Fig. 1. Simulation of absorption spectra of CO₂ and interfering gases at air volume fraction

Fig. 2. Schematic diagram of detection system

Fig. 3. Temperature change in detection chamber under skin fit. (a) Results without temperature control; (b) results under temperature control

Fig. 4. Physical device diagram of the skin CO₂ detection system based on QEPAS technology. (a) Equipment; (b) sample absorption pool; (c) detection chamber; (d) analog PID control circuit; (e) temperature controlled circuit

Fig. 5. Signals of the first harmonic frequency locking system. (a) Locking process of the first harmonic signal of the sample absorption cell; (b) second harmonic signal of QTF

Fig. 6. Gas distribution system

Fig. 7. Test results of system response time. (a) Second harmonic signal response curve for variation of CO₂ volume fraction from 0 to 2000×10^-6; (b) results of exponential fitting of the volume fraction change process

Fig. 8. Linear analysis of the system

Fig. 9. Allen analysis of variance

Fig. 10. Schematic diagram of skin detection. (a) Detection site; (b) detection method

Fig. 11. Results of the skin CO₂ detection by the system on the left arm, left palm, left cheek, and left armpit with linear fitting results for the four sites

Fig. 12. Linear fit results obtained from four identical sites of skin CO₂ testing on two adjacent days for the same volunteer

Fig. 13. Schematic diagram of human detection sites and statistical results of CO₂ release rates from four sites in three volunteers