In the theory of

photography Photography is the art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employe ...

, tone reproduction is the mapping of scene luminance and

color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are assoc ...

to print

reflectance The 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 the boundary. Reflectance is a component of the response of the electronic ...

display Display may refer to: Technology * Display device, output device for presenting information, including: ** Cathode ray tube, video display that provides a quality picture, but can be very heavy and deep ** Electronic visual display, output devi ...

luminance, with the aim of subjectively "properly" reproducing

brightness Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. The perception is not linear to luminan ...

and "brightness differences". The reproduction of color scenes in

black-and-white Black-and-white (B&W or B/W) images combine black and white in a continuous spectrum, producing a range of shades of grey. Media The history of various visual media began with black and white, and as technology improved, altered to color. ...

tones is one of the long-time concerns of photographers. 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

, 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 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 ...

was developed as a way to illustrate and combine curves, to study and explain the photographic process. The luminance range of a scene maps to the focal-plane

illuminance In photometry (optics), 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 w ...

and exposure in a camera, not necessarily directly proportionally, as when a

graduated neutral density filter A graduated neutral-density filter, also known as a graduated ND filter, split neutral-density filter, or just a graduated filter, is an optical filter that has a variable light transmission. Typically half of the filter is of neutral density whi ...

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 Sensitometry is the scientific study of light-sensitive materials, especially photographic film. The study has its origins in the work by Ferdinand Hurter and Vero Charles Driffield (circa 1876) with early black-and-white emulsions. They determine ...

; this plot of optical density of the developed negative versus the

logarithm In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a number to the base is the exponent to which must be raised, to produce . For example, since , the ''logarithm base'' 10 of ...

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 Dodging and burning are terms used in photography for a technique used during the printing process to manipulate the exposure of select areas on a photographic print, deviating from the rest of the image's exposure. In a darkroom print from a fil ...

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

range. In

digital photography Digital photography uses cameras containing arrays of electronic photodetectors interfaced to an analog-to-digital converter (ADC) to produce images focused by a lens, as opposed to an exposure on photographic film. The digitized image ...

image sensor 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 (as they pass through or reflect off objects) into signals, small bursts of c ...

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

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 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 ...

of a particular printing method.{{cite book , title = Digital Color Imaging Handbook , chapter = Digital Color Halftones , author = Charles Hains, editor = Gaurav Sharma , publisher = CRC Press , year = 2003 , isbn = 0-8493-0900-X , chapter-url = https://books.google.com/books?id=AkByHKRGTsQC&dq=%22tone+reproduction+curve%22+%22dot+gain%22&pg=RA1-PA431 , display-authors=etal 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

of the print closely approximates a proportionality to the luminance intent implied by the electronic image. It is easier to demonstrate the need for a TRC using halftoned printing methods such as

inkjet Inkjet printing is a type of computer printing that recreates a digital image by propelling droplets of ink onto paper and plastic substrates. Inkjet printers were the most commonly used type of printer in 2008, and range from small inexpens ...

, or

xerographic Xerography is a dry photocopying technique. Originally called electrophotography, it was renamed xerography—from the roots el, ξηρός, label=none ''xeros'', meaning "dry" and -γραφία ''-graphia'', meaning "writing"—to emphasize ...

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 A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of colorwhether such representation entails an analog or a digital represen ...

conversion. For example, by default, when transforming from L*A*B* to

CMYK The CMYK color model (also known as process color, or four color) is a subtractive color model, based on the CMY color model, used in color printing, and is also used to describe the printing process itself. The abbreviation ''CMYK'' refers ...

Photoshop Adobe Photoshop is a raster graphics editor developed and published by Adobe Inc. for Windows and macOS. It was originally created in 1988 by Thomas and John Knoll. Since then, the software has become the industry standard not only in raster ...

applies an ICC profile for SWOP standard

inks Ink is a gel, sol, or solution that contains at least one colorant, such as a dye or pigment, and is used to color a surface to produce an image, text, or design. Ink is used for drawing or writing with a pen, brush, reed pen, or quill. Thicke ...

and 20% dot gain for coated paper.

References

Printing Photographic processes

In photography

In printing

See also

References