Identifications from Video Surveillance Cameras

On several occasions we have been asked to examine and comment upon the quality of the still photo images extracted from security video surveillance cameras. Most video surveillance systems use an inexpensive single VHS recorder, one or more video cameras and a control unit which records from the individual cameras in a pre-determined controlled sequence. As the control unit cycles through its program, a camera is activated and records a selected number of frames of its field of view. The pictures are stored on the VCR tape in an electronic format known as the analog form. The analog format is prone to noise pick up and distortion that results in image quality deterioration, it is accepted that a 5% loss in image quality accompanies each copying or duplication of a videotape in the analog format. Many video camera surveillance systems use a single tape on which all the video images are stored in a serial sequence. When the tape is full it may be stored for a period of time and then re-used to record current data. The constant re-use of the tape also contributes to image degradation. Extraction of an image from the recorded data and any re-copying to tape or to a paper or "hard copy" format then gives rise to a numerical value for the resultant image quality degradation. Data is often taken from very poor video surveillance recordings and processed with image manipulation software such as "Photoshop" to alter image contrast and remove distracting background so as to produce a better image. Such data has been used to identify individuals and place them at a given location at a defined time.


Exploding Microwaves

The mechanism by which foods and water are heated in a microwave oven is very different from the normal contact heating of a stovetop element or the radiant heating of baking. Stovetops and conventional ovens are a source of high energy that must "flow" into the cooler object being heated. Heat flow requires time for energy to be transferred from the hot object to the cold. The container that holds the material being warmed restricts the rate at which the heat can be transferred.

Microwave heating energy does not "flow" but is "created" in the water of the material being warmed. The container holding the material being warmed does not absorb heat from a hot element or oven but is heated by absorbing heat from what it contains. The electronics of the microwave oven generate "microwaves" (the original microwaves were called Radar Ranges) which cause only the water in a foodstuff to heat. The generation of heat in the water component of a food takes place virtually instantaneously. If sufficient microwave energy is applied to a food the rapidly generated heat can make the water boil or turn to steam so quickly that articles like potatoes, eggs and beans explode. When microwave energy is used to heat water itself care must be exercised so that the container does not boil over.

Chemists learn about "boiling stones" early on in their practical laboratory work. In simplified terms the action refered to as "boiling" requires a site on which the transition from a liquid to a gas can take place. Tiny pieces of broken glass, or ceramics can provide a large number of microscopic sharp points and edges on which gas bubbles can form and the chemist observes a gradually increasing stream of bubbles evolving from liquids being heated as the boiling point is approached. If a very clean, smooth container of water is placed in a microwave and excessive energy applied, the liquid may be heated virtually to its boiling point but not have a point or edge on which to start boiling. In such a heated state the liquid may be referred to as being "superheated". If the vessel is vibrated or an object such as a speck of dust, coffee crystals, tea bag or spoon, drops into the superheated liquid a veritable explosion can follow as large numbers of sites are provided at which boiling can simultaneously take place.