topic 1 - image acquisition Flashcards
image acquisition
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.
how is an image acquired through the lens?
object image relation:
1/s + 1/s’ = 1/f
magnification:
- s’/s
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.
how is an image acquired using an infrared filter?
The eye is not sensitive to infrared light.
Camera sensor is sensitive to infrared light.
Infrared filter reduces the amount of IR that reaches.
How is an image acquired using a colour filter array?
Two common configurations:
- Stripe
- Bayer
Twice as many green filter elements as red or blue.
This ratio mimics human sensitivity to green light.
how is an image acquired using a colour filter array pt 2?
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 the pixel.
how is an image acquired using a sensor?
main types:
- CCD - charged couple device
- CMOS - complimentary metal oxide semi-conductor - used more often due to modern technology
Both types consist of tightly packed light sensitive photodiodes constructed on a silicon 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.
what are the advantages and disadvantages of CCD sensors?
Charged Couple Device.
Invented in 1969 in Bell Labs.
Advantages of CCD
Proven record of technologies and commercialization.
Low noise, high S/N.
Disadvantages of CCD
High power consumption, slower speed.
On-chip peripheral circuits difficult to manufacture.
what are the advantages and disadvantages of CMOS sensors?
Complementary Metal Oxide Semiconductor.
Advantages of CMOS
Manufacture can be simpler and less expensive.
Use less power than CCD.
Disadvantages of CCD
Relatively high noise
how is an image acquired using a DSP module?
Voltages are read 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.
image acquisition (demosaicing) Green Channel
Green Channel: bilinear interpolation
image acquisition (demosaicing) Red Channel
Red channel: bilinear interpolation
