivity, contrast and resolution of the film.

In digital photography, image sensors tend to be nearly linear, but these nonlinear tone reproduction characteristics are emulated in the camera hardware and/or processing software, via "curves".

Digital photography commonly used term for photography with a digital camera

Digital photography uses cameras containing arrays of electronic photodetectors to capture images focused by a lens, as opposed to an exposure on photographic film. The captured images are digitized and stored as a computer file ready for further digital processing, viewing, digital publishing or printing.

Image sensor device that converts an optical image into an electronic signal

An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves into signals, small bursts of current that convey the information. The waves can be light or other electromagnetic radiation. Image sensors are used in electronic imaging devices of both analog and digital types, which include digital cameras, camera modules, medical imaging equipment, night vision equipment such as thermal imaging devices, radar, sonar, and others. As technology changes, digital imaging tends to replace analog imaging.

Curve (tonality) tonality map in image editing

In image editing, a curve is a remapping of image tonality, specified as a function from input level to output level, used as a way to emphasize colours or other elements in a picture.

In printing

In printing, a tone reproduction curve is applied to a desired output-referred luminance value, for example to adjust for the dot gain of a particular printing method. [6] Dot-based printing methods have a finite native dot size. The dot is not square, nor any other shape that when stacked together perfectly fills an image area; rather, the dot will be larger than its target area and overlap its neighbors to some extent. If it were smaller than its target area, it would not be possible to saturate the substrate. A tone reproduction curve is applied to the electronic image prior to printing, so that the reflectance of the print closely approximates a proportionality to the luminance intent implied by the electronic image.

Dot gain, or tonal value increase, is a phenomenon in offset lithography and some other forms of printing which causes printed material to look darker than intended. It is caused by halftone dots growing in area between the original printing film and the final printed result. In practice, this means that an image that has not been adjusted to account for dot gain will appear too dark when it is printed. Dot gain calculations are often an important part of a CMYK color model.

Substrate is used in a converting process such as printing or coating to generally describe the base material onto which, e.g. images, will be printed. Base materials may include:

Reflectance capacity of an object to reflect light

Reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at an interface. The reflectance spectrum or spectral reflectance curve is the plot of the reflectance as a function of wavelength.

It is easier to demonstrate the need for a TRC using halftoned printing methods such as inkjet, or xerographic technologies. However, the need also applies to continuous-tone methods such as photographic paper printing.

As an example, suppose one wants to print an area at 50% reflectance, assuming no ink is 100% reflective and saturated black ink is 0% (which of course they aren't). The 50% could be approximated using digital halftoning by applying a dot of ink at every other dot target area, and staggering the lines in a brick-like fashion. In a perfect world, this would cover exactly half of the page with ink and make the page appear to have 50% reflectivity. However, because the ink will bleed into its neighboring target locations, greater than 50% of the page will be dark. To compensate for this darkening, a TRC is applied and the digital image's reflectance value is reduced to something less than 50% dot coverage. When digital halftoning is performed, we will no longer have the uniform on-off-on-off pattern, but we will have another pattern that will target less than 50% of the area with ink. If the correct TRC was chosen, the area will have an average 50% reflectance after the ink has bled.

A TRC can be applied when doing color space conversion. For example, by default, when transforming from L*A*B* to CMYK, Photoshop applies an ICC profile for SWOP standard inks and 20% dot gain for coated paper.

See also

Related Research Articles

CMYK color model subtractive color model, used in color printing

The CMYK color model is a subtractive color model, used in color printing, and is also used to describe the printing process itself. CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key.

Halftone

Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size or in spacing, thus generating a gradient-like effect. "Halftone" can also be used to refer specifically to the image that is produced by this process.

High-dynamic-range imaging high dynamic range (HDR) technique used in imaging and photography

High-dynamic-range imaging (HDRI) is a high dynamic range (HDR) technique used in imaging and photography to reproduce a greater dynamic range of luminosity than is possible with standard digital imaging or photographic techniques. The aim is to present a similar range of luminance to that experienced through the human visual system. The human eye, through adaptation of the iris and other methods, adjusts constantly to adapt to a broad range of luminance present in the environment. The brain continuously interprets this information so that a viewer can see in a wide range of light conditions.

Darkroom workshop used by photographers make prints and otherwise handle photographic film

A darkroom is a workshop used by photographers working with photographic film to make prints and carry out other associated tasks. It is a room that can be made completely dark to allow the processing of the light-sensitive photographic materials, including film and photographic paper. Various equipment is used in the darkroom, including an enlarger, baths containing chemicals, and running water.

In digital photography, computer-generated imagery, and colorimetry, a grayscale or greyscale image is one in which the value of each pixel is a single sample representing only an amount of light, that is, it carries only intensity information. Grayscale images, a kind of black-and-white or gray monochrome, are composed exclusively of shades of gray. The contrast ranges from black at the weakest intensity to white at the strongest.

Digital printing

Digital printing refers to methods of printing from a digital-based image directly to a variety of media. It usually refers to professional printing where small-run jobs from desktop publishing and other digital sources are printed using large-format and/or high-volume laser or inkjet printers. Digital printing has a higher cost per page than more traditional offset printing methods, but this price is usually offset by avoiding the cost of all the technical steps required to make printing plates. It also allows for on-demand printing, short turnaround time, and even a modification of the image used for each impression. The savings in labor and the ever-increasing capability of digital presses means that digital printing is reaching the point where it can match or supersede offset printing technology's ability to produce larger print runs of several thousand sheets at a low price.

Frederic Eugene Ives American inventor

Frederic Eugene Ives was a U.S. inventor, born at Litchfield, Connecticut. In 1874–78 he had charge of the photographic laboratory at Cornell University. He moved to Philadelphia, Pennsylvania, where in 1885 he was one of the founding members of the Photographic Society of Philadelphia. He was awarded the Franklin Institute's Elliott Cresson Medal in 1893, the Edward Longstreth Medal in 1903, and the John Scott Medal in 1887, 1890, 1904 and 1906. His son Herbert E. Ives was a pioneer of television and telephotography, including color facsimile.

Photogravure printmaking technique

Photogravure is an intaglio printmaking or photo-mechanical process whereby a copper plate is grained and then coated with a light-sensitive gelatin tissue which had been exposed to a film positive, and then etched, resulting in a high quality intaglio plate that can reproduce detailed continuous tones of a photograph.

Color printing or colour printing is the reproduction of an image or text in color. Any natural scene or color photograph can be optically and physiologically dissected into three primary colors, red, green and blue, roughly equal amounts of which give rise to the perception of white, and different proportions of which give rise to the visual sensations of all other colors. The additive combination of any two primary colors in roughly equal proportion gives rise to the perception of a secondary color. For example, red and green yields yellow, red and blue yields magenta, and green and blue yield cyan. Only yellow is counter-intuitive. Yellow, cyan and magenta are merely the "basic" secondary colors: unequal mixtures of the primaries give rise to perception of many other colors all of which may be considered "tertiary."

The Zone System is a photographic technique for determining optimal film exposure and development, formulated by Ansel Adams and Fred Archer. Adams described the Zone System as "[...] not an invention of mine; it is a codification of the principles of sensitometry, worked out by Fred Archer and myself at the Art Center School in Los Angeles, around 1939–40."

Photoengraving is a process that uses a light-sensitive photoresist applied to the surface to be engraved to create a mask that shields some areas during a subsequent operation which etches, dissolves, or otherwise removes some or all of the material from the unshielded areas. Normally applied to metal, it can also be used on glass, plastic and other materials.

Tone mapping

Tone mapping is a technique used in image processing and computer graphics to map one set of colors to another to approximate the appearance of high-dynamic-range images in a medium that has a more limited dynamic range. Print-outs, CRT or LCD monitors, and projectors all have a limited dynamic range that is inadequate to reproduce the full range of light intensities present in natural scenes. Tone mapping addresses the problem of strong contrast reduction from the scene radiance to the displayable range while preserving the image details and color appearance important to appreciate the original scene content.

LightJet is a brand of hardware used for printing digital images to photographic paper and film. LightJet printers are no longer manufactured but are however remanufactured and resold; and their lasers are still manufactured.

The Neugebauer equations are a set of equations used to model color printing systems, developed by Hans E. J. Neugebauer. They were intended to predict the color produced by a combination of halftones printed in cyan, magenta, and yellow inks.

CcMmYK color model

CcMmYK, sometimes referred to as CMYKLcLm or CMYKcm, is a six color printing process used in some inkjet printers optimized for photo printing. It complements the more common four color CMYK process, which stands for Cyan, Magenta, Yellow and Key (black), by adding light cyan and light magenta. Individually, light cyan is often abbreviated to Lc or c, and light magenta is represented as Lm or m.

The following outline is provided as an overview of and topical guide to photography:

Photo quality printing is the ultra high resolution reproduction of digital artwork onto printable materials such as paper, vinyl, film, polyester, etc.

Error diffusion

Error diffusion is a type of halftoning in which the quantization residual is distributed to neighboring pixels that have not yet been processed. Its main use is to convert a multi-level image into a binary image, though it has other applications.

Stochastic screening or FM screening is a halftone process based on pseudo-random distribution of halftone dots, using frequency modulation (FM) to change the density of dots according to the gray level desired. Traditional amplitude modulation halftone screening is based on a geometric and fixed spacing of dots, which vary in size depending on the tone color represented. The stochastic screening or FM screening instead uses a fixed size of dots and a distribution density that varies depending on the color’s tone.

References

  1. ↑ John Sturge; Vivian Walworth & Allan Shepp (1989). Imaging Processes and Materials. John Wiley and Sons. ISBN   0-471-29085-8.
  2. ↑ L. A. Jones (July 1920). "On the Theory of Tone Reproduction, with a Graphic Method for the Solution of Problems". Journal of the Franklin Institute. The Franklin Institute of the State of Pennsylvania. 190 (1): 39–90. doi:10.1016/S0016-0032(20)92118-X.
  3. ↑ "A New Photographic Process". American Engineer and Railroad Journal. XLVIII (4): 183. April 1894.
  4. ↑ L. A. Jones (March 1921). "Photographic Reproduction of Tone". Journal of the Optical Society of America. OSA. V (2): 232. doi:10.1364/josa.5.000232.
  5. ↑ Leslie D. Stroebel; Ira Current; John Compton & Richard D. Zakia (2000). Basic Photographic Materials and Processes. Focal Press. pp. 235–255. ISBN   0-240-80405-8.
  6. ↑ Charles Hains; et al. (2003). "Digital Color Halftones". In Gaurav Sharma. Digital Color Imaging Handbook. CRC Press. ISBN   0-8493-0900-X.
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