Photographic Digital Imaging

Frequently Used Terms
& Glossary

Note: If a term is underlined it links to more information - this may be of further benefit.
1-bit colour
The lowest number of colors per pixel in which a graphics file can be stored. In 1-bit color, each pixel is either black or white.
8-bit colour/greyscale
In 8-bit color, each pixel is has eight bits assigned to it, to provide 256 colors or shades of grey, as in a greyscale image.
24-bit colour
In 24-bit color, each pixel has 24 bits assigned to it, to represent some16.7 million colors. 8 bits - or one byte - is assigned to each of the red, green, and blue components of a pixel.
32-bit colour
A display resolution setting that is often referred to as true color and offers a color palette of over 4 billion colors.

Refers to displays of bitmapped images, where both curved lines and diagonal straight lines appear to be jagged due to the way they are composed of square pixels (sometimes referred to as ‘staircasing’ or ‘jaggies’).

Aliasing also can result from low-resolution scanning-in or from low resolution camera images. A small file size will show ‘aliasing’ when image size increases.

Alpha Channel
The fourth channel of an RGB image that creates transparency in the image.
In Photoshop it's a 'saved selection'. This 'selection' can be viewed and edited when revealed in the 'Alpha Channel' palette.

Analogue (sometimes, analog)
A signal that represents sound or vision by electrical analogy, e.g., variations in a DC voltage producing corresponding variations in luminance, or vice versa. Similarly, a silver photographic negative is an analogue representation of the subject.


A method of reducing the effect of aliasing by averaging the densities of pixels at the edges of items such as text, thereby softening their appearance.

When using text, this is the 'invisible' line upon which the type 'sits'.
A method of interpolation whereby to, in order to increase image resolution, the value of the new pixel is determined by averaging from all those surrounding it. It is the most accurate form of interpolation. It can be selected as the preferred 'default' setting in Photoshop.

A numerical coding system using only two digits, 0 and 1.

Two binary digits, or 'bits', can give four possible combinations:

00, 01, 10, 11

Three bits can give 8 (23) possible combinations:

000, 001, 010, 011, 100, 110, 111, 101


Short for 'binary digit'. A single number having the value either zero or one, which may represent the states of 'on' or 'off'. Eight bits make up one byte.


Bit depth, also known as bits per pixel
The number of bits used to represent the colour value of each pixel.

1 bit per pixel displays 2 colours, 2 bits gives 4 colours, 3 bits gives 8 colours and so on. Continuing to 24 bit, we have 16.7 million colours.


Windows bitmap file format.

Blending Mode
A function within 'Photoshop' (and other imaging software) that changes the behavior of a tool or layer, allowing it to interact in a variety of ways with the underlying image.

Compact Disk. Used for storing large amounts of data (700mb or so per disk)

High-speed memory chips, which store frequently used instructions. This is much faster that using conventional RAM.
A designated part of the computer RAM that holds the last item copied or cut from a file. An item held in the clipboard can be pasted into other files

Loss of shadow or highlight detail due to the conversion of grey tones lighter than a certain value to white, or darker than a certain value to black.


Clock speed
The effective rate at which the central processing unit (CPU) in a computer communicates with the various elements within it. The processor runs at a fixed clock-speed, regulated by the pulses of a quartz or other crystal. The speed is rated in megahertz (MHz) – one megahertz representing one million instructions per second.


The selection and duplication of groups of adjacent pixels within an image. For example, an area of skin tone can be cloned from another area to hide a skin blemish. The cloning tooling in most imaging programs can be varied to select different sized areas of pixels to be cloned and the manner in which they are cloned.


(cyan, magenta, yellow,black)
The three colours and black used by printers to produce printed colour illustrations using inks or dyes.


Colour Temperature
This refers to the predominant colour of the light source. It's measured in degrees Kelvin (K), which takes 'Absolute Zero' as its zero point. The redder the light source, the lower the colour temperature and conversely, the bluer the light source, the higher the colour temperature.

Photographic tungsten ('Photoflood') light = 3,200 degrees K
Electronic flash = 5,500 degrees K
Average daylight = 5,500 degrees K

Note, daylight is extremely variable and can be anything from the orange/red glow of a sunset to a dull overcast day (which could be 10,000 degrees K or more)

'Daylight' (as opposed to 'Tungsten') colour films are balanced for a colour temperature of sunlight in mid June at midday.


A digital process which enables data to be stored or transmitted using less than the normal number of bits. Compression can be 'Lossy', 'Lossless' or 'Virtually Lossless'. There are many types of compression, JPEG (Joint Photographic Experts Group) is one of the most commonly used, however it is 'Lossy', which means that once compressed some information is lost, this may or may not be visually apparent, depending upon the image content.


Desktop Colour Separations a special version of the EPS file format.

Dots per inch.
A measure of the resolution of a scanner or printer.
Digital Video Disk.
Used for storing large amounts of data. Of larger capacity than CD, 4.7gb is a typical DVD size.
A method of simulating a greater range of colours than a monitor can correctly display, this is achieved by placing small dots of colour adjacent to one another.

Dye Sub
Dye Sublimation.
An output/printing method that produces a high quality continuous tone by heating cyan, magenta and yellow (CMY) dyes until they become gaseous (without going through the liquid stage) The output of a dye-sub printer appears as a continuous tone glossy print, very similar to a photographic print by tradition chemical process.


ncapsulated Post Script. File format widely used in desk top publishing (DTP) software.

A number that denotes Exposure Value. It is determined by the brightness of the subject and sensitivity (speed) of the film/CCD array. Doubling the amount of brightness increases the EV by 1, halving the amount of brightness decreases the EV by 1. (See reference to 'f Stops).

Eye Candy
A set of creative plug-in filters for use with Adobe Photoshop, produced by Alien Skin.

File Transfer Protocol - the most common method for transferring files across the Internet.
A method of controlling the amount of light reaching the film plane/sensor of a camera by halving or doubling the amount in one-stop increment..

File Formats
The overall format in which any digital data file is saved.
Choosing the correct format for saving images is important so as to ensure that files are compatible with various software packages. Some formats compress the data to reduce space taken for storage. File formats include TIFF, EPS and PICT.


File size
Generally speaking, the file size of an image can be calculated by the formula:-

Total No of pixels x No bits per colour = 'X', divide by 8

This gives total No of bytes

An image has 1200 x 800 pixels = 960,000 pixels and is stored as 24 bit colour

Therefore we have 960,000 x 24 = 23,040,000 divided by 8

Which gives 2,880,000 bytes or 2.8Mb

We can readily see that as the total number of pixels increases, thus improving definition, the file size will increase. Also, as 'bit depth' increases, improving colour rendering and dynamic range, the file size will, again, likewise increase.

The file size will be dependent upon the file format used to save the image.

The total number of pixels is divided by 8 because there are 8 'bits' making up one 'byte'.

Just to make life more interesting, a 4Mb file stored to different file formats becomes in:

Photoshop = 2.1Mb

TIFF with LZW compression = 1.8Mb

JPEG settings vary from 'good' (104Kb) to 'best' (1.1Mb)

EPS = 5.6Mb.

A very fast external bus that supports data transfer rates of up to 400 Mbps. Firewire was developed by Apple and falls under the IEEE 1394 standard. Other companies follow the IEEE 1394 but have names such as Lynx and I-link.
A method of loading multiple web pages into a single browser window.

In photographic sensitometry, gamma refers to the slope of the straight-line portion of the characteristic curve of an emulsion. In digital imaging, it is a measure of midtone image contrast, the relationship between input data from an electronic image, and output data telling the monitor how to display an image.


The range of colours (or tones) which can be displayed or printed by a particular colour system. Many programs and colour management systems have ‘out of gamut’ warnings, indicating when colours displayed on a monitor are not reproducible by the printing system in use.


Graphics Interchange Format.
Developed by Compuserve in 1987 it allows compression of 8 bit images (original intention for transmission via modems). GIF uses LZW compression and is limited to 256 colours

Hyper Text Markup Language - this is the standard language used to create web pages.
Hyper Text Transfer Protocol - a method allowing web browsers and servers to communicate.
Image Map
A part of an image that has been made to be used as an HTML link or 'rollover'.
Joint Photographic Experts Group
A widely used 'Lossy' compression for still images. The amount of compression may be selected.
See also, Compression, File Size, Lossy


Java Script
A programming language to make web pages more 'alive' and interactive.
Jaz drive
A removable hard disk system using 3˝" diameter disks of 1Gb capacity.


ai’s Power Tools)
Originated by Kai Kreuse
- a series of plug-in filters for Adobe Photoshop. They include special effects filters, texture generators and fractal generators.


The adjustment of spacing between letters of type to create consistent letter-spacing.

A facility in Photoshop and other image programs for overlaying successive images on the same picture. Each layer may be treated separately or as part of a group. Think of layers as transparent sheets that may be shuffled into the required order.

Pronounced 'ledding', this is the spacing of a paragraph as measured from one baseline to the next. Originally, in the printing industry, strips of lead were used to increase the separation between lines of type.

Linear Array
A single row of picture elements which traverses across the image projected by a lens (in a digital camera) or across a print or transparency during 'scanning'. The process converts the analogue picture information into digital data.
Tri-linear arrays have three lines of picture elements each coated with a primary colour filter i.e. red, green and blue.
Resolution is quoted in ppi or ppc (pixels per inch or cm).


A ‘Lossless’ compression routine developed in the 1970s, and incorporated into the TIFF and GIF file formats.

Lossless Compression
A form of compression that maintains the fidelity of an image.
Lossy Compression
The term used to describe any sort of compression that results in a lower quality image.

If you wish to impress your friends, this stands for microreciprocal degree and represents 1 million divided by the Kelvin colour temperature of the light source. It's used as a reference number by manufactures of colour correction filters. Daylight @ 5,550 K corresponds to 182 Mired.

Originally devised to create graphic images with Macintosh computers.

Pixels Per Inch
A standard unit of measurement for the spatial resolution of scanned images. This is the usual way of stating the definition of a digital image.


Ink commonly used when printing with fewer than four inks or when colours are needed that cannot be reproduced using CMYK inks (including metallic, fluorescent vivid greens, intense blues, etc). Each Pantone colour is represented by a specific reference number which high-end commercial printing machines recognise in this way a high accuracy of colour is maintained.

Parallel Port
A socket in a computer most often used for connection to a printer. Parallel ports have 8 parallel wires which send 8 bits (1 byte) of information simultaneously, in the same amount of time it takes a serial port to send '1 bit'.
The term derived from Picture Element. The smallest area capable of resolving image detail in a pick-up device such as a CCD array. Pixels are usually square but can be rectangular, triangular or circular.
A unit for measuring text size. A 'point' is 1/72 inch (there are 12 points to a pica and 6 picas per inch). The size is an 'indicator' as it will vary from font style to font style.

Primary colours
The three primary colours of white light are red, green and blue (RGB).


Process colours
Cyan, magenta, yellow and black (CMYK).


Process inks
The four specific inks used in the four colour printing process, namely cyan, magenta, and black.


Random Access Memory
Temporary memory created and used only when the computer is switched on. The size of images, which can be opened, is dependent on how much RAM is installed in the computer.

This is image data that the camera 'array' records prior to internal processing by the camera. Users can choose to process the data later using a computer program / RAW file reader. In this way high quality image fidelity can be maintained.

Changing the resolution of an image, either by discarding unwanted pixels, or by interpolating new ones.


Changing the size of an image without altering the resolution. Increasing the size will lead to a decrease in image quality.

When moving the cursor around an image/graphic, sections of the image/graphic can be made to change when the cursor passes over. It's a feature of Java Script that allows the original image to be (temporarily) replaced by another - such as a 'button' appearing to be depressed.

The process of converting analogue value varying continuously with time into discrete values.


The strength or purity of a colour.

Small ComputerSystems Interface
A port input in a computer that allows high-speed communication between the computer and a peripheral device. The devices are linked or 'daisy-chained' together. Each device requires an identity number (I.D.) so that data raveling along the 'bus' (a common pathway) can be recognised.
Scan resolution
The resolution at which an original is scanned. Usually quoted in pixels per inch (ppi), samples per inch (spi), or Res (pixels per millimetre).
Serial Port
A multipurpose port on a computer frequently used for connecting a 'mouse', a graphics pen tablet, a modem or other device. Data is sent 'in series' i.e.1 bit at a time and is thus 'slow' in comparison to a parallel port. However for devices such as the mouse etc, speed is not an essential requirement. Modems connected to a standard telephone line can only handle one signal at a time.
Scratch disk
A reserved area of a hard disk used for temporarily storing a copy of an image being processed.
A square or rectangular area of an image that can be saved as a separate file. A 'slice' may have special properties associated with it. Slices can be used for 'rollovers' or HTML 'links' and so on.
Spot Channel
A special sort of colour channel that allows an image to be made from other inks or in addition to CMYK inks. Spot Channels are usually associated with Pantone inks.
The process by which a solid becomes a gas without first passing through a liquid state.
agged Image File Format
Originated by Microsoft and Aldus in the late 1980's this is the most widely used format today for bitmapped images.
echnology Without an Interesting Name
A cross platform interface for acquiring images with scanners and frame grabbers.
niform Resource Locator
The standard naming and addressing system on the World Wide Web. For example the URL of Kodak’s digital imaging page is:
Universal Serial Bus
An external input to a computer that is about 10 times faster than a typical serial connection.

An improved, faster version of USB

Vector graphics
A type of computer graphics where images (geometric shapes such as curves, arcs and lines) are defined as a series of mathematical formulae rather than a grid of pixels. Also referred to as object-oriented graphics.
A standard format used on 'Windows' for saving audio data ('.WAV' is the file extension). Playback is either by 'Media Player' (Windows) or 'QuickTime Player (Macintosh).
Web Safe Colours
A special set of 216 colours that are common to both Windows and McIntosh operating systems that will not become 'dithered' when viewed using 256-colour displays.
White balance
The relative amounts of red, green and blue light in a light source. Digital and video cameras can be ‘white balanced’ i.e. the signal adjusted so that the light reflected from a white or neutral grey surface can be neutralised.
Zip drive
A form of removable hard disc storage using 3˝" diameter discs of 100 Mb capacity.

For extended terms and a more detailed glossary a really useful booklet is:

The A to Z of Digital Imaging by Adrian Davies, published by Focal Press.