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Tuesday, February 5, 2008

Glossary of Image Analysis Terms - G to M

Glossary of Image Analysis Terms - G to M


Gamma – The numeric value for the degree of contrast in a video signal. The exponent in the power law relating output to input signal magnitude. Non-linear camera tube. Glints – Shiny, specular reflections from smooth objects or surfaces. Global Method – An image processing operation uniformly applied to the whole image. Gradient – The rate of change of pixel intensity (first derivative). Gradient Space – A matrix containing values for the rate of change of pixel values or gray level

intensity of the image. Gradient Vector – The orientation and magnitude of the rate of change in intensity at a point or pixel location in the image.

Grating – An optical element with an even arrangement of rods or stripes with spaces between them for light to pass. Its ability to separate wavelengths is expressed in line pairs per millimeter, for example. A moiré grating of parallel dark and light stripes is an example. Also used for structured light projection.

Gray Level – A quantized measurement of image irradiance (brightness), or other pixel property typically in the range between pure white and black.

Gray Scale – Variations of values from white, through shades of gray, to black in a digitized image with black assigned the value of zero and white the value of one.

Grayscale Image – An image consisting of an array of pixels which can have more than two values. Typically, up to 256 levels (8 bits) are used for each pixel.

Guidance – Deriving properties in an image to describe a position at various points in time.


Halogen Lamp – An incandescent lamp with a gas similar to iodine inside which is constantly evaporated then redeposited on the filament.

Hardware – Electronic integrated circuits, boards and systems used by the system.

HDTV – High Definition TV proposed broadcast standard to double the current 525 lines per picture to 1,050 lines, and increasing the screen aspect ratio from 12:9 to 16:9. The typical TV of 336,00 pixels would increase to about 2 million.

Height/Range – Object profile is usually measured by changes in range or distances from the sensor. 3D techniques are usually used.

High-Pass Filter – Passes detailed high frequency image information, while attenuating low frequency, slow changing data.

High Speed Imaging – Image capture near, at or above 1800 parts per minute.

Histogram – A graphical representation of the frequency of occurrence of each intensity or range of intensities (gray levels) of pixels in an image. The height represents the number of observations occurring in each interval.

Histogram Analysis – Determination of the presence or absence of a feature or flaw based on the histogram values in a certain gray scale region.

Histogram Equalization – Modification of the histogram to evenly distribute a narrow range of image gray scale values across the entire available range.

Holography – Optically recording of the interference pattern from two coherent waves which forms a three-dimensional record or hologram.

Horizontal Blanking – The blanking signal that is produced at the end of each scanning line.

Horizontal Scan Rate – The speed at which the scanning beam in a display or a camera moves from side to side.

Horizontal Sync – The portion of a video signal indicating the end of a line of video information. This sync pulse is used by video equipment in order to maintain line synchronization with the incoming video signal.

Hough Transform – A global parallel method for locating both curved and straight lines. All points on the curve map into a single location in the transform space.

HSI (Hue-Saturation-Intensity) – A color representation. A mathematical conversion from RGB.

HSI Conversion – A mathematical conversion from the color RGB space to hue, saturation and intensity values.

HSV (Hue-Saturation-Value) – See HSI.

HSYNC – See Horizontal Sync.

HueOne of the three properties of HSI color perception. A color attribute used to express the amount of red, green, blue or yellow a certain color possesses. White, gray and black do not exhibit any hue.

Hueckel Operator – An edge finding operator which fits an intensity surface to the neighborhood of each pixel and selects surface gradients above a specified threshold.

Hybrid Electro-Optic Sensor – A silicon sensor fabricated in a configuration to match spatial information generated by the imaging system, such as a PSD (position sensitive detector), concentric rings, pie shapes and others.

Hz – An abbreviation for Hertz or cycles per second. Often used with metric prefixes such as kHz or MHz for kilohertz and megahertz respectively.

Identification – The process of specifically identifying an object from a large class of objects

through reading symbols. Illumination – Normally a wavelength or range of wavelengths of light or visible used to enhance a scene so the detector, normally a camera, can produce an image.

Image – Projection of an object or scene onto a plane (i.e. screen or image sensor). Image Analysis – Evaluation of an image based on its features for decision making. Image Capture – The process of acquiring an image of a part or scene, from sensor irradiation to

acquisition of a digital image. Image Compression – Used to reduce the amount of memory required to store an image. Image Distortion – A situation in which the image is not exactly true to scale with the object


Image Enhancement – Image processing operations which improve the visibility of image detail and features. Image Formation – Generation of an image of an object or scene on the imaging sensor. It

includes effects from the optics, filters, illumination and sensor itself.

Image Intensifier – Usually an electron tube equipped with a light sensitive electron emitter at one end and a phosphor screen at the other. Used to provide electron gain for imaging in low light conditions such as night vision.

Image Memory – An internal, high speed, large capacity storage area on a frame grabber card or

in a computer dedicated to image retention. Image Plane – The plane surface of the imaging sensor, perpendicular to the viewing direction, at which the optics are focused.

Image Preprocessing – Methods of conditioning an image before it is acquired, such as a Look-

Up Table (LUT) or attenuation. Image Processing – Digital manipulation of an image to aid feature visibility, make measurements or alter image contents.

Incandescent Lamp – An electrical lamp in which the filament radiates visible light when heated in a vacuum by an electrical current.

Incident Light – Light that falls directly onto an object.

Index of Refraction – A property of a medium that measures the degree that light bends when passing between it and a vacuum.

Infrared – The region of the electromagnetic spectrum adjacent to the visible spectrum, just beyond red with longer wavelengths.

Infrared Imaging – Image formation using wavelengths just below the visible spectrum.

Inspection – Non-destructive examination of a workpiece to verify conformance to some criteria.

Intensity – The relative brightness of a portion of the image or illumination source. Also called brightness.

Interlaced – A system in which two (or more) interleaved fields are used to scan out one video

frame. Interlaced Scanning – A scanning process in which all odd lines then all even lines are alternately scanned. Adjacent lines belong to different fields.

Interline Transfer CCD (Charge Coupled Device) – Data (charge) is transferred simultaneously out by odd and even lines or fields directly from the image sensors to their corresponding sensor registers. The output from the camera is always one field (frame) behind the image being captured.

Interpolation – A mathematical method of regenerating missing or needed information.

I/O (Input/Output) – Data either entering or leaving a system.

J Jitter – A time-based error caused by instabilities in synchronization and timing circuits. JPEG (Joint Picture Experts Group) – An image compression standard.

LAB – An acronym for the LAB color coordinate system. Laplacian Operator – The sum of the second derivatives of the image intensity in both the x and

y directions is called the Laplacian. The Laplacian operator is used to find edge elements by locating points where the Laplacian in zero. Laser Illumination – Lighting an object with a laser source for frequency selection, pulse width

(strobe) control or for accurate positioning. Laser Radar – See LIDAR. LED (Light Emitting Diode) – Often used as a strobe for medium speed objects. Lens – A transparent piece of material, usually glass or plastic, with curved surfaces which either

converge or diverge light rays. Often used in groups for light control and focusing.

Lens Types – Commonly used lenses are: 35mm, CCTV, Copying, Cylindrical, Enlarger, Micrographic, Video, and Wide Angle. LIDAR (Light Detection And Ranging) – A system that uses light instead of microwaves for

range and tracking measurements. LADAR uses a laser light source to measure velocity, altitude,

height, range or profile Light Tent – An arrangement of diffusing surfaces above the object to create a horizon to horizon diffuse illumination.

Lightpen – A pen on a cable used to select items from a display screen.

Line(s) of Light – One or more light stripes projected at a known angle onto the object. Deformation of this type of structured light results in 3D information in a 2D image. Line Scan Camera – A solid state video camera consisting of a single row of pixels. Also called a

Linear Array Camera. Linear Array Camera – See Line Scan Camera. Linearity – A basic measurement of how well an A/D or DAC is performing. The smaller the

number, the better. Linearity is typically specified as a range or percentage of LSBs (Least Significant Bits). Lighting – See Illumination.

Location – The point in X and Y image space where a recognized object is found. Look-Up Table (LUT) – High speed digital memory used to transform image input values to outputs for thresholding, windowing and other mappings such as pseudo-color.

Low Angle Illumination – See Darkfield. Very useful to enhance and highlight surface texture

features. Low-Pass Filter – A digital or optical filter which passes slow changing, low frequency information, while attenuating high frequency, detailed edge information.

Luma – See Luminance.

Luma Bandpass – A type of filter that separates the luminance part of the NTSC or PAL signal from the chrominance part.

Luminance – The black-and-white part of a video signal. Also called Luma.


Magnification – The relationship between the length of a line or size of a feature in the object plane with the length or size of the same in the image plane.

Mask – 1) Setting portions of an image ate neighbors to a constant value; 2) A filter matrix used as a convolution operator; 3) A logical or physical structure placed in an optical system to prevent viewing or passing of information in a certain spatial or frequency region.

Material Handling – Hardware systems that provide motion, indexing and/or orientation both during manufacture and the inspection process.

Matrix Array Camera – See Area Array Camera.

Measurement – Verification that a workpiece conforms to specified tolerances, such as dimensions, colors or textures.

Median Filter – A method of image smoothing which replaces each pixel value with the median grayscale value of its immediate neighbors.

Memory – The internal, high-speed, large capacity working storage in a computer where data and images may be both stored and retrieved.

Micron – One millionth of a meter, also called a micrometer.

Mirror – A smooth, highly polished surface, for reflecting light. It may be plane or curved. Mirrors are fabricated by depositing a thin coating of silver or aluminum on a glass substrate. First surface mirrors are coated on the top surface, thus avoiding a second ghost image produced when light is reflected off the back surface after passing through the glass twice.

MIPS (Millions of Instructions per Second) – A measure for computer processing speed.

Modulation Transfer Function (MTF) – The ability of a lens or optical system to reproduce (transfer) various levels of detail (modulation) of an object to the image as the frequency (usually sinusoidal) increases.

Moiré – A type of image artifact when a pattern occurs on the screen – generated when two oscillators of different frequencies beat together to create a new, unwanted frequency.

Moiré Interferometry – A method to determine 3D profile information of an object or scene, using interference of light stripes. Two identical gratings of known pitch are used. The first creates a shadow of parallel lines of light projected on the object. The second is placed in the imaging train, and superimposed on the shadow cast by the first grating, forming a moiré fringe pattern. Distance between the fringes or dark bands is directly related to range or profile. Varying the gap between the lines changes the sensitivity.

Moiré Pattern – A pattern resulting from the interference of light when gratings, screens or regularly spaced patterns are superimposed on one another. Two stacked window screens create this effect.

Moiré Topography – A contour mapping technique in which the object is both illuminated and viewed through the same grating. The resulting moiré fringes form contour lines of object elevation or profile.

Monochromatic – Refers to light having only one color or a single wavelength of radiation.

Monochrome – Refers to a black and white image with shades of gray but no color. A monochrome signal is a video source having only one component.

Monotonic – An A/D or DAC is monotonic if for every increase in input signal, the output increases also and does not back up.

Morphology – Image algebra group of mathematical operations based on manipulation and recognition of shapes. Also called mathematical morphology. Operations may be performed on either binary or gray scale images. Parallel processors are useful to implement.

MOS Array – Metal Oxide Semiconductor camera array sensor with random addressing capability, rows and columns of photodiodes and charge sent directly from the photodiode to the camera output.

Mouse - A hand operated pointing device used to select items from a display screen.

MPEG (Moving Picture Experts Group) – A compression algorithm for video sequences. Differs from JPEG in that MPEG takes advantage of the redundancy on a frame-to-frame basis of a motion video sequence, whereas JPEG does not.

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