Abstract
<title>Abstract</title> <p>This paper presents a laboratory feasibility study of non-spectral, direct time-of-flight (dToF) singlephoton avalanche diode (SPAD) sensing for liquid purity assessment through sealed packaging, using homogenised milk as a complex-fluid exemplar. Adulteration and degradation can alter transient photon-arrival histograms even when visible appearance is unchanged. Material impurities are organised by matter state (solid, liquid, gas) and mixture attributes, and non-material pathways (temperature abuse, expiration) are included. At 20–30 cm randomised standoff under balanced bright and dark passive illumination, eight experiments are reported: homogenised versus inhomogeneous signatures; nineteen-way multi-class screening with bottle-disjoint evaluation (∼90%); binary pure/impure discrimination on held-out bottles from separate purchases (∼93.33%); frozen-model evaluation on invisible adulterants (∼96%); and manufacturer, fat-content, and product-type transfer checks without retraining. The results support non-spectral transient sensing as a complementary first-line screen when appearance-based cues fail.</p>