Input technologies

• All require some form of data acquisition– e.g. Image scanner, Microphone

• Once acquired, if the signal is not already digital, it will need to be converted using a A to D converter.

• Image scanners for scanning fingerprints for example, tend to use one of two scanning technologies: optical and capacitive. – What is capacitive?

General recognition block diagram

Input Electronics

A to D Pre-proc Reco Output Electronics

Image capture

Sound capture

Pen capture

Monitor

Brail pins

Printer

Speaker

Intro to common components• Hardware is used at the extreme input and

output so a basic understanding is necessary• Resistor (Ohms)

– Current directly proportional to Voltage– Current inversely proportional to resistance

• Capacitor (Farads)– Charge storage– Current is proportional to capacitance and rate of

change of voltage– Capacitance is directly proportional to plate area and

inversely proportional to plate distance

Common components

• Voltage amplifier– Typically inverts so Vout = -A.Vin

• Charging a capacitor via a resistor– Capacitor charges non-linearly– http://www.lon-capa.org/~mmp/kap23/RC/app.htm

Resistor Capacitor

CR

t

eIi

ˆ :current ChargingCR

t

eIi

ˆ :current Discharge

CR

t

eV

ˆ v:Voltage Disharging )1(ˆ v:Voltage Charging CR

t

eV

Amplifiers

R1

-

+

Vout

C

Vin

R1

-

+Vin

R2

Vout = -Vin.R2/R1

dVCR

Vt

inin

out 0

1

Capacitive scanning

• Skin has electrical properties such as resistance and capacitance so can be used to control electric current

• A touch switch for example relies on either the resistance of the skin to bridge two contacts to pass an electric current or to bridge two contacts to connect a capacitance in a circuit.

• An array of electrical contacts placed at a resolution of less than the width of a finger print ridge

• The nearer the skin is to the contact, the higher the capacitance value – so a ridge would create a high capacitance value at each set of contacts it touches

• Each set of contacts connects the feedback loop of an integrating amplifier

R

i

n

P-

+

V

out

V

i

n

C

• Integrating amplifier. Keep R and t constant so Vout is dependant on C

dVCR

Vt

inin

out 0

1

Rin

P-

+

Vout

Vin

C

.

      

Microphone

• 3 main types– Dynamic

• Electromagnetic using moving coil

– Condenser• Electrostatic. Needs a voltage source

– Electret• Similar to condenser but does not need voltage

source

Dynamic microphone

• Like a loudspeaker in reverse

Condenser microphoneArea

CapacitanceDistance

•Sound cards provide voltage through connector

Electret microphone

• The electret condenser mic uses a special type of capacitor which has a permanent voltage built in during manufacture. This is can be likened to a permanent magnet, in that it doesn't require any external power for operation. Otherwise it operates in the same way as a condenser microphone

Capacitance scanner operation

• Finger is placed on the scanner• All capacitances are short circuited to

discharge any residual charge in the contacts

• The resistance is constant and a DC voltage is applied to the input for a fixed time

• The voltages at the outputs of the integrators is input to an A to D converter

Loudspeaker

Optical Scanning

• Generally use a Charge Coupled Device (CCD). – Array of photodiodes which generate an

electric current in response to being struck by light photons – i.e. output is analogue.

– An A to D converter converts the analogue variation from each cell into a binary number representing brightness

Image Sensor Operation

• Charge Coupled Device image sensor

• Background is image sensor zoomed in

• CCD measures brightness• Tiny lenses direct light onto filtered

photosensitive regions• More green than red to better match the eye

Bayer filter

• Eye responds mostly to green so as many green filters as red+blue

• Demosaic – interpolate a single pixel colour by interpolating nearest neighbours as each pixel only records one colour so the actual colour at that point is the average of it and the surrounding pixels

CCD Operation

CCD Operation

• Charge is moved down 1 row at a time then clocked out to an amp and A to D converter

• CCD device used to photograph the finger when placed on the glass

• Light source is usually an array of LEDs to enable sharp capture of fingerprint ridges– Image processing techniques (covered later in

the module) check the contrast and sharpness of the captured image and adjusts the light source (or generates instructions to the user) if necessary

Pros, Cons and Limitations

• Capacitance scanner can not be shown a picture of a fingerprint for recognition so is not as easily fooled as CCD

• Capacitance scanners tend to be smaller• Both can be fooled by latex copy or by chopping

off someone's finger • Some systems have heat / pulse sensors

– Can put latex copy over real finger to fool these

• Better to use additional biometrics / password

Fingerprint Recognition

• Comparing one whole fingerprint to another by direct comparison is infeasible as the contrast may be different, pressure and orientation may be slightly different …

• Most recognition technologies look for specific features whether this is fingerprint or speech recognition as a lot of the data defining uniqueness is redundant

Fingerprint characteristics• The scanned image shows ridges as black

and valley’s as white areas

• Characteristics are called minutiae (mi-NOO-shee-ee )– Ridge ends (A)– Bifurcations (B) ridge splits from 1 to 2

• Image needs to be pre-processed to enhance it

• Algorithms needed to used to identify minutiae

General recognition block diagram

Input Electronics

A to D Pre-proc Reco Output Electronics

Image capture

Sound capture

Pen capture

Monitor

Brail pins

Printer

Speaker

Summary

• As will be seen with most recognition technologies:– data is captured– analogue to digital conversion– enhanced (e.g. contrast enhance)– Feature extraction (in this case minutiae)– Recognition algorithm