Biometrics & Security

Tutorial 7

• 1 (a) Please compare two different kinds of biometrics technologies: Retina and Iris. (P8:2-3)

• 1 (b) Understand the retina recognition system. Please give the differences between retinal identification and retinal authentication. (P8: 10-11)

• 1 (c)P8:20 show a representative example of identification system. Please give another example of verification system.

• 1 (d) Please understand the iris recognition system. (P8: 22-36)

• 1 (e) What is texture feature (P8: 30)? Why we can use texture feature in iris recognition (P8: 31)?

• 2. There are four steps in the Daugman’s approach (P8: 32-36). The third step generates IrisCode with 512 bytes. If 2 bits represent a feature, please compute the total number of features. (512*(8/2)=2,048)

• Hamming distance: the distance between two vectors A and B is ∑ | Ai - Bi |.

General idea of Iris Code

• 3. In P8:37, an example of iris verification distributions is given. Notice that Hamming distance is defined to measure the similarity of two IrisCodes.

• Which value in the figure can separate two parts, authentics and imposters?

• If the similarity between two IrisCodes is taken as 0.5, can you say they are identical?

• How about the comparison result when the similarity is 0.2?

• What conclusion will you get from the hamming distance used to compute similarity of IrisCodes? (0.38; no; yes)

True Acceptance Rate

• A genuine individual is accepted. (TAR)

T hre shold (M a tc h ing sc ore s)

T A R

0

1

Total True AcceptanceTAR

Total True Attempts

Threshold=0 means that the attempts whose matching score >0 will be accepted; (everybody will be accepted)

Threshold=∞ means that the attempts whose matching score > ∞ will be accepted (nobody will be accepted)

False Rejection Rate

• A genuine individual is rejected. (FRR)

Total False RejectionFRR

Total True Attempts

T hre shold (M a tc h ing sc ore s)

F R R

0

1

Threshold=0 means that the attempts whose matching score <0 will be rejected;(nobody will be rejected)

Threshold=∞ means that the attempts whose matching score < ∞ will be rejected(everybody will be rejected)

True Rejection Rate

• A impostor is rejected. (TRR)

Total True RejectionTRR

Total False Attempts

T hre shold (M a tc h ing sc ore s)

T R R

0

1

Threshold=0 means that the attempts whose matching score <0 will be rejected;(nobody will be rejected)

Threshold=∞ means that the attempts whose matching score < ∞ will be rejected(everybody will be rejected)

False Acceptance Rate

• A impostor is accepted. (FAR)

Total False AcceptanceFAR

Total False Attempts

T hre shold (M a tc h ing sc ore s)

F A R

0

1

Threshold=0 means that the attempts whose matching score >0 will be accepted; (everybody will be accepted)

Threshold=∞ means that the attempts whose matching score > ∞ will be accepted (nobody will be accepted)

ROC

• There are some trade off among TAR, FRR, TRR, FAR.

• We need a particular threshold to keep the TAR and TRR as high as possible and to keep FAR and FRR as low as possible.

• We need the ROC curve to find the optimal threshold.

• We also use ROC curve to evaluate the system

ROC(2)

T A R

0

1

1

R O C C urve

F A R

F A R

0

1

1

R O C C urve

F R R

Find the optimal point (threshold)

• 4. P8:27 shows the corresponding Fourier transforms of iris images with different quality. Please give the property of Fourier transform of good quality iris images. (Even distribution)

Fourier Transform

• Any signal can be expressed as a weighted sum of a series of sine or cosine wave

• a is the original signal• a = w1∙b + w2∙b + w3∙d• b is a low frequency component• d is a high frequency component

=

3 sin(x) A

+ 1 sin(3x) B A+B

+ 0.8 sin(5x) CA+B+C

+ 0.4 sin(7x) DA+B+C+D

A sum of sines and cosines

sin(x) A

• 5. In Daugman’s approach iris is regarded as a circular pattern (P8: 32-36). In fact we can also take the iris as other pattern, as shown in P8: 29. Please try to design the corresponding scheme.