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• Glossary, Technical Notes

Technical Notes - CCTV Lenses

The following are the main terms used when specifying CCTV lenses, useful in selecting the most suitable product for your application. Should further clarification be needed, please contact MidOpt technical support at 847-359-3550.

Aberration
The failure of an optical lens to produce an exact point-to-point correspondence between the object and its resulting image. Various types are chromatic, spherical, coma, astigmatism and distortion.

Absorption
The loss of light of certain wavelengths as it passes through a material and is converted to heat or other forms of energy.

Active Illumination
Lighting a scene with a light source coordinated with the acquisition of an image. Strobed flash tubes, pulsed lasers and scanned LIDAR beams are examples.

Ambient Light
Light which is present in the environment of the imaging front end of a vision system and generated from outside sources. This light, unless used for actual scene illumination, will be treated as background noise by the vision system.

Angle of View
The angle of view of a lens depends on both the focal length and the sensor format size – the larger the format size, the larger the angle of view for a given focal length.

Angular Field of View
This angle is defined by two rays of light crossing at the secondary principal point, and depends on the focal length and on the sensor format size.

Formula: w = 2 x tan-1 D/2EFL
D = object dimension
EFL = effective focal length

Aperture Ratio
This is the ratio between the effective diameter and the focal length of the lens, and indicates the actual brightness of the image. The aperture ratio is specified by the f/# – the smaller the f/#, the brighter the image.

Example: A lens specified with an aperture of f/1.3 is brighter (allows in more light) than another at f/1.8.

Formula: f = EFL/A
f = f/# (brightness)
EFL = effective focal length
A = aperture = effective diameter

Area
Portion or area of the image to be analyzed. Area analysis measures the number of pixels which fall in a specified range of gray levels for the feature of interest.

Area Array Camera
A solid state imaging device with both rows and columns of pixels, forming an array which produces a 2D image.

Aspect Ratio
The ratio of the width to the height of a frame of a video image. The U.S. television standard is 4:3 or 1.333

Astigmatism
A defect in a lens which causes blur or imperfect image results, since the rays from a given point fail to meet at the focal point.

Back focal length (BFL)
The distance between the apex of the last element of a lens group and the focal point is the back focal length.

Backlighting
Placement of a light source behind an object so that a silhouette of that object is formed. It is used where outline information of the object and its features is important rather than surface features.

Baffle
A type of shield that prohibits light from entering an optical system.

Bandpass Filter
An optical filter which allows a known range (band) of wavelengths to pass while blocking those of lower and higher frequencies.

Beamsplitter
An optical device which divides one beam into two or more separate beams. A simple coated piece of glass in the optical path might reflect 60% of the light down onto the object, while allowing the other 40% to pass.

Brightness
The total amount of light or incident illumination on a scene or object per unit area. Also called intensity.

C- and CS-mount
There are two standard mounts for CCTV cameras: C- and CS- mounts. The only difference is the flange focal length: for C-mount lenses it is 17.256mm, and for CS-mount lenses it is 12.5mm. C-mount lenses can be used on cameras having CS-mount standards with the use of C/CS (5mm) extension tubes or rings, but CS-mount lenses can only be used with cameras having CS-mount standards.

CCD
Charge Coupled Device. A photo-sensitive image sensor implemented with large scale integration technology.

Chroma
The quality of a color including both the hue and saturation. Not present in gray. (-)

Close-up Focusing
When the object to be viewed is closer than the minimum focusing distance of the lens, the rays of lights will be focusing on a point beyond the image sensor surface, therefore the image is not focused on the sensor. Two methods can be used to focus the image:

a) Extension tube set
These are rings which are inserted between lens and camera, thus positioning the lens at a greater distance from the camera sensor, bringing the nearer object in focus. In this way, the point where the rays of light are focused is shifted towards the sensor surface. However, depth of field is reduced so only objects within a certain distance range can be focused properly. By changing the object distance, the length of the extension tube should also be changed. Extension tubes are not recommended for zoom lenses.

b) Close-up lenses
Additional lenses, added onto the front of camera lenses themselves, are used to correct the convergence of rays of light and thus reduce the minimum focusing distance. These close-up lenses have a given strength measured in diopters, defined as the reciprocal of the focusing distance (in meters).

Formula: D=1/d
D = diopters
d = focusing distance (in meters)

A close-up lens with a strength of +4 diopters, will always focus an object at a distance of 25 cm (1/4m), regardless of the focal length of the lens.

Diopters can be added together for different distance focusing. For example, threading together a +1 and a +4 close-up diopter will give the same result as a single +5 diopter lens.

Coaxial Illumination
Front lighting with the illumination path running along the imaging optical axis and usually introduced with a 45 degree angle beam splitter.

Color
A visual object attribute which may be described by a "coordinate system" such as hue, saturation and intensity (HSI), CIE or LAB. Wavelengths in the visible part of the electromagnetic spectrum to which retinal rods respond.

Color Space
A two or three dimensional space used to represent an absolute color coordinate. RGB, HSI, LAB and CIE are all representations of color spaces.

Color Temperature
A colorimetric concept related to the apparent visual color of a source, but not its actual temperature.

Colorimetry
Techniques used to measure color of an object or region and to define the results in a comparison or coordinate system.

Condenser Lens
Used to collect and redirect light for the purpose of illumination. Often used to collect light from a small source and project even light onto an object.

Contrast
The difference of light intensity between two adjacent regions in the image of an object. Often expressed as the difference between the lightest and darkest portion of an image. Contrast between a flaw or feature and its background is the goal of illumination.

Contrast Enhancement
Stretching of the gray level values between dark and light portions of an image to improve both visibility and feature detection.

Darkfield Illumination
Lighting of objects, surfaces or particles at very shallow or low angles, so that light does not directly enter the optics. Objects are bright with a dark background. This grazing illumination causes specular reflections from abrupt surface irregularities.

Depth of Field
Depth of field is the range of distance over which an image of an object can be focused by an optical system.

The depth of field is:
a) inversely proportional to the focal length.
b) inversely proportional to the aperture ratio (f number).

Consequently:
a) Wider angle lenses have larger depth of field. Eventually, the image always remains focused at shorter focal lengths. This is the reason why in most wide angle lenses there is no focusing ring and only an iris ring.
b) Closing the iris increases the depth of field. For this reason, the focus setting should be made when the iris is completely open (as in lower light conditions). Objects focused in this way will also then be at best focus when a more closed iris position is used.

Depth Perception
The perception of solidity of a visual object and its location in the spatial field, through the fusion in the brain of the two slightly dissimilar images from the two eyes.

Dichroic Filter
A filter used to transmit light based on its wavelength, rather than on its plane of vibration. Transmits one color, while reflecting a second when illuminated with white light. Often used in heads-up displays.

Diffuse Reflection
Light which bounces off an object surface in many different directions. Light radiated from a matte surface is highly diffused.

Diffused Lighting
Scattered soft lighting from a wide variety of angles used to eliminate shadows and specular glints from profiled, highly reflective surfaces.

Electro-Magnetic Spectrum
The total range of wavelengths, extending from the longest (audio) to the shortest (gamma rays) which can be physically generated. This entire spectrum is potentially useful for imaging, well beyond just the visible spectrum.

Extension Tube
A cylindrical threaded tube used to change the magnification, effective focal length and field of view of a lens when inserted between the lens and imaging sensor.

Field-of-View
The 2D area which can be seen through the optical imaging system.

Filter
In optics, the material either reflects or absorbs certain wavelengths of light, while passing others.

Filtering
The use of an optical filter for picture or color enhancement in front of the camera lens or light source.

Flange Focal Length (FFL)
The distance from the flange surface of the lens (contact point between camera and lens) and the focal point of the lens is the flange focal length. It is always fixed and dependent on mount type.

Examples of mount types and their FFL:
C-mount = 17.526mm
CS-mount = 12.50mm
F-mount = 46.50mm
T-mount = 55mm

Fluorescence
The emission of light or other electromagnetic radiation at longer wavelengths by matter as a result of absorption of a shorter wavelength. The emission lasts only as long as the stimulating irradiation is present.

F-number or F-stop
The ratio of the focal length to the lens aperture. The smaller the f- number, the larger the lens diameter and brighter the image and narrower the depth-of-field.

Focal Length (Effective Focal Length or EFL)
The rays from an infinitely distant object are directed by the lens to a common point of the optical axis, called the focal point.

A lens has two principal nodal points: a primary and secondary principal point. The distance from the secondary principal point to the focal point on the optical axis (where the imager is placed) is the focal length of the lens.

Focal Plane
Usually found at the image sensor, it is a plane perpendicular to the lens axis at the point of focus.

Focus
The point at which rays of light converge for any given point on the object in the image. Also called the focal point.

Front End System
The object, illumination, optics and imager blocks of a vision system. Includes all components useful to acquire a good image for subsequent processing.

Front Lighting
The use of illumination on the camera side of an object so that surface features can be observed.

Glints
Shiny, specular reflections from smooth objects or surfaces.

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 moire grating of parallel dark and light stripes is an example. Also used for structured light projection.

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

Hue
One 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.

Illumination
Normally a wavelength or range of wavelengths of light or visible light 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 (ie 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 Format Size
There are four main image sensor format sizes commonly used in CCTV cameras: 1", 2/3", 1/2", 1/3". These dimensions are the width plus height, rounded to the nearest convenient fraction of an inch.

A lens designed for a specific format can always be used with a smaller format sensor, but should not be used with a larger format device. For instance: a 1/2" format lens can be used with a camera having a 1/2" or a 1/3" imager but should not be used with 2/3" or 1" format cameras.

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

Incident Light
Light which 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 above the visible spectrum.

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

Laser Illumination
Lighting an object with a laser source for frequency selection, pulse width (strobe) control or for accurate positioning.

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.

Line Scan Camera
A solid state video camera consisting of a single row of pixels. Also called a linear array camera.

Lighting
See illumination.

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.

Machine Vision
The use of devices for optical non-contact sensing to automatically receive and interpret an image of a real scene, in order to obtain information and/or control machines or processes.

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.

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.

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.

Object Features
Any characteristic that is descriptive of an image or region, and useful for distinguishing one from another. A feature may be any measurable item such as length, size, number of holes, surface texture amount or center of mass.

Object Plane
An imaginary plane at the object, which is focused by the optical system at the image plane on the sensor.

Oblique Illumination
A lighting direction at an angle which emphasizes object features by shadows produced.

Opaqueness
Degree to which an object does not transmit light.

Photometry
Measurement of light which is visible to the human eye.

Photopic Response
The color response of the eye's retinal cones.

Pixel
An acronym for "picture element." The smallest distinguishable and resolvable area in an image. The discrete location of an individual photo-sensor in a solid state camera.

Polarized Light
Light which has had the vibrations of the electric or magnetic field vector typically restricted to a single direction in a plane perpendicular to its direction of travel. It is created by a type of filter which absorbs one of the two perpendicular light rays. Crossing polarizers theoretically blocks all light transmission.

Polarizer
An optical device which converts natural or unpolarized light into polarized light by selective absorption of rays in one direction, and passing of rays perpendicular to the polarizing medium. Usually fabricated from stretched plastic sheets with oriented, parallel birefringent crystals. The first polarizers were constructed with parallel wires.

Prism
An optical device with two or more non-parallel, polished faces from which light is either reflected or refracted. Often used to redirect light as in binoculars.

Reflection
The process by which incident light leaves the surface from the same side as it is illuminated.

Refraction
The bending of light rays as they pass from one medium (ie air) to another (ie glass), each with a different index of refraction.

Repeatability
The ability of a system to reproduce or duplicate the same measurement. The total range of variation of a dimension is called the 6-sigma repeatability.

Resolution, Pixel Grayscale
The number of resolvable shades of gray (ie: 256).

Resolution, Image
The number of rows and columns of pixels in an image.

Resolution, Spatial
A direct function of pixel spacing. Pixel size relative to the image field of view is key.

Resolution, Feature
The smallest object or feature in an image which may be sensed.

Resolution, Measurement
The smallest movement measurable by a vision system.

Reticle
An optical element with a pattern located in the image plane to assist in calibration, measurement or alignment of a system or instrument. Examples are cross lines or grids.

RGB
An acronym for the Red-Green-Blue color space. This three primary color system is used for video color representation.

Ringlight
A circular lamp or bundles of optical fibers arranged around the perimeter of an objective lens to illuminate the object in the field below it. A wide variety of sizes are available on both a stock and custom basis.

Saturation
The degree to which a color is free of white. One of the three properties of color perception along with hue and intensity (HSI).

Shutter
An electrical or mechanical device used to control the amount of time the imaging surface is exposed to light. Often used to stop blur from moving objects.

Simple Lens
A lens with only a single element.

Sinusoidal Projection
Use of a grating in which the dark stripes vary in their density sinusoidally across each one, rather than constant black. Improved profile or range discrimination is possible when used in a moire type configuration.

Spectral Analysis
Evaluation of the wavelength composition of object irradiance.

Spectral Characteristics
The unique combination of wavelengths of light radiated from a source or transmitter or reflected from an object.

Spectral Response
The characteristic of a sensor to respond to a distribution of light by wavelength in the electromagnetic spectrum.

Specular Reflection
Light rays that are highly redirected at or near the same angle of incidence to a surface. Observation at this angle allows the viewer to "see" the light source.

Strobe Duration
The amount of time, expressed in microseconds, during which the flash lamp (strobe) is at 90% intensity.

Strobed Light
Brief flashes of light for observing an object during a short interval of time, typically used to "stop" movement and resulting image blur. Strobes may use xenon flash tubes, banks of LEDs or a laser to illuminate the scene.

Structured Light
Points, lines, circles, sheets and other projected configurations used to directly determine shape and/or range information by observing their deformation as it intersects the object in a known geometric configuration.

Subpixel Resolution
Mathematical techniques used on gray scale images to resolve an edge location to less than one pixel. A one tenth pixel resolution is reasonable in the factory.

Thickness
The measurement in the third dimension (length and width being the other two) from one object surface to another using one or two 3D range sensors or other technique.

Translucent
An object characteristic in which part of the incident light is reflected and part is transmitted. The transmitted light emerges from the object diffused.

Ultraviolet
The region of the electromagnetic spectrum adjacent to the visible spectrum, but of higher frequency (shorter wavelength) than blue ranging from 1 to 400 nm. UV A ranges from 320 to 400 nm while UV B falls between 280 and 320 nm.

Visible Light
The region of the electromagnetic spectrum in which the human retina is sensitive, ranging from about 400 to 750 nm in wavelength.

Wavelength
The distance covered by one cycle of a sinusoidally varying wave as it travels at or near the speed of light. It is inversely proportional to frequency.

Xenon Strobe
A gas filled electronic discharge tube, useful for high speed, short duration illumination for inspection.