![]() With modern camera technologies, aggressive and excessive cooling is unnecessary due to the short exposure times. When using a highly-sensitive camera, a sufficient signal can be collected in very short exposures (well below 1 second), and the dark current buildup is negligible in these cases. The important part to consider here is: How long are your exposure times? Typical fluorescence imaging, low-light imaging, and high-speed imaging all feature low signals and short exposure times, well below a second and often below 100 ms. For example, the typical dark current of a Teledyne Photometrics Prime BSI sCMOS is 0.5 e –/p/s, meaning that a two second exposure would result in one electron of dark current per pixel. The sensor is cooled absolutely and evenly, is an optimal distance from the window, and heat is effectively expelled.īut while dark current is temperature dependent, it is also dependent on the exposure time (the ‘second’ part in e/p/s). The left image shows typical temperature levels within a CMOS camera with Citadel chamber design principles (logo right). Ideally, the dark current should be reduced to a point where its contribution is negligible over a typical exposure time.įigure 2: Scientific camera cooling and Citadel Chamber Technology. This is why scientific cameras often feature fans and will state their operating temperature in the datasheet, in order to reduce the effects of noise sources such as dark current. In general, for every 6-7 ☌ the sensor can be cooled, the effects of dark current are halved. Scientific cameras can use thermoelectric (TE) or Peltier cooling in combination with forced air or liquid cooling in order to reduce the temperature of the sensor during operation, as seen in Fig.2. However, as dark current is dependent on the temperature its effects can be reduced using cooling. The higher the dark current, the less able a camera is to perform long-exposure imaging. This indicates how many electrons build up on each pixel for every second of exposure, typically shown as e –/p/s. This is known as the dark current.Įvery model of scientific camera, whether using a CCD, EMCCD, or CMOS sensor, will have a dark current specification. Unfortunately, the camera sensor doesn’t know the difference between these types of electrons, and so any thermal electrons that accumulated in the sensor pixel wells (along with the photoelectrons) are counted as signal upon readout, despite not being part of the signal from the sample. These thermal electrons are independent of the photoelectrons generated proportional to the photons (light intensity) falling on the sensor. As a camera sensor is exposing an image, the electronics in the camera will heat the sensor, and this accumulation of thermal energy causes thermal electrons to build up on the sensor. Dark Currentĭark current arises from thermal energy within the camera sensor. This noise will spread across the camera as the heat increases over long exposures and will impact image quality. Figure 1: Thermal build-up and dark current noise at the edges of a camera sensor. ![]()
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