Time-Resolved SPAD Transients for Milk Purity Assessment: Non-Spectral Optical Sensing, Contamination Sensitivity, and a Laboratory Feasibility Case Study

This paper reports a laboratory feasibility case study of non-spectral, direct time-of-flight (dToF) single-photon avalanche diode (SPAD) sensing for liquid purity assessment, using homogenised milk as a complex-fluid exemplar. Adulteration and degradation can alter transient photon-arrival histograms even when visible appearance remains unchanged. Material impurities are categorised by matter state (solid, liquid, gas) and mixture attributes (colour change, homogeneity), and non-material degradation pathways (temperature abuse, expiration) are included in the protocol. Within 20–30 cm, randomised range, with balanced bright and dark passive illumination, eight laboratory experiments are reported: homogenised versus inhomogeneous signatures (Experiment 1); nineteen-way multi-class screening with bottle-disjoint evaluation (Experiment 2, ∼90%); binary pure/impure discrimination with the primary held-out metric on evaluation bottles from separate purchases (∼93.33%, Experiment 3); frozen-model evaluation on new invisible-adulterant bottles (Experiment 4, ∼96%); and manufacturer (Experiment 5), fat-content (Experiment 6), and product-type (Experiment 7) checks without retraining. Training-set fit for Experiment 3 (∼98%) is reported separately and is not treated as a genera