CMOS Sensors : Comparative Study of CCD & CMOS

CCD AND CMOS SENSORS: PROS AND CONS

1. Fabrication Process

CCD sensors use specialized fabrication that uses dedicated and costly manufacturing processes, whereas CMOS sensors rely on standard CMOS technology (used for IC fabrication like microprocessors, memory, etc.). As CMOS sensors can also integrate required electronics on the same chip, CMOS sensors results in compact and cost effective system

2. Dynamic Range

Dynamic range of CCD is roughly twice as that of CMOS sensor. This implies that if better colour depth is required, CCDs are likely to offer better results. On the other hand, CMOS are marginally more photosensitive.

3. Power Consumption

CMOS cameras have lower power consumption than CCDs but other CMOS circuitry may require more power. Low end CMOS sensors have low power requirements, but high speed CMOS cameras typically require more power than CCDs.

4. Noise

Two types of noise affect sensors’ performance: Temporal Noise and Fixed pattern noise. Fixed pattern Noise is more in CMOS, compared to CCDs because charge is converted to voltage at each pixel as compared to single point charge-voltage conversion in CCDs. In terms of temporal noise, CMOS sensors are better as the bandwidth of amplifiers at each pixel is lower than the output amplifier in case of CCD.

5. Image Quality

Due to poor fill factor of CMOS, photosensitivity of CMOS sensors is poor in low light conditions.

6. Uniformity of response

CCDs use single amplifier for all pixels and CMOS use separate amplifiers for each pixel. Pixel-to-pixel amplification differences lead to non-uniformity. Response of CCDs is pretty uniform.

7. Speed

CMOS sensors have higher speed due to the fact that it uses active pixels and ADCs on same chip leading to lesser propagation delays.

8. Readout area

CMOS sensors allow any region or even multiple regions to be read off the sensor. CCDs are limited by vertical scan read out

9. Smart functions

With the integration of signal processing circuitry on the CMOS sensor chip, functions like auto gain control, auto exposure control etc., anti-jitter, image compression, color encoding, motion tracking, etc. can be incorporated on-chip.

10. Overexposure effect

Overexposure can cause smearing around over-exposed pixels. Smearing is caused by spilling of charge into the shift register. Due to absence of shift registers in CMOS sensors, they are immune to this effect.