Most of TSI’s research has used mobile phones as the source of the time series to be analyzed. However other devices such as photoplethysmograph, NIR (near infrared) and thermography can be used instead because they are all passively acquired time series. Contrast this with imaging which is based on active ultrasound time series.
There’s substantial research into these technologies with respect to time series analysis as evidenced from the following studies:
Source: Espejel-Rivera, M. A., Toxqui-Quitl, C., Padilla-Vivanco, A., & Castro-Ortega, R. (2025). Dynamic Thermography-Based Early Breast Cancer Detection Using Multivariate Time Series. Sensors (Basel, Switzerland), 25(24), 7649. https://doi.org/10.3390/s25247649
Pulse computation from thermal imaging data. (a) Collection point on the carotid arteriovenous complex, the frontotemporal region, and the wrist of the subject. (b) Temperature profile after removing frequency signals lower than 0.67 Hz (40 bmp) and higher than 1.67 Hz (100 bmp) (adapted from [9]).
Source: Cardone D, Pinti P, Merla A. Thermal Infrared Imaging-Based Computational Psychophysiology for Psychometrics. Comput Math Methods Med. 2015;2015:984353. doi: 10.1155/2015/984353. Epub 2015 Aug 3. PMID: 26339284; PMCID: PMC4538766.
Source: Carroll D, Raymond Chieng S, Howden W, et al. Spectral Doppler (ultrasound). Reference article, Radiopaedia.org (Accessed on 23 Feb 2026) https://doi.org/10.53347/rID-67204
Source: Wittenberg, Thomas & Koch, R. & Pfeiffer, Norman & Lang, N. & Struck, Matthias & Amft, O. & Eskofier, B.. (2020). Evaluation of HRV estimation algorithms from PPG data using neural networks. Current Directions in Biomedical Engineering. 6. 505-509. 10.1515/cdbme-2020-3130.