The theory and application of the Integral Equation Method (IEM) in an innovative non-iterative fluorescence lifetime sensing algorithm, is presented. The IEM allows direction calculations and can be integrated on-chip with sensors, advantageous when compared to lifetime based sensing systems using iterative-based algorithms, for example least square methods. Insensitivity to background noise means when integrated with higher dark count; or with low noise sensors in extremely low light conditions, the IEM algorithm is very powerful. The IEM has been adopted in the large CMOS single-photon avalanche diode (SPAD) sensors of Professor Edoardo Charbon. Contact: Professor Edoardo Charbon, University of Delft, Tel: +31-15-278-3667, Email: e.charbon@tudelft.nl.