Despite the convenience of CMOS imagers, CCD imagers still have their place. And if you really need low-light performance, consider a device with EMCCD technology, such as the KAE-02152.

CCDs and CMOS image sensors both convert light into electrical signals, but there is a lot that can be said about how these two technologies differ in terms of performance and implementation details. This article is not the place for an in-depth comparison. Instead, I’ll offer a summary that I think is fairly standard, despite the fact that it probably constitutes a dangerous oversimplification: CMOS sensors are easier to use, but in general their imaging quality is inferior to that of CCDs.
 

CCD stands for charge-coupled device; as you can see, the term “CCD,” strictly speaking, has nothing to do with imaging. It simply refers to the movement of charge.

 

The curvy lines in this diagram from the KAE-02152 datasheet convey the fact that charge from the CCD pixels is indeed moving toward the periphery of the imager and eventually to the output pin.

 

However, “CCD” has come to be associated almost exclusively with CCD imagers, perhaps because imagers are the only widespread devices that depend on CCD technology.

The essence of CCD functionality is the movement of electric charge from the interior of the imager to the image-data output pin, which generates an analog signal governed by the light intensity at each pixel location. Digital imagery is produced by sampling this analog signal. (Obviously there are a lot of details involved in sampling at the right moment and keeping track of which sample corresponds to which pixel—not to mention the carefully generated control signals that are needed to keep the charge moving.)

The following diagram gives you an idea of how a CCD output signal changes according to the charge packets that are moving one by one through the device. You can see that the movement of charge is synchronized with the H2L clock signal.

 

Diagram taken from the KAE-02152 datasheet.

 

When CCD Isn’t Good Enough

The main selling point for the KAE-02152 is its versatility with respect to lighting conditions. ON Semi’s surprisingly poetic marketing catchphrase is “from sunlight to starlight”—in other words, the KAE-02152 can maintain image quality despite enormous variations in the intensity of the ambient illumination.

Low-light performance is enhanced by the use of something called an electron-multiplying CCD, or EMCCD. EMCCD technology does complicated things that I’m not qualified to expound upon, and the end result is lower noise that allows for higher signal-to-noise ratio (SNR) in low-light situations (because low light means low signal levels).

The charge packets from the CCD pixels can be routed through either the conventional CCD output or the EMCCD output. How do you know which one to use?

Well, the first thing to understand is that the EMCCD is only beneficial if you’re dealing with low light levels; sending higher-value pixel data to the EMCCD actually results in lower SNR compared to using the normal CCD. So you have to choose a threshold that will determine which CCD is used, and page 32 has some specific guidance on threshold selection.

 

Keeping It Cool

If you really need to maximize image quality, you have to find a way to reduce the temperature of your CCD. The datasheet for the KAE-02152 says that a lower temperature is beneficial because it reduces dark current noise and image defects.

 

Diagram taken from the KAE-02152 datasheet.

 

It seems to me that cooling the CCD would also reduce thermal noise associated with various other portions of the device, but maybe I’m wrong, or maybe those noise sources are included in the reference to “dark current noise.”

In any event, you can see in the above plot that the dark signal diminishes rapidly as the temperature decreases from room temp to 0°C. Clearly, then, room-temperature operation is far from ideal. But it’s also less than ideal to restrict your imaging endeavors to wintry nature scenes.

Fortunately, ON Semi has a solution: one of the two KAE-02152 package options includes a thermoelectric cooler. Just keep in mind that your power supply will have to be a little heftier if you plan on seeking optimal image quality during photo shoots to the tropics:

 

Plots taken from the KAE-02152 datasheet.

 


 

Do you have any experience with EMCCDs? Feel free to compensate for my ignorance by leaving an informative comment.

 

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