Digital Imaging

Digital Imaging Category

Clarifies scanner and digital camera jargon

White Point

  • The colour that when scanned produces values of 255, 255, 255 in an 8-bit scanner. Ideally the white point is 100% neutral reflectance or transmittance. See also Reflectance.


  • Developed by a consortium of imaging hardware and software manufacturers, TWAIN is a cross-platform interface for acquiring electronic images captured by TWAIN-compliant scanners, digital cameras and still-frame video capture boards.

Tonal Resolution

  • The number of bits per pixel used in the digital representation of an image. The intensity and colour of each pixel in the image are represented by an integer value or set of integer values. Tonal resolution is a measure of a scanner’s resolution capability for small changes in intensity. Also called pixel depth or bit depth.

Stepper Motor

  • A type of motor that moves in discrete amounts with each electrical pulse. Steppers were originally the most common type of actuator engine, since they can be geared to advance a read/ write head one track per step. However, they are not as fast, reliable, or durable as voice coil actuators.

Shutter Speed

  • The shutter speed controls how long the digital sensor is exposed to light. The faster it is, the less susceptible the camera is to movement.


  • An option on some scanners that emphasises detail by increasing the contrast of the boundaries between light and dark areas of an image.


  • A device that reads a printed page and converts it into a graphics image for the computer. It works by digitising an image – converting everything on the page (text and graphics objects) – into one raster graphics image. The resulting matrix of bits, or bitmap, can then be stored in a file, displayed on a screen and manipulated by programs.


  • To enlarge or reduce an image by increasing or decreasing the number of scanned pixels, or the sampling rate, relative to the number of samples per inch needed by the printer or other output device. See also Interpolation.


  • A technique used to recalculate an image’s pixel data values to produce a different size of image. It involves a process similar to “interpolation”, this being the term used for resampling to a larger image.


  • The fraction of the light incident on a surface that is reflected and varies according to the wavelength distribution of the light. Also called reflectivity.

Raster Line

  • A thin horizontal strip across an image. Raster lines are captured one at a time by the CCD elements in a scanner. When displayed or printed in sequence, raster lines make up the image. Raster lines in a TV or monitor work the same way.


  • Photo Response Nonuniformity: pixel-to-pixel variation in the response of a CCD array to a fixed-intensity light. Ideally, the response to each CCD element in the array is identical; deviations from that response are caused by PRNU.


  • Pixels Per Inch: ppi is often used interchangeably with dpi, although a dot is a bilevel entity, either on or off, and a pixel can hold multiple levels of information. For instance, for an 8-bit scanner, 1 pixel has 256 possible values (0 to 255).


  • Photomultiplier Tube: a vacuum tube that converts light into electrical energy and amplifies it. Photomultiplier tubes are used in high-end drum scanners, because they are more sensitive to light than the CCD elements used in desktop devices.

Pixel Dropping

  • Subsampling to reduce the number of pixels in an image by dropping every nth pixel from the scan.

Pixel Replication

  • Creating more pixels than are actually scanned by replicating every nth pixel to create the n+1 pixel.


  • Print Image Matching: a system created by Epson in partnership with major digital camera manufacturers. PIM embeds colour space, gamma level, sharpness, brightness, shadow and highlight information into the digital file’s JPEG header, thereby allowing PIM-enabled devices to work together at their highest level of performance.


  • An instrument for measuring luminous intensity, luminous flux, illumination, or intensity. See also Correlated Noise.


  • Scanning at more than an optimum sampling rate. See also Subsampling.

Optical Frequency Response

  • A scanner’s capability for capturing a given frequency or range of frequencies.


  • Optical Character Recognition: a technology that allows dots or pixels comprising characters in a bitmapped image to be converted into ASCII text. OCR is frequently combined with scanners to scan documents into a computer and convert the resulting information into textual data.

Nyquist Frequency

  • The maximum frequency that can be sampled by a digital sampling device such as a scanner. The Nyquist frequency of any digital sampling device is 1/2 the sampling rate of the device. To capture full information about a signal, the frequency content of the signal must be significantly below the Nyquist frequency of the device.

Noncorrelated Noise

  • A random distortion in an analogue signal causing snow or speckles – random spots throughout the image. The distortion can be the result of electronic noise in the amplifiers, electrical spikes somewhere in the system, or random fluctuations in the scanner lights. Also called random noise. See also Correlated Noise.


  • Modulated Transfer Function: a test that measures the optical frequency response of a scanner or other optical system.

Macro Mode

  • A setting that allows a camera to focus on objects which are very near. It is a feature found on some 35mm, APS, and digital cameras.


  • A unit used to measure light.

Line Replication

  • Creating more raster lines than are actually scanned by replicating every nth raster line from the scan. See also Pixel Replication.

Line Dropping

  • Subsampling to reduce the number of raster lines in an image by dropping every nth raster line from the scan. See also Pixel Dropping.


  • The process of averaging pixel information when scaling an image. Used in scanners and digital cameras to produce an image resolution greater than the resolution of the device’s CCD array.

Instrument Metamerism

  • A phenomenon in a scanner in which two colours that look the same to an observer look different to the scanner, or two colours that look different to an observer look the same to a scanner. Instrument metamerism is an nonrecoverable error because based on the output, there is no way to determine what the input was.


  • The amount of light reflected or transmitted by an object with black as the lowest intensity and white as the highest intensity.

Image Type

  • The different representations of an original that can be captured by a scanner For instance: 24-bit colour, 8-bit greyscale, or 1-bit drawings.


  • A video recording and playback system that uses 8mm video cassettes and the S-video technology.

Dynamic Range

  • The range of the lightest to the darkest object that a scanner can distinguish. Also called density range.

Dropout Colour

  • A colour that is invisible when scanning a colour object in greyscale mode, causing any detail in this colour to disappear.


  • Dots Per Inch: a unit of measure used to describe the resolution of images produced by printers, scanners or other output devices. Generally, more dots per inch mean a higher resolution, a greater amount of visible detail in the image, and a larger file size.

Digital Camera

  • A camera that stores images on an internal memory chip, removable PC cards, or other digital media. Images can be transferred electronically to a computer for manipulation, e-mailing or website creation.

Digital Imaging

  • A field of computer science covering digital images – images that can be stored on a computer, particularly bit-mapped images. Digital imaging is a wide field that includes digital photography, scanning, and composition and manipulation of bit-mapped graphics.


  • To straighten a crooked page or image. Some scanners automatically deskew pages placed on the scanner bed at less than a 10 degree angle.


  • Design Rule for Camera File System: an industry standard for saving digital images based on the EXIF (Exchangeable image file format) standard. Drawn up for the purpose of simplifying the interchange of image files and related files on digital still cameras and other equipment and in particular to support the higher resolution images capable of being handled by modern-day devices.

DC Jack

  • Allows portable electronic devices to be connected to an external power source via an AC adapter.

Dark Voltage

  • The voltage from a CCD when no light is incident on the CCD. Also called dark current.

Correlated Noise

  • A recognisable pattern of change in an image file. The change is an increase or a decrease in the brightness of the pixels compared to what they should be. The pattern can be horizontally across a raster line, vertically down through the raster lines, or diagonally down and across the raster lines. Vertical correlated noise is often called streak noise and is a common problem with CCD technology. Also called periodic noise. See also Noncorrelated Noise.


  • The mathematical relationships that assign a weight to red, green, and blue colour separations to reproduce the original colour. 3-by-3 matrixing is the use of a colour-matching function available on some scanners.


  • The act of gathering information in the form of analogue or digital input through devices such as CCDs, scanners, microphones, camcorders, TV cameras, pressure and temperature transducers and keyboards.


  • Contact Image Sensor: a type of scanning sensor used in low-cost scanners that is smaller than a CCD. Although it allows for smaller, lighter scanners to be built, colour fidelity and image quality is not as good as CCD.


  • Charge-Coupled Device: a miniature photometer that detects light intensity and represents the intensity with an analogue voltage. A CCD array is made up of CCD elements, the smallest discrete CCD. Used in many digital cameras, video cameras and scanners.


  • A scanner’s imaging head that moves down a page to capture an image. Also called scanning head, optical imaging head.


  • A video camera that takes continuous pictures and generates a signal for display or recording. It captures images by breaking down the image into a series of lines. PAL (625 scan lines) and NTSC (525 scan lines) are the most common video formats. Most video cameras are analogue, but digital video cameras are becoming increasingly popular.

Black Point

  • The colour that when scanned produces values of 0, 0, 0 in an 8-bit scanner. Ideally, the black point is 0% neutral reflectance or transmittance. See also White Point.

Beat Frequency

  • A periodic variation in a signal resulting when two signals of unequal frequencies are combined. Where the beat is maximum the signal and the samples line up perfectly and the resolution is high. But where the beat is minimum the signal and the samples do not line up and the resolution is low.


  • A device that determines how much light is allowed to pass onto the film of a 35mm camera or onto the digital array of a digital camera while the shutter is open.


  • Automatic Document Feeder: a separate part that attaches to a scanner and feeds multiple page documents to it.


  • A menu item typically under File that allows you to start the scanning software directly from the application. Acquire is only available if the software is TWAIN-compliant.

Accessory Connector Socket

  • The circular socket labelled ADF/ACC on the back of a scanner where the connector cable is inserted.


  • The failure of a mirror, refracting surface, or lens to produce exact, point-to-point correspondence between an object and its image.

3-by-3 Matrixing

  • A mathematical operation that takes the RGB components of a colour separation and creates a new RGB output based on the relative values for the input components. Also called colour matrixing, colour mixing. See also Colour-Matching Functions.

X-direction Sampling Rate

  • A scanner’s sampling rate in the horizontal direction (across the page). The x-direction sampling rate is determined by the number of CCD elements in the CCD array.

Y-direction Sampling Rate

  • A scanner’s sampling rate in the vertical direction (down the page). The y-direction sampling rate is determined by the mechanical motion of the scanner’s carriage as it moves down the page. Some scanners vary the y-direction sampling rate in steps of lines pairs per inch (Lppi), offering more sampling rates to scale a document. Scanners with fixed, y-direction sampling rates offer fewer sampling rates or use interpolation, line dropping, or line replication to supply more sampling rates.


  • A universal direct-print standard adopted by most digital camera manufacturers, which allows digital camera users to connect directly to printers which support the feature, to allow printing without the use of a computer liaison.

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