Charge Coupled Device Cameras

Question 1

What were CCDs originally developed for use in?

Answer 1

Computer memory devices.

Question 2

What field first realised the potential for CCDs in image production and for what reason?

Answer 2

Astronomy due to the low light intensity.

Question 3

What are the five advantages of CCDs over conventional photographic film?

Answer 3

• Much more sensitive to light
• Image may be built up over a long time (long exposure length)
• Image consists of electrical signals and thus may be enhanced electronically
• Can be used to produce moving images (videos) without the need for chemical processing.
• Digitised image may be converted to an on screen image, making surgery easier

Question 4

What part of a medical endoscope do CCDs replace?

Answer 4

The magnifying lens.

Question 5

What is the common usage of CCDs, other than endoscopy?

Answer 5

Digital cameras.

Question 6

What type of electrical components makes up the photosites on a CCD chip?

Answer 6

A metal-oxide-semiconductor capacitor array (MOS array).

Question 7

How much of the overall image does an individual photosite contribute to?

Answer 7

A single picture element (1 pixel).

Question 8

Photosites are also known as what?

Answer 8

Photodiodes.

Question 9

What is the purpose of a photosite?

Answer 9

To store the charge produced when incident photons liberate electrons from atoms.

Question 10

How is the image represented quantitatively in each photosite?

Answer 10

By the distribution of electrons that build up at different photosites during exposure.

Question 11

What does the quality/resolution of the final image depend on, and how would you increase the image resolution?

Answer 11

The number of photosites in the area where the image is formed. Improve the image by increasing the number of photosites.

Question 12

What is quantum efficiency?

Answer 12

The percentage of photons incident on the image forming area used to produce an image.

Question 13

To prevent blurring/overexposure of parts of the image, what must be constant across all photosites and why?

Answer 13

The quantum efficiency must be the same across all photosites.
This ensures the relative intensity of all parts of the image remains the same as the scene being recorded.

Question 14

What is the quantum efficiency of the retina?

Answer 14

Around 1%.

Question 15

What is the quantum efficiency of a conventional photographic plate?

Answer 15

4%.

Question 16

What is the quantum efficiency of a standard CCD chip?

Answer 16

70%.

Question 17

The high quantum efficiency of a CCD has what two practical advantage over conventional photographic film in cameras?

Answer 17

• Allows images to be formed in low light conditions
• -Enables much shorter exposure times under normal lighting conditions therefore many more images may be taken within a given time.

Question 18

Outline the five steps in transferring the charge on each photosite into an image on a screen.

Answer 18

1) Clock pulse shifts charge on each site vertically downwards so that charge on each photosite is transfered to the one below
2) Bottom row is fed into a shift register where the charge is measured
3) Shift register outputs a series of pulses representing the charge level of the photosite.
4) Another clock pulse shifts the charge out of the shift register horizontally, emptying it.
5) Electrical system identifies which charge level corresponds to which pixel on the screen, allowing the image to be built up. The cycle repeats.

Question 19

What three colours are mixed to produce coploured images?

Answer 19

Red, blue and green.

Question 20

What does the final colour of an image depend on?

Answer 20

The relative intensity of each of the primary colours.

Question 21

How do cameras used for astronomy and expensive video cameras produce coloured images?

Answer 21

• Three different CCDs produce three different images
• Mirros and lenses direct light to each CCD
• In front of the CCD is placed a filter (either red, green or blue) so each image depends on just two primary colour
• These individual images are processed, producing the final overall picture