CMOS Luminescence Imager With Ambient Light Compensation and Lifetime to Frequency Conversion

This paper presents a novel CMOS image sensor for luminescence imaging with direct lifetime-dependent digital pulse frequency modulated output. Recently reported parasitic insensitive multicycle charge modulation scheme is applied to accumulate photon-generated charges in discrete programmable time windows over multiple exposures. An autoreset pulse serving as the digital output is generated by comparing the multicycle charge integrated output with a reference threshold. The detected luminescence's lifetime is extracted by monitoring the frequency in this digital pulse. To compensate for the background photocurrent generated by ambient light and built-in offset, a charge pump based calibration circuitry is also proposed. Driven by a 10-KHz clock signal with 20-μs pulse width as the integration time window, the proposed circuitry can achieve responsivity and resolution at 575 nm wavelength. It has lifetime resolution of 8 ns. The proposed sensor chip was applied for lifetime measurement of a Ru(dpp)₃(PF₆)₂ fluorescent sample whose lifetime was estimated to be 4.2 μs. A two-dimensional luminescence intensity and lifetime images of a single white LED have also been obtained to further validate its functionality.