EMCCD Tutorial
EMCCD Tutorial
WHY CHOOSE AN EMCCD CAMERA?
Low light imaging takes place in various fields, from the infinitely small to the infinitely large. It can occur while tracking down the molecular dynamics of brain synapses with fluorescent markers or studying the atmosphere of a faraway extrasolar planet through spectroscopy. However, in all circumstances, when photons are scarce, the signal reaching the imaging device may be weak enough to blend with the background noise. A strategy to recover the photon signal is thus much needed.
Electron-multiplying CCD (EMCCD) technology, sometimes referred to as low light level CCD (L3CCD), is designed to beat down the intrinsic electronic noise of the readout process, which is comparable to a signal of a few photons per exposure. In this way, EMCCD cameras address the challenges of most low light imaging. They also support faster frame acquisition rates than their CCD counterparts, making them highly suitable for live imaging. Better still, EMCCD cameras can offer ultimate sensitivity for the observation of the darkest scenes by becoming wide-field real-time photon counting imaging devices.
This series of pages has the goal to inform you about the various specificities of EMCCDs; from noise sources & operation modes to advanced photon counting operation.
THE TECHNOLOGY AT THE HEART OF THE EMCCD
A relative of the charge-coupled device (CCD) technology, the EMCCD is a frame-transfer CCD to which is added a special output register. It operates by first transforming incident photons into photoelectrons in the detector’s light-sensitive region—the imaging region—located within the camera’s cooled enclosure. The sensor’s silicon body is meticulously organized in a matrix of potential wells, or pixels, that trap photoelectrons as they are created through the photoelectric effect during exposure. Following the collection of these negative charges, the application of a series of voltages across the sensor forces the transfer of all electrons from the imaging to the storage region of the detector. Doing so ensures the processing of the acquired image while performing a new acquisition.
In the storage region, the electrons at the detector’s bottommost row travel pixel by pixel into the multiplication register, which comprises several hundred electrodes. When a photoelectron encounters an electrode, there is a 1–2% probability that it will generate a secondary electron by impact ionization, a type of avalanche effect. As a result, an incoming signal of a few photons can be amplified up to several thousand times. The charges then reach the output amplifier where they are converted into an electric impulse subsequently digitized to form an image.
