QMFLIM2 is the cmos sensor made specifically for pco.FLIM. QMFLIM2 , has been developed by the CSEM (Centre Suisse d’Electronique et de Microtechnique SA, Zürich, Switzerland) and PCO. Each pixel of the image sensor has two charge collection sites, called tap A and tap B, and a switch, which can be controlled by an external signal.
This configuration acts like a charge swing, which is illustrated in figure 1 . The external two-level voltage signal, called modulation signal, selects whether tap A or tap B is active. If tap A is active, the photogenerated charge carriers are directed to the tap A charge bucket, when tap B is active, the carriers drift into the tap B bucket. If this switching of the signal corresponds to the zero-crossing of a sinusoidal or rectangular signal (with a duty cycle of 50%) at constant frequency, the tap A corresponds to a phase angle of 0°, while tap B corresponds to a phase angle of 180°. This is shown in figure 2, which is a repetition of figure 1 just with the additional information of the corresponding phase angle. Whenever the control signal for tap A (rectangular signal in figure 2 above the left bucket) is active (respectively high) the light generated charge carriers flow into the Φ = 0° (tap A) bucket, while tap B is inactive. Whenever the tap B signal is active, the charge carriers are collected in the Φ = 180 ° (tap B) bucket.
This mechanism can be used to integrate over a half period of the sinusoidal signal in the given example. The 0° information of the modulated light signal is collected during the first half period and the 180° information of the modulated light signal is collected during the second half period
(see figure 14). The QMFLIM2 image sensor has following general characteristics. With a resolution of 1024 x 1024 pixels equipped with microlenses it is the only directly modulatable CMOS image sensor with such a high resolution. The pixel pitch of 5.6 μm is a good fit for a combination with a microscope, and the intra-scene dynamic of 10 bit is well suited for many luminescence imaging applications.
When the image sensor is read out two images are generated simultaneously, a tap A and a tap B image. Due to the image sensor architecture the light signal has to be switched off or suppressed, when the recorded double images (frames) are read out, otherwise the light falling onto the sensor will cause additional noise in the read out images. Any asymmetry can be calibrated out. Alternatively the phase can alternate so that tap A receives phase 0° half of the time and phase 180° degrees for the other half. This causes the asymmetry to average out but the effective frame rate is cut in half.