Fluorescence lifetime of molecules can be considered as an additional color, revealing crucial invisible information in a large range of applications, spanning from microscopy of biological cells to image guided surgery. Users are longing for cameras that can image this fluorescence lifetime at a high resolution and at video-rate speed. The detector of choice to achieve this is the Single-Photon Avalanche Diode (SPAD) because it can deliver directly a digital output that includes the precious time-of-arrival (ToA) of each received photon. Current state-of-the-art systems communicate these received ToAs from the sensor to an external processor for computing the lifetime estimation. However, more than thousands of ToAs/pixel/frame are required for good accuracy and precision. For mega-pixel-resolution at video-rate operation, data congestion forms however an imminent showstopper.
During my PhD I investigate how we can use an elegant in-pixel, partly analog circuits, to provide for a lifetime estimation method that only occupies a small silicon area, supports good fill-factor, low power operation, and that has a large dynamic range for the incoming fluorescence light.