A visual guide to CCD vs. EM-CCD vs. CMOS

THIS GUIDE IS FOR:

Persons who are curious about scientific cameras.

THIS GUIDE OFFERS:

A quick look inside the camera to understand how technology affects performance and capabilities.

The latest improvements in semiconductor technology have greatly increased the capabilities of CMOS sensors, giving scientific CMOS (sCMOS) the sensitivity for use in all but the lowest light situationswhile providing resolution, field-of-view, and frame rates.

 

Here’s how the CMOS sensor compares to the venerable CCD technology.

CCD AND EM-CCD IMAGE SENSORS

Figure 1. Schematic of a CCD image sensor.

Each pixel of the CCD image sensor (Figure 1) is composed of a photodiode and a potential well, which can be thought of as a bucket for photoelectrons.

 

A wavelength-dependent quantity of light hitting the sensor is converted into electric charge (photoelectrons). This wavelength dependent conversion of light to photoelectrons is conveyed through the quantum efficiency (QE) specification.

Figure 2. Photoelectrons are relayed down each row of pixels and charge gathered pixel-by-pixel, serially, in a container at the end of the relay.

Photoelectrons accumulate in each bucket until it’s time for readout, when all of the photoelectrons are relayed from one bucket to the next down each row of pixels (Figure 2). The charge is gathered pixel-by-pixel—serially—into a container at the end of the relay. Once in the container, the photoelectrons are converted into voltage and processed into an image on the camera circuit board.

 

Because the photoelectrons are converted into signal (voltage) at a common port, the speed of image acquisition is limited.

Figure 3. Schematic of an EM-CCD sensor.

EM-CCD sensors (Figure 3) have an additional component, the multiplication register, that multiplies the photoelectrons before the readout of the sensor (read more about the multiplication register and noise here). However, this process introduces another noise source called noise factor (or excess noise) that renders the effective QE of these cameras to about half of the native QE.

 

Because the photoelectrons are converted into signal (voltage) at a common port, the speed of image acquisition is limited.

CMOS IMAGE SENSOR

Figure 4. Schematic of a CMOS sensor.

In contrast with CCD and EM-CCD sensors, each pixel of a CMOS image sensor (Figure 4) is composed of a photodiode-amplifier pair.

 

As in a CCD sensor, light hitting the CMOS sensor is converted into photoelectrons, with conversion efficiency (QE) dependent on wavelength. But unlike a CCD sensor, photoelectrons are converted into voltage by each pixel’s photodiode-amplifier pair. Because conversion to voltage happens in parallel instead of serially, as is the case for the CCD sensor, image acquisition can be much faster for CMOS sensors.

 

The optimized architecture of scientific CMOS sensors combines high QE with fast frame rates and low noise—without the noise factor introduced by the multiplication register of EM-CCDs.1 This combination of capabilities translates into high speed, high-resolution biological images, even in low light situations.

References

  1. Huang, F. et al. Video-rate nanoscopy using sCMOS camera-specific single-molecule localization algorithms. Nat. Methods 10, 653–658 (2013).

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