Cooled CCDs--DALSA Life Sciences

Cooled CCD and CMOS Imagers

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In medical imaging, cooler imagers prevail

Why cool image sensors?

lower dark current
longer exposure times
better images

For more on how CCD and CMOS sensors work, see these documents on the DALSA website:
• CCD Technology Primer (PDF 195k)
• CCD vs CMOS comparison

Very simply, CCD and CMOS image sensors are cooled to improve image quality. Both CCD and CMOS sensors capture images by accumulating charge from electrons knocked free by incident photons, but incoming light is not the only way to create charge in a pixel well. CCD and CMOS sensors are sensitive to heat, and even in total dark, a sensor will accumulate "thermally induced charge" or dark signal (usually measured in electrons per pixel per second) over time. The higher the temperature, the more dark signal will accumulate. Dark signal doubles approximately every six Celsius degrees, and can signficantly degrade an image, particularly in medical applications, in which image fidelity, dynamic range and signal-to-noise ration are crucial. Cooling reduces the dark signal, allowing better, more uniform images and longer exposure times.

While you can subtract dark signal from an output signal, preventing it to begin with is less error-prone and preserves more dynamic range. Sensors are also subject to dark noise, which is random and cannot be subtracted; cooling is therefore the only way to reduce dark noise.

DALSA Life Sciences cameras can cool sensors to 35 Celsius degrees below ambient, allowing dark signal as low as 1 e-/pixel/sec.

How are DALSA Life Sciences Sensors cooled?

thermo-electric cooling (TEC), dissipated by forced air
TEC dissipated by liquid cooling

Thermoelectric cooling by the Peltier effect Thermo-electric cooling uses the Peltier effect to transfer heat away from the sensor. When an electric current is passed through the the thermocouple of two dissimilar conductors joined in two places, one junction will absorb heat and the other will dissipate it. With the "cold junction" in the right place, a sensor can be cooled with nothing more than a current. TE cooling is compact and reliable, and used in many DALSA Life Sciences cameras.

With Forced air cooling, a fan or compressed air dissipates the heat. For the most stringent requirements, liquid cooling (e.g. using a glycerol/water or methanol/water solution) provides the best cooling performance. Although it adds cost and complexity, liquid cooling can bring the sensor temperature to -45°C, all but eliminating dark signal for short to medium exposures. DALSA Life Sciences uses liquid cooling in products for x-ray crystallography. It should be noted that liquid cooling may not be cost-effective for most applications, because even with negligible dark signal, there are other limiters to image quality (e.g. read noise, shot noise).

DALSA Life Sciences and cooling

experience + expertise + efficiency = effectiveness

Through years of experience, DALSA Life Sciences has developed considerable expertise in cooling imagers to maximize image quality. Our designs are effective, efficient, and robust, and with a range of technology available to us, we can help you choose the right option for your application.

Cooling is just one of the factors in capturing good images. If the rest of the camera electronics are poorly designed, the advantage of proper cooling will be lost. DALSA Life Sciences's tightly integrated designs preserve the high S/N ratio and sensitivity made possible by cooled imagers.

For more information, contact Sales.