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iXon 3

Features & Functionality

3.7 - COUNT CONVERT

OneofthedistinctivefeaturesoftheiXon3isthecapabilitytoquantitativelycaptureandpresentdatainunitsof

electrons or photons, the conversion applied either in real time or as a post-conversion step. Photons that are incident

on pixels of an array detector are captured and converted to electrons. During a given exposure time, the signal in

electrons that is collected in each pixel is proportional to the signal intensity. In EMCCDs, the signal in electrons is further

multiplied in the EM gain register. The average multiplication factor is selected in software from the RealGain™ scale. It

can be desirable to directly quantify signal intensity either in terms of electrons per pixel or in terms of incident photons

per pixel. However, during the readout process, array detectors must rst convert the signal in electrons (the multiplied

signal in the case of EMCCDs) into a voltage which is then digitized by an Analogue to Digital Converter (ADC). Each

Analogue to Digital Unit (ADU) is presented as a ‘count’ in the signal intensity scale, each count corresponding to an

exact number of electrons. Furthermore, the signal value in counts will sit on top of an electronic bias offset value. In the

iXon3 this ‘baseline’ can be clamped at 100 counts.

Therefore, in order to back calculate to the original signal in electrons, electron to ADU conversion factor must be very

accurately stored by the camera (which varies depending on the pre-amplier gain selection chosen through software).

Calculation of the signal as absolute electrons also requires knowledge of the bias offset and the EM Gain. The

calculation path is shown in gure 40 below:

Furthermore, knowledge of the Quantum Efciency (QE) at each wavelength and light throughput properties of the

camera window enables this process to be taken a step further, allowing the signal to be estimated in photons incident

ateachpixel.Forthisstep,theusermustinputthesignalwavelength.Inuorescencemicroscopyforexample,this

wouldcorrespondtothecentralwavelengthdenedbyanarrowbandemissionltermatchedtotheuorophoreof

interest. If the spectral coverage of the signal on the detector is too broad, such that the QE curve varies signicantly

throughout this range, then the accuracy of the incident photon estimation would be compromised.

TheCountConvertfunctionalityoftheiXon3providestheexibilitytoacquiredataineitherelectronsorincident

photons, with negligible slow down in display rate. Furthermore, the option exists to record the original data in counts

and perform this important conversion to either electrons or photons as a post-conversion step, while retaining the

original data.

Figure 40: Count Convert calculation path

Andor Technology iXon3 Series User Manual

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