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1

What is the first principal of the ACPO guidelines?

No action taken by law enforcement agencies should change data held on a computer or storage media which may subsequently be relied upon in court.

2

What is the second principal of the ACPO guidelines (competence)?

In circumstances where a person finds it necessary to access original data held on a computer or on storage media, that person must be competent to do so and be able to give evidence explaining the relevance and the implications of their actions.

3

What is the third principal of the ACPO guideline?

An audit trail of all processes applied to computer-based electronic evidence should be created and preserved.

An independent third party and should be able to examine those processes and achieve the same result.

4

What is the fourth principal of the ACPO guidelines?

The person in charge of the investigation has overall responsibility for ensuring that the law and these principles are adhered to.

5

What are the key sections of the Computer Misuse Act?

  • Section 1: Unauthorised access to computer material.
  • Section 2: Unauthorised access with intent to commit or facilitate the commission of a further offence.
  • Section 3: Unauthorised modification of computer material.

6

What are the key sections of the Protection of Children Act (1978)?

Section 1:

  • Taking, making or possessing;
  • Distributing;
  • Possessing with a view to distributing;

An indecent photograph of a child.

7

What did the Criminal Justice and Public Order act add to POCA?

Amended S1 of POCA to include pseudophotographs.

8

What did the Sexual Offences Act add to POCA?

  • Increased age of a child from 16 to 18
  • Added a defence where an incident photograph of a child over the age of 16 was created by the child's long-term partner
  • Added a defence where it is necessary to create an indecent image of a child for criminal investigation.

9

What principle is facial reconstruction based on?

  • Facial reconstruction is based on the principle that there is a predictable relationship between the skull and overlying soft tissues.
  • This form of ID is used when facial features are severely damaged or decomposed beyond recognition.

10

What are the five steps of the Enhanced Cognitive Interview?

  1. Rapport building
  2. Recreate the context of the original event
  3. Open-ended narration
  4. Questioning
  5. Closure

11

What is Rapport Building?

  • Sets witness at ease, placing them in the correct frame of mind.
  • Transfer control to the witness.
    • Witness holds the key to a successful interview and should be made aware of this fact.
    • Let the witness choose at which point to start.
    • Listen and allow for pauses.

12

What is Context Reinstatement?

  • Most effective component to the CI
  • Invite witness to close their eyes and place themselves back at the scene.
  • Remember associated events including sights, smells and sounds.
  • Record any emotions.

13

What is the Open-Ended Narration?

  • Referred to as free-flowing narration
  • Witness describes the event of their own accord without prompts from the operator/interviewer.

14

What is the questioning phase of the CI?

  • Interviewer pays a more central role than in open-ended narration but the witness remains in control.
  • Focused retrieval: Open-ended questions, do not interrupt, allow for long pauses.
  • Witness compatible questioning: Conduct the interview at the witness's pace rather than dictating the pace.
  • Avoid biased questions.

15

What is Closure?

  • Brief the witness as to what will happen next
  • Exchange contact details if they remember anything else.
  • Leave the witness in a positive frame of mind.

16

What are the differences between E-FIT and EFIT-6?

  • E-FIT is a feature-based composite system; face is constructed by selecting and arranging individual features.
  • E-FIT 6 is a holistic-based composite system; adapt a whole-face approach and appeal to the human capability to recognise faces.
    • Works on the notion of face space in which a face is represented as a point in a multidimensional space.

17

Why are holistic composites better than feature-based systems?

Holistic composites are better than feature-based systems as it has been shown that we recognise faces holistically rather than a sum of individual features.

It uses the notion of face space in that a face is represented as a point in multidimensional space, which helps memory recall.

18

What are the stages of image acquisition?

  • The lens focuses the image on the sensor
  • Sensitivity to infrared light is reduced by the filter.
  • CFA compensates for 'colour blindness' of the sensor.
  • Sensor turns light into a recordable image.
  • DSP unit performs some basic image processing before the image is saved.

19

What role does the lens play?

The lens focuses light onto the sensor.

s = distance of object from the lens

s' = distance of focused image from the sensor

f = focal length

20

What is the role of the infrared filter?

The infrared filter reduces the amount of IR that reaches the sensor, as the camera is sensitive to IR.

The eye is not sensitive to IR light.

21

What is the role of the Colour Filter Array?

  • The two common configurations are the stripe and Bayer
  • In both configurations, there are twice as many green filter elements as there are blue and red; mimics human sensitivity to green light.
  • The CFA separates natural light into its individual colour components.

22

What is the role of the sensor?

  • Sensors are made from tightly packed light-sensitive photodiodes constructed on a silicon wafer.
  • Photodiodes are semiconductor devices that generate an electrical charge in proportion to the number of photons that reach them.
  • Electrical charges are converted into voltages that determine pixel brightness.

23

What are the advantages and disadvantages of the CCD sensor?

  • Advantages:
    • Low noise, high S/N ratio.
    • Proven records of technologies and commercialisation
  • Disadvantages:
    • High power consumption
    • Slower speed
    • On-chip peripheral circuits are difficult to manufacture.

24

What are the advantages and disadvantages of the CMOS sensor?

Advantages:

  • Manufacture is simple and cheaper.
  • Uses less power than CCD

Disadvantages:

  • Relatively high noise

 

25

What is the role of the DSP module?

  • Voltages are read from the image sensor and are fed into an onboard image processing module.
  • Image processing module contains algorithms for improving perceived quality e.g. WB, smoothing, demosaicing and compression

26

What is the process of demosaicing?

  • Reconstructs a full-colour image from incomplete colour samples output from the CFA.
  • Uses interpolation.

27

What are the three types of data redundancy?

  • Coding redundancy: caused by suboptimal codewords for symbol encoding - symbol represents a grey-level
  • Interpixel redundancy: due to grey-level correlations between neighbouring pixels - justifies image transformation.
  • Psycho-visual redundancy: information contained within an image that is superfluous to the interpretation/aesthetics of an image. Clarity depends on spatial frequency and amplitude.

28

What are the basics of JPEG image compression?

  1. Split the image into 8x8 blocks and treat each block separately.
  2. Apply DCT to each block.
  3. Quantise the DCT coefficients (transformed 8x8 block).
  4. Apply Huffman coding scheme to quantised coefficients.
  5. Save the result as a .jpg compressed file.

29

What is PRNU and its evidential use?

  • Photo Response Non-Uniformity (PRNU) is variation in pixel sensitivity caused by manufacturing defects and natural NU of the silicon used in the image sensor.
  • Pattern noise is consistent, and unique to the image sensor and can be used to match an evidential image to its source camera. It occurs in the same pixel locations in every image.

30

What is Hashing?

  • A hash is a transformation of data into a message digest that is unique to the data - process is not reversible
  • Typical algorithms are Secure Hashing Algorithm (SHA-1) and Message Digest 5 algorithm (MD5)