EMCCD CAMERA

F1 : Relative SNR in Photon Counting Mode

Photon etc has developed the world’s most sensitive EMCCD camera, now commercialized by NÜVÜ Cameras. Based on a new CCD controller for counting photons (CCCP), this camera is more precise and less time-consuming in low luminosity applications, such as astronomy. This camera will not only help to better understand the depths of the universe but also to perceive weak optical signals coming from the human body. These signals can reveal the early signs of several diseases such as macular degeneration and certain types of cancer.

A first prototype has been developed by Olivier Daigle, physics PhD student and Photon etc employee, with the participation of the Laboratoire d’astrophysique expérimentale de l’Université de Montréal, of the Centre de recherche en astrophysique du Québec, and of the Laboratoire d’astrophysique de Marseille. Impressive results led NASA to buy the first commercial EMCCD controller placed on the market.

How does it work?

A CCD (Charge-Coupled Device) based camera transforms photons into electrons using the photoelectric effect induced by the contact of incoming photons with a semiconductor material. The semiconductor surface is structured by an array of potential wells to be filled by electrons. The rendering of an image is performed by the application of cyclic electrical potential variations on the wells, forcing the electrons to flow from one well to the next until they reach an amplifier which counts electrons and transforms them into numerical data. The first noise interference in the process comes from the intrinsic instability of the wells. The photoelectric effect triggered by the incident photons would ideally be the only elements creating electrons in the wells, but since the optimization of this process requires a material from which electrons are easily extracted, these electrons may also jump to other levels of energy spontaneously and create the characteristic dark current of CCDs. In other words, even if the camera’s shutter is on, the wells are filled by random electrons, giving the false impression that an exposure has taken place. To resolve this problem, it is possible to lower the temperature of the CCD, the solution offered by most manufacturers. While results obtained this way are satisfactory for applications in which the number of incoming photons is sufficient, in low-light observation conditions such as astronomy or any other night observation, another source of noise is necessary.

The number of electrons collected is normally proportional to the amount of light received; however, when the observation conditions offer only a few incident photons, a lengthy wait time is required before the wells are enough charged to create an interesting image. For some phenomenon, this time-consuming process is too onerous to capture the process under investigation. Furthermore, there is noise associated with the reading of the electrons at the output of the array of wells. This noise is generally very weak, around 2 to 10 electrons / 1000. On the other hand, in low light situations, for example when only one electron per well is created, this noise is paramount. In order to increase CCD’s sensitivity, an EMCCD (Electron-Multiplying CCD) offers an amplification system at the output of the CCD through high voltages. This system allows for considerable amplification, with one electron transforming into 1000 or even 2000 available for reading. Thus, the readout noise discussed above becomes negligible with EMCCDs. These detectors permit observation of phenomenon under very dark conditions since every photon is multiplied a thousand times. At this level, a new source of noise becomes a factor, which limits the performance of these cameras; this is a challenge uniquely suited to Photon etc’s innovative tools.

The high voltages and high frequencies involved in EMCCDs cause a phenomenon called Clock-Induced Charges (CIC). These charges are created in the very short period of stabilization of the potential, but may be limited by a more precise and versatile electronic controller. The CCCP (CCD Controller for Counting Photons) developed by Olivier Daigle at Photon etc and Université de Montréal serves this purpose. Finally, while transferring charges at high speed from one well to its neighbour, a fraction of the electrons remain in the first well. This is called the Charge Transfer Efficiency (CTE). The electrons which did not overcome the gap between the wells create “false” light areas on the image. While lowering CICs, CCCP also allows a better CTE, improving significantly the signal-to-noise ratio.