Image Acquisition

Question 1

Image Acquisition

Answer 1

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

Question 2

Image Acquisition (lens)

Answer 2

Focuses light onto sensor
Simple convex single lens example
-F2 the second focal point - a point on the optic axis where parallel light rays converge
f is the focal length of the lens

Question 3

Infrared filter

Answer 3

The eye is not sensitive to infrared
Camera sensor is sensitive to infrared light
Infrared filter reduces the amount of IR that reaches the sensor

Question 4

Colour filter array

Answer 4

Two common configurations:
- Stripe
- Bayer

Twice as many green filter elements as red or blue
This ratio mimics human sensitivity to green light

Red filter elements only let red light through
In a digital image a pixel is assigned an RGB value
- Must interpolate between the RGB values in the neighbourhood of the pixel

Question 5

Sensor

Answer 5

Two main types - CCD, CMOS
Both types consist of lightly packed light sensitive photodiodes constructed on a silicon wafer
Electrical changes converted into voltages that determine pixel brightness

Question 6

Photodiodes

Answer 6

Semiconductor devices that generate in electrical charge in proportion to the number of photons that reach them

Question 7

CCD Sensors

Answer 7

Charged Couple Device
Advantages of CCD
- Proven record of technologies and commercialization
Low noise, high S/N

Disadvantages:
High power consumption, slower speed
On-chip peripheral circuits difficult to manufacture

Question 8

CMOS Sensors

Answer 8

Complementary Metal Oxide semiconductor

Advantages:
Manufacture can be simpler and less expensive
Use less power than CCD

Disadvantages:
Relatively high noise

Question 9

DSP module

Answer 9

Voltages are read out from the image sensor and fed into an onboard image processing module
The image processing module contains propriertary algorithms for improving the percieved quality , demosaicing and compression

Question 10

Demosaicing

Answer 10

Green channel: bilinear interpolation
Blue values are interpolated in the same manner
Digital cameras often contain proprietary interpolation algorithms
In this context interpolation is often referred to as demosacing.