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CCTV Products and Reviews Directort of Companies and Shops Image Capture
Image Processing
Glossary of Terms
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Imaging for CCTV, Security and SurveillanceImage CaptureThis section relates to the aquisition of a digital
video image from the surveillance target. It covers the physical aspects
of acquiring an image and the equipment used. Lens and OpticsThe human eye and vision system. |
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An image is a representation of how the human eye and brain see a scene. The source of the image does not have to be limited to conventional visible light reflected or radiated from objects but extends to include aftificial images created from data such as medical images from an MRI scanner of X rays. Infra-red and ultra-violet light are often used to reveal information normally unseen by the human eye. Even audio sound waves (sonar) and microwaves (radar) are used to gather data which can be represented as images. The image is a presentation of information in a form suitable for the human eye. Whatever the image source, the information is presented as varying intensities of visible light. See the section # 'the digital image' for details on common digital representations of image information.
An interresting example of the use of infra-red is for capturing images of vehicle number plates or license plates. The varying illumination from visible light is removed by filters and the scene is illuminated from an infra-red source close to the camera lens. License plates are made from an auto-reflective material like the high visibility reflector stripes on safety clothing. This means that the license plate reflects the light back from the direction it came. The result is an image of the license plate with very little else to confuse the image making OCR or Optical Character Recognition much simpler. There are of course many ways to solve this problem.
The diagram below shows the structure of the human eye. It can be thought of as the camera or image sensor while parts of the brain can be thought of as providing image processing to interpret the visual information. Briefly, the iris is the coloured disk which give you those big blue eyes. Its purpose is to block light, leaving only the pupil to pass light. The pupil is a varying diameter hole in the iris which is used to control the exposure level or amount of light entering the eye. The pupil enables the eye to work over widely varying lighting levels. The lens focuses the light onto the retina at the back of the eye. Its shape changes to focus the light depending on how far the object is from the eye. The retina contains millions of light sensitive cells which turn the light falling on them into tiny electrical signals. These signals are caried by the optical nerve to the brain for interpreting. Two types of light sensitive cells are found in the retina, these are rods and cones. Rods are the most abundant and the most sensitive with a broad spectral response making them good for sensing in low light but unable to resolve different colours. Cones are less abundant and less sensitive but come in 3 colour sensitive varieties. Red, green and blue sensitive cones are consentrated at the fovea which is the part of the retina where light from looking straight ahead falls. The distribution of rods and codes explains why our low light night vision is best just off the straight ahead axis and why our colour vision is best for a brightly illuminated object straight ahead.
[logarithmic intensity response, mach band effect, optical illusions to demonstrate perception]
[diagrams - eye, distribution of rods and cones, log intensity response, mach band effect, optical illusions]
The purpose of a lens is to collect and focus
light from the monitored scene onto the image sensor surface. Light behaves
like waves and its path can be bent by passing through gradients of different
refractive index. The refractive index is a measure of the speed which light
travels through the medium and therefore by how much it is bent. Air changes
its refractive index as its temperature changes. This is what causes stars
to twinkle or the horizon to shimmer as turbulent air at slightly different
temperatures causes a small random bending of the light. A lens uses a material
such as glass or plastic with a different refractive index to the surrounding
air. The lens shape then defines by how much light is bent when passing through
it.
Before seeing how a lens works, it is worth mentioning two basic measures
of a light source. The total amount of light generated by a source is measured
in candela. This represents the total light energy available from the source
in any direction. The amount of light falling on a particular area is called
the flux and is measured in lumens. Two similar light sources will generate
the same total light energy or candela but one may provide double the luminous
intensity of the other by focusing the light generated more than the other.
Consider two imaginary points (A and B) on an image source plane such as the filament of a light bulb and two imaginary points (C and D) on an image sensor plane such as a CCD array or the eye's retina. Light from point A on the filament spreads out in all directions including some falling on point C and some on point D. The same applies for point B of the filament. There is no distinction possible between light from different parts of the filament. The whole sensor is evenly illuminated and every part of the image is the same as the rest. A lens is used between the source and sensor to focus the light from different parts of the source onto different parts of the sensor.
If an obstruction is placed between source and
sensor with a tiny hole to allow light through…