EMCCD Tutorial

SENSITIVITY PARAMETERS

• THERMAL NOISE

  Ideally, an EMCCD sensor would only detect photoelectrons. However, the mere thermal agitation within the EMCCD chip is sufficient to eject electrons from the silicon body, which are then collected by the potential wells. Even in total darkness, the wells slowly gather these charges, also referred to as dark electrons, and do so in an increasing amount with prolonged exposure times. It is as if the camera had been exposed to a faint light source.

  The EMCCD detector is cooled down to temperatures nearing -90 °C to counter such noise, known as either dark current or thermal noise. Doing so substantially decreases the number of dark electrons within the chip, hence improving dramatically the photoelectron signal. Performing acquisitions in the inverted mode of operation (IMO) further reduces the dark current contribution. Refer to the Modes of Operation section later in this tutorial for more information.

  Nüvü™ addresses thermal noise with either liquid nitrogen or a thermoelectric cooling (TEC) unit to absorb the detector’s heat. To that end, Nüvü Camēras has developed a revolutionary camera packaging that can maintain the detector’s temperature at -85 °C with an extraordinary precision of 0.01 °C even when performing fast acquisitions and reading the entire detector frame. Such stability ensures the uniformity of the signal amplification through the electron-multiplying register, as the number of dark electrons remains the same See the EM gain subsection for more information.

  Acquisition conditions: Sensor operating temperature of -85 °C; 10 MHz readout speed; EM gain of 5,000 for Nüvü Camēras, and 1,000 for other EMCCDs.

• READOUT NOISE

  Readout noise occurs whenever the charges are amplified and digitized. Negligible in high illumination conditions, as in the order of 2 to 10 electrons, the readout noise becomes comparable to the photoelectron signal in near-darkness scenes.

  EMCCD technology includes a charge amplification stage, the electron-multiplying (EM) register, which boosts the photoelectron signal before digitization. As this stage increases the incoming signal by a factor up to a few thousand, the readout noise becomes negligible.

• EM GAIN

  The factor associated with the photoelectron amplification is known as the EM gain. Boosting the photoelectron signal, however, comes at a price: the EM registry is a sensitive component that may easily saturate, and saturation may lead to its premature aging or damage.

  Note that the EM registry boosts not only the photoelectron signal but also dark electrons and clock-induced charges (see the next subsection). Nüvü™ addresses such issue with both the innovative CCD Controller for Counting Photons (CCCP) and unrivaled cooling performances. The first minimizes the clock-induced charge contribution while the latter cuts down dark current levels. Therefore, Nüvü™ is the sole EMCCD camera manufacturer that offers cameras supporting EM gain up to 5000.

  For most applications, Nüvü™ recommends employing EM gains below 1,000 to avoid dynamic range losses. The camera’s dynamic range, defined as the ratio between the full-well electron capacity and the camera overall noise level, declines with increasing EM gains as a result of the charge amplification register capacity. For increased gains, the EM register accepts fewer electrons before saturation due to the amplification of the charges contained in an entire row and not individual pixel charges. But lower gains lead to better dynamic ranges, which refine the image contrast.

  Nevertheless, for applications requiring photon counting —further on this page—Nüvü Camēras strongly recommends EM gain values of more than 3,000 to optimize the sensitivity and dynamic range.

• CLOCK-INDUCED-CHARGES

  Shifting the photoelectrons at high readout rates requires the application of both high voltages and high-frequency clocks, the latter signal coordinating the photoelectrons displacement within the EMCCD sensor. With greater clock frequencies are associated a supplementary noise source known as the clock-induced charges (CIC).

  Fortunately, Nüvü Camēras innovative and patented CCD Controller for Counting Photons (CCCP) generates finer and more adjustable clock signals than those produced by other controllers. As such, it drastically limits the creation of CIC while reading the EMCCD at high speeds.

• CHARGES TRANSFER EFFICIENCY

  Some photoelectrons may be left behind during the charge transfer process, especially at high readout speeds: the electron shifting process from one potential well to the next may be incomplete, thus creating so-called deferred charges in the imaging region. Consequently, the leftover electrons artificially increase the brightness of certain pixels and decrease the overall image quality of the EMCCD camera.

  Nonetheless, on top of lowering CIC, the CCD Controller for Counting Photons enhances charge transfer efficiency (CTE), and does so at any operating temperature and readout frequencies, hence leading to images of highly superior quality.

  Acquisition conditions: Sensor operating temperature of -85 °C; 10 MHz readout speed; EM gain of 5,000 for Nüvü Camēras, and 1,000 for other EMCCDs.

• EXCESS NOISE FACTOR

  The avalanche process that boosts the photoelectron signal before readout is unpredictable in nature: one can only ascertain the mean value of the EM gain, never its exact value. The signal follows a Poisson probability distribution, as seen in the figure below.

  Electron distribution after the avalanche process in the EM register. The output probability distribution is plotted against the output electrons for a value of the EM gain of 3,000. The n values correspond to the number of electrons in a pixel before traveling through the EM register.

  Uncertainties arise when attempting to determine the exact number of electrons that contributed to the signal before amplification: as shown above, the electron distributions overlap considerably despite the number of electrons n, thus making it impossible to ascertain how many electrons entered the EM register first. Such ambiguity results in the contamination of EMCCD-acquired images by the excess noise factor (ENF), which has a √2 value at high EM gain. This stochastic noise source, noticeable in low light conditions, affects an image’s signal-to-noise ratio (SNR) as if the sensor’s quantum efficiency was cut in half.

  Performing photon counting measurements eliminates the ENF by assessing the number of photoelectrons per pixel and attributing to each a value of 1 or 0. The charge amplification registry, therefore, cannot change the pixels’ output and thus not alter their values with the electron-multiplication associated uncertainty. Further explanations are provided in the Photon Counting section.