Presentation is loading. Please wait.

Presentation is loading. Please wait.

Digital Imaging with Charge- coupled devices (CCDs)

Similar presentations


Presentation on theme: "Digital Imaging with Charge- coupled devices (CCDs)"— Presentation transcript:

1 Digital Imaging with Charge- coupled devices (CCDs)

2 Capacitor

3 Q = CV

4 Example The capacitance of two parallel plates is 4.5pF. Calculate the charge on one plate when a voltage of 8.0 V is applied to the plates.

5 Example The capacitance of two parallel plates is 4.5pF. Calculate the charge on one plate when a voltage of 8.0 V is applied to the plates. Q = CV = 4.5 x x 8.0 = 3.6 x C = 36 pC

6 Charged-coupled device (CCD)

7 Silicon chip varying from 20 mm x 20 mm to 60 mm x 60 mm. Surface covered in pixels (picture elements) varying from 5 x m to 25 x m.

8 Charged-coupled device (CCD) Each pixel releases electrons (by the photoelectric effect) when light is incident on it. We may think of each pixel like a small capacitor.

9 Charged-coupled device (CCD) The electrons released in each pixel constitute a certain amount of charge Q, and so a potential difference V develops at the ends of the pixel (V = Q/C)

10 Charged-coupled device (CCD) The number of electrons released, and the voltage created across the pixel is proportional to the intensity of light.

11 Charge-coupled device (CCD) The charges on each row of pixels is pushed down to the next row until they reach the bottom row (the register)

12 Charge-coupled device (CCD) The charges are the moved horizontally, where the voltage is amplified, measured, and and passed to a digital converter.

13 Charge-coupled device (CCD) The computer processing this information now knows the position and voltage on each pixel. The light intensity is proportional to the voltage so a digital (black and white image) is now stored.

14 Charge-coupled device (CCD) To store a colored image, the pixels are arranged in groups of four, with 2 green filters, a red and a blue.

15 CCD uses - Telescopes

16 CCD uses – Digital Cameras

17 CCD uses – Endoscopes

18 Quantum efficiency The ratio of the number of emitted electrons to the number of incident photons. About 70% for a CCD (4% for photographic film and 1% for the human eye.

19 Example The area of a pixel on a CCD is 8.0 x m 2 and its capacitance is 38 pF. Light of intensity 2.1 x W.m -2 and wavelength 4.8 x m is incident on the collecting area of the CCD for 120 ms. Calculate the p.d. across the pixel, assuming that 70% of the incident photons cause the emission of an electron.

20 Example The area of a pixel on a CCD is 8.0 x m 2 and its capacitance is 38 pF. Light of intensity 2.1 x W.m -2 and wavelength 4.8 x m is incident on the collecting area of the CCD for 120 ms. Calculate the p.d. across the pixel, assuming that 70% of the incident photons cause the emission of an electron. The energy incident on a pixel = 2.1 x x 8.0 x x 120 x = 2.0 x J

21 Example The area of a pixel on a CCD is 8.0 x m 2 and its capacitance is 38 pF. Light of intensity 2.1 x W.m -2 and wavelength 4.8 x m is incident on the collecting area of the CCD for 120 ms. Calculate the p.d. across the pixel, assuming that 70% of the incident photons cause the emission of an electron. The energy incident on a pixel = 2.1 x x 8.0 x x 120 x = 2.0 x J The energy of one photon = hf = hc/λ = (6.63 x x 3.0 x 10 8 )/4.8 x = 4.1 x J

22 Example The area of a pixel on a CCD is 8.0 x m 2 and its capacitance is 38 pF. Light of intensity 2.1 x W.m -2 and wavelength 4.8 x m is incident on the collecting area of the CCD for 120 ms. Calculate the p.d. across the pixel, assuming that 70% of the incident photons cause the emission of an electron. The energy incident on a pixel = 2.0 x J The energy of one photon = 4.1 x J The number of incident photons is then = 2.0 x / 4.1 x = 4.9 x 10 5

23 Example The area of a pixel on a CCD is 8.0 x m 2 and its capacitance is 38 pF. Light of intensity 2.1 x W.m -2 and wavelength 4.8 x m is incident on the collecting area of the CCD for 120 ms. Calculate the p.d. across the pixel, assuming that 70% of the incident photons cause the emission of an electron. The number of incident photons is then = 2.0 x / 4.1 x = 4.9 x 10 5 The number of absorbed photons is therefore = 0.70 x 4.9 x 10 5 = 3.4 x 10 5

24 Example The area of a pixel on a CCD is 8.0 x m 2 and its capacitance is 38 pF. Light of intensity 2.1 x W.m -2 and wavelength 4.8 x m is incident on the collecting area of the CCD for 120 ms. Calculate the p.d. across the pixel, assuming that 70% of the incident photons cause the emission of an electron. The number of absorbed photons is therefore = 0.70 x 4.9 x 10 5 = 3.4 x 10 5 The charge corresponding to this number of electrons is 3.4 x 10 5 x 1.6 x = 5.4 x C

25 Example The area of a pixel on a CCD is 8.0 x m 2 and its capacitance is 38 pF. Light of intensity 2.1 x W.m -2 and wavelength 4.8 x m is incident on the collecting area of the CCD for 120 ms. Calculate the p.d. across the pixel, assuming that 70% of the incident photons cause the emission of an electron. The charge corresponding to this number of electrons is 3.4 x 10 5 x 1.6 x = 5.4 x C The p.d. is thus V = Q/C = 5.4 x /38 x = 1.4 mV

26 Resolution Two points are resolved if their images are more than two pixel lengths apart.

27 Finished!


Download ppt "Digital Imaging with Charge- coupled devices (CCDs)"

Similar presentations


Ads by Google