Question

Comprehension

Direction: Read the Passage and Answer Questions

Charge Couple Device (CCD) can be used either as an image sensor or as a shift register. As a photo detector it has also been called charge - coupled image sensor or charge - transfer image sensor. Most common area images use either interline transfer or frame transfer readout architecture. The spacing between CCDs should be small for efficient charge transfer.

CCDs have the following characteristics

(1) Low dark current, high-noise

(2) High - voltage operation, low-noise

(3) Poor dynamic range, low-dark current

(4) Good linearity, low - dark current

1. 1
2. 2
3. 3
4. 4

Answer 1

Correct Answer - Option 2 : 2

Explanation:

CCDs have the highest light sensitivity of any light detection technique available, but they aren’t the best sensor for every application. Below is a table demonstrating some of the advantages of the more common detectors available.

• CCDs are capable of holding a large signal in any single pixel, and also have a very low read noise and dark current, thus they have a very large dynamic range.
•  CMOS parts have more read noise, and generally smaller pixels than CCDs and thus don’t quite compare to CCDs.
• A photomultiplier tube (PMT) can generate an electric output after a photon strikes the photocathode in just a few nanoseconds - not even high speed CMOS devices can match this.

 In a general sense, CCDs are much better where low noise is essential, but CMOS can be better in applications where speed is important and cooling is not needed

	PMT	Film	CCD	CMOS
Dynamic Range	> 1000	< 100	> 10000	> 5000
Detection speed	fastest	Slow	Slow	Fast
Quantum Efficiency	5 – 20%	5 – 20%	25 – 95%	15 – 35%
Multi-channel	No	Yes	Yes	Yes
Real time	Yes	No	Yes	Yes
Spectral Sensitivity	300 – 900 nm	200 – 1300	300 – 1100	400 – 1100
Dark Signal	Good	Poor	Best	Best
Read noise	Good	Good	Best	Best

 

Conclusion:

From the table it is clear that CCD have lower dark current and read noise, high voltage operation since it is having the capability of holding large signal.

Extra points:

• CCDs turn up to 95% of incident photons into usable signal, which can be read with low noise electronics.
• CMOS devices can read an entire array of silicon very quickly, but at the expense of noise in comparison to CCDs.
• Film, CCDs and CMOS can make pictures with one input event, while a PMT is a single source (one pixel) detector.
• Film must be processed in order to see the image that was detected which can take some time – but CMOS, CCDs and PMTs can be actively monitored and recorded by computers or other display devices.