Image Acquisition Flashcards
(12 cards)
How does taking a photo work?
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.
How does the lens work?
Focuses light onto sensor.
What do the letters y, y’, s, s’, and fa stand for on a lens acquisition diagram?
Y= height of object
Y’ = height of image
S = distance of object from lens
S’ = distance of focused image from lens
F = the focal length
Equation for object image relation
1/s + 1/s’ = 1/f
(1/distance of object from lens + 1/ distance of focused image from lens = 1/ the focal length).
Equation for magnification
Y’/y
(Height of image/height of object)
Or
-(s’/s)
(-(distance of focused image from lens/distance of object from lens).
What does the infrared filter in a camera do?
The eye is not sensitive to infrared light
Camera sensor is sensitive to infrared light
Infrared filter reduced the amount of IR that reaches the sensor.
Colour filter array
Two common types:
- stripe
- bayer
Twice as many green filter elements as red or blue
Its ratio mimics human sensitivity to green lights.
Red filter elements only let red light through etc.
In a digital image a pixel is assigned an RGB value.
- must interpolate between the RGB values in the neighbourhood of pixels.
Sensor
Main types:
- CCD
-CMOS
Both types consist of tightly packed light sensitive photodiodes constructed on a silicone wafer.
Photodiodes
- semiconductor devices that generate an electrical charge in proportion to the number of photons that reach them.
Electrical charges converted into voltages that determine pixel brightness.
Advantages and disadvantages of CCD sensors
Advantages:
- proven record of technologies and commercialisation.
- low noise, high S/N.
Disadvantages
- high power consumption, slower speed
- on-chip peripheral circuits difficult to manufacture.
Advantages and disadvantages of CMOS sensors
Advantages
- Manufacture can be simpler and less expensive.
- use less power than CCD
Disadvantages
- relatively high noise.
DSP module
Voltages are red out from the image sensor and fed into an onboard image processing module.
The image processing module contains proprietary algorithms for improving the perceived quality (e.g. white balance and smoothing), demosaicing and compression.
Demosaicing
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 demosaicing.
