Does Your CCD Camera Need Cooling?

David W. Gardner

To those in the light-starved world of astronomy, the result of cooling cameras to ultralow temperatures is obvious: Truly amazing images can be produced in situations where only a few photons are available. The need for cooling may not be so obvious in applications such as web inspection, laser profiling and medical imaging. Yet even here, cooling the camera (albeit not to cryogenic levels) can often provide a dramatic boost to image quality.

In most cases, the benefits of cooling come from its ability to reduce the dark current generated by the camera. This pesky current represents the collection of all unwanted free electrons that thermal noise generates in the CCD. In applications such as astronomy and spectroscopy, it can mean the difference between a good image and no image at all. In more conventional industrial applications, the amount of dark current correlates with a lack of image clarity.

Decoding dark current

The term “dark current” comes from the fact that the current has nothing to do with the incident light and is generated equally well in complete darkness. Depending on where the unwanted electrons are generated in the silicon, some of the charge will collect in the individual CCD pixels and contaminate signal electrons (related to the image). At the CCD output, dark-current-generated electrons appear identical to signal-generated electrons, so dark current appears as noise in the image.

At a given temperature, the average dark current in a given pixel is relatively constant, which results in a fixed offset that is added to each pixel value. But the generation rate for that current typically varies spatially across the array. This process produces a fixed background pattern that will be superimposed on top of the desired image. By capturing an image in complete darkness, a pixel-by-pixel representation of the average dark current can be obtained for that temperature and exposure period. Subtracting this dark reference image from all subsequent real images partially eliminates the effects of the spatially varying dark-current offset. Another name for the subtraction process is dark-field correction.

Dark-field correction can dramatically reduce the contamination effect that dark noise has on an image.

Unfortunately, there is a second component of dark-current generation — a randomly varying component related to shot noise, which scientists approximate as the square root of the dark current collected in a given pixel. For example, a CCD that generates 4000 such electrons in a pixel during a one-second exposure will have an offset of 4000 electrons that can be subtracted, plus a random noise equal to roughly 63 electrons rms that cannot.

Consider an application involving a 12-bit camera where each pixel can hold a maximum of 40,000 electrons (the full-well capacity). If properly calibrated, each analog-to-digital count of the camera output would represent 40,000 electrons divided by 2¹², or a total of 10 electrons. An increase of 10 photoelectrons captured by a given pixel would increase the camera’s digital output by one count.

If this camera has a dark current of 4000 electrons per second, and a one-second exposure is taken, the average pixel would have an offset of 4000 electrons that could be subtracted and, for the most part, eliminated. Keep in mind, however, that the dark current has consumed 4000 of the 40,000 maximum, so the signal full well has been reduced to 36,000 electrons.

Furthermore, the process of subtracting images takes time and computational power. Added to this 4000-electron offset is random noise from the dark current of 63 electrons rms. With the camera biased so that one analog-to-digital count represents 10 electrons, this means that your camera has about 6.3 counts of rms noise and that any signal lower than this will never be seen. As a result, the 12-bit camera is providing a real dynamic range of only (40,000 — 4000)/63 = 571:1, or about 9 bits.

Just as dark current grows with increasing temperature, it decreases if the sensor is cooled. This allows some camera manufacturers to reduce dark current to extremely low levels. One field that requires ultracooled cameras is astronomy. Because of low light levels, researchers may wish to cool to as low as 2100 °C and take an exposure that is 30 minutes to an hour long.

Ultracooling is tricky

Cooling a camera to this level involves significantly more than simply pouring liquid nitrogen over the sensor while the camera is running. Cameras of this type can be quite complex and often use liquid nitrogen or multistage thermoelectric coolers combined with vacuum enclosures and closed-loop feedback to lower sensor temperatures.

An extensive knowledge of heat transfer, materials science and semiconductor physics is key to successfully cooling a camera in applications where dark current must be virtually eliminated. Only a few highly skilled companies, such as Spectral Instruments in Tucson, Ariz., and Roper Scientific in Trenton, N.J., offer products that are geared toward the ultracooled market.

One might ask whether high frame rates make dark current insignificant and therefore eliminate the need for cooling. After all, because the amount of dark charge captured is directly proportional to the exposure time, a shorter exposure (higher frame rate) will reduce its effect at a given temperature. However, the user should not assume that running at high frame rates alone will provide immunity to dark current and other thermal issues.

CCD and camera manufacturers typically specify dark current for a certain temperature, often around 25 °C. Higher-frame-rate cameras have significant self-heating within the sensor because of higher amplifier bias currents and higher-speed clocking of the gate capacitance. Therefore, it is not unusual to find a commercial camera running at 30 frames with a sensor temperature of 45 °C. A rough approximation is that dark current doubles for every 6 °C rise. Using this rule of thumb, the sensor at 45 °C would produce dark current roughly 10 times greater than the specification shows. The dark current not only is significant on this high-frame-rate camera, but also changes by an order of magnitude between the time the sensor is turned on (cool) and the time it reaches thermal equilibrium (perhaps 30 minutes later).

To further complicate things, the dark current will also change in an uncooled camera when there is a change in operating modes, such as binning, frame rate, time delay and integration shift rate, and region of interest. Each time the dark current changes, the noise level and “black level” also change. To calibrate out these variations, the camera system would have to store dark-field calibration frames for every possible operating mode, and these would not be valid until the camera had reached thermal equilibrium.

New challenges

With the current push for ever-increasing speeds, many camera manufacturers have moved toward multiple-output sensors. This is particularly true for time delay and integration and area-scan sensors used in high-speed inspection where composite data rates may exceed 1 billion pixels per second. For these multichannel cameras, varying sensor temperature adds a whole new set of challenges.

In sensors with multiple outputs, the gain and offset for each individual video channel must be precisely matched to all the other channels. Moreover, these gains and offsets must not change over time and temperature. If they do, the resultant image will show “tiling” effects, where the image section from a given output varies in contrast and brightness with respect to adjacent image areas.

Both the gain and offset for these areas are affected by the operating temperature. This, in turn, is influenced by an uncooled camera’s operating mode, ambient temperature, frame rate and other factors. Significant problems related to maintaining a good gain and offset balance can result. Attempts to correct this mismatch can be both time-consuming and expensive for the customer — and, in the end, a futile effort because the next change in the operating mode or temperature will again create imbalance.

The cost of cooling

Clearly, low dark current and extreme thermal stability offer significant advantages in light-starved applications such as spectroscopy, crystallography and astronomy. In higher-frame-rate applications, the performance enhancement from a cooled camera also can be substantial. Reduced dark current noise, coupled with a dramatic increase in stability and repeatability, directly benefits applications ranging from semiconductor and web inspection to laser profiling and medical imaging. The problem is that such inspection applications often don’t warrant the cost and complexity of cryogenically cooled sensors.

Fortunately, in the majority of these applications, most of the performance improvement can be accomplished simply by lowering the sensor temperature to a few degrees below ambient and stabilizing it. Decreasing to just 20 °C reduces dark current in most inspection applications to an insignificant level. The thermal stabilization ensures that performance characteristics remain constant regardless of frame rate or operating mode. For higher-speed cameras, this low-level cooling and thermal stabilization provide significant gains in imaging performance with little or no impact on camera cost.

Meet the author

David W. Gardner is president of Summit Imaging Inc. in Colorado Springs, Colo., which specializes in high-performance area-scan camera design. He was the founder and former president of Silicon Mountain Design Inc.