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How to... Reduce Thermal Noise in Industrial Imaging Cameras
Cameras and imaging systems for industrial and manufacturing markets are traditionally manufactured for high throughput and image quality, often at the expense of speed, cost and system robustness. As a result, many system design challenges have emerged. A primary concern in camera system design is noise, or the camera’s signal-to-noise ratio (SNR): the ratio of the measured signal to the overall noise at a given pixel. High SNRs are particularly important in applications requiring precise measurement of light.
A fraction of light incident on the camera’s image sensor gets converted to photoelectrons within the silicon layer. Photoelectrons comprise the signal while carrying a statistical variation of fluctuations in the photon arrival rate at a given point, known as “photon noise.” Evaluate camera features by examining three general types of noise:
- Photon noise. As discussed above, this is the inherent noise or variation of the incident photon flux (light).
- Read noise. The uncertainty introduced during the process of quantifying the electronic signal on the CCD. The faster a given sensor is clocked, the higher the read noise will be.
- Dark noise. The statistical variation of thermally generated electrons (heat) within the CCD.
Some heat reduction procedures are achieved through the following:
- Thermal conduction. Conducts heat away from the image sensor, reducing CCD temperatures and improving dark noise.
- Convection. Utilizes a simple fan to dissipate heat, but results in minimal cooling if a camera is not designed to benefit from it.
- Active cooling. The CCD is cooled via thermoelectric (TECs) or liquid coolers.
TEC or Peltier coolers use a current flow to transport heat and are stacked together for excessive cooling, circulating liquid or air through a large fan.
In research grade cameras, combined TECs coupled to a liquid cooling system transfer heat away from the TEC. However, liquid cooling systems are generally not considered robust enough to be practical outside the realm of research cameras.
