How do motion sensing lights and Burglar Alarms work?

T­here are many different ways to create a motion sensor. For example:

• It is common for stores to have a beam of light crossing the room near the door, and a photosensor on the other side of the ­room. When a customer breaks the beam, the photosensor detects the change in the amount of light and rings a bell.
• Many grocery stores have automatic door openers that use a very simple form of radar to detect when someone passes near the door. The box above the door sends out a burst of microwave radio energy and waits for the reflected energy to bounce back. When a person moves into the field of microwave energy, it changes the amount of reflected energy or the time it takes for the reflection to arrive, and the box opens the door. Since these devices use radar, they often set off radar detectors.
• The same thing can be done with ultrasonic sound waves, bouncing them off a target and waiting for the echo.

All of these are active sensors. They inject energy (light, microwaves or sound) into the environment in order to detect a change of some sort.

The "motion sensing" feature on most lights (and security systems) is a passive system that detects infrared energy. These sensors are therefore known as PIR (passive infrared) detectors or pyroelectric sensors. In order to make a sensor that can detect a human being, you need to make the sensor sensitive to the temperature of a human body. Humans, having a skin temperature of about 93 degrees F, radiate infrared energy with a wavelength between 9 and 10 micrometers. Therefore, the sensors are typically sensitive in the range of 8 to 12 micrometers.

The devices themselves are simple electronic components not unlike a photosensor. The infrared light bumps electrons off a substrate, and these electrons can be detected and amplified into a signal.

You have probably noticed that your light is sensitive to motion, but not to a person who is standing still. That's because the electronics package attached to the sensor is looking for a fairly rapid change in the amount of infrared energy it is seeing. When a person walks by, the amount of infrared energy in the field of view changes rapidly and is easily detected. You do not want the sensor detecting slower changes, like the sidewalk cooling off at night.

Your motion sensing light has a wide field of view because of the lens covering the sensor. Infrared energy is a form of light, so you can focus and bend it with plastic lenses. But it's not like there is a 2-D array of sensors in there. There is a single (or sometimes two) sensors inside looking for changes in infrared energy.

If you have a burglar alarm with motion sensors, you may have noticed that the motion sensors cannot "see" you when you are outside looking through a window. That is because glass is not very transparent to infrared energy. This, by the way, is the basis of a greenhouse. Light passes through the glass into the greenhouse and heats things up inside the greenhouse. The glass is then opaque to the infrared energy these heated things are emitting, so the heat is trapped inside the greenhouse. It makes sense that a motion detector sensitive to infrared energy cannot see through glass windows.

Fingerprint Verification

What is the fingerprint verification technology?

A fingerprint in its narrow sense is an impression left by the friction ridges of a human finger. In a wider use of the term, fingerprints are the traces of an impression from the friction ridges of any part of a human or other primate hand. 

Since the early 20th century, fingerprint detection and analysis has been one of the most common and important forms of crime scene forensic investigation. More crimes have been solved with fingerprint evidence than for any other reason. This fact necessitated the need for assailants to cover their hands during the commission of their crimes; thus designating gloves to be the most essential and crucial tool for any successful criminal perpetrator.

Fingerprint verification method?

There are two types of method, optical and capacitance. 

Optical fingerprint imaging involves capturing a digital image of the print using visible light. This type of sensor is, in essence, a specialized digital camera. The top layer of the sensor, where the finger is placed, is known as the touch surface. Beneath this layer is a light-emitting phosphor layer which illuminates the surface of the finger. The light reflected from the finger passes through the phosphor layer to an array of solid state pixels (a charge-coupled device) which captures a visual image of the fingerprint. A scratched or dirty touch surface can cause a bad image of the fingerprint. A disadvantage of this type of sensor is the fact that the imaging capabilities are affected by the quality of skin on the finger. For instance, a dirty or marked finger is difficult to image properly. Also, it is possible for an individual to erode the outer layer of skin on the fingertips to the point where the fingerprint is no longer visible. It can also be easily fooled by an image of a fingerprint if not coupled with a "live finger" detector. 

Capacitance sensors use principles associated with capacitance in order to form fingerprint images. In this method of imaging, the sensor array pixels each act as one plate of a parallel-plate capacitor, the dermal layer (which is electrically conductive) acts as the other plate, and the non-conductive epidermal layer acts as a dielectric

Coverage

It is widely used in access control, building management, bank, airport information system, etc

Video Transmission & Compression

During the past 18 years, traffic and freeway management agencies have been integrating the use of CCTV cameras into their operational programs. The heavy use of this technology has created a need to deploy very high bandwidth communication networks. The transmission of video is not very different from voice or data. Video is transmitted in either an analog or digital format. Video transmitted in an analog format must travel over coaxial cable or fiber optic cable. The bandwidth requirements cannot be easily handled by twisted pair configurations.

Video can be transmitted in a digital format via twisted pair. It can be transmitted in a broadband arrangement as full quality and full motion, or as a compressed signal offering lower image or motion qualities. Via twisted pair, video is either transmitted in a compressed format, or sent frame-by-frame. The frame-by-frame process is usually called "slow-scan video".

Full color broadcast analog video requires a substantial amount of bandwidth that far exceeds the capacity of the typical twisted pair analog voice communication circuit of 4 KHz. Early commercial television networks were connected via Coaxial cable systems provided by AT&T Long Distance. These networks were very costly to operate and maintain, and had a limited capability.

Transmission of analog video requires large amounts of bandwidth, and power. The most common use of analog video (outside of commercial broadcast TV) is for closed circuit surveillance systems. The cameras used in these systems use less bandwidth than traditional broadcast quality cameras, and are only required to send a signal for several hundred feet. For transmission distances (of analog video) of more than 500 feet, the system designer must resort to the use of triaxial cable, or fiber optics. Depending upon other requirements, the system designer can convert the video to another signal format. The video can be converted to a radio (or light) frequency, digitized, or compressed.

Cable companies have traditionally converted television broadcast signals to a radio frequency. With this technique, they can provide from 8 to 40 analog channels in a cable system using coaxial cable (more about multiplexing later in this chapter). Cable company operators wanting to provide hundreds of program channels will convert the video to a radio frequency, and then digitize. The cable company is able to take advantage of using both fiber and coaxial cable. These are called HFC (hybrid fiber coax) systems. Fiber is used to get the signal from the cable company main broadcast center to a group of houses. The existing coaxial cable is used to supply the signal to individual houses.

Early freeway management systems used analog video converted to RF and transmitted over coaxial cable. Later systems used fiber optic cable with either RF signal conversion, or frequency division multiplexing (see Multiplexing in this chapter).

With the introduction high bandwidth microprocessors and efficient video compression algorithms, there has been a shift from analog video transmission systems to digital systems. New processes such as Video over IP (Internet Protocol) and streaming video allow for the broadcast of video incident images to many user agencies via low (relatively) cost communication networks. Before looking at the systems, let's take a look at the various types of video compression schemes.

Video Compression

Compressed Video – Since the mid-1990s, FMS system designers have turned to digital compression of video to maximize resources, and reduce overall communication systems costs. The digital compression of video allows system operators to move video between operation centers using standard communication networks technologies.

Video compression systems can be divided into two categories – hardware compression and software compression. All video compression systems use a Codec. The term Codec is an abbreviation for coder/decoder. A codec can be either a software application or a piece of hardware that processes video through complex algorithms, which compress the file and then decompress it for playback. Unlike other kinds of file-compression packages that require you to compress/decompress a file before viewing, video codecs decompress the video on the fly, allowing immediate viewing. This discussion will focus on hardware compression technologies.