SNR (Signal-to-Noise Ratio) is the ratio of the analytical signal to the noise level — $SNR = S / N$ where $S$ is the signal intensity and $N$ is the noise amplitude, quantifying the quality and reliability of a measurement. Higher SNR means more reliable measurements.

SNR in Analytical Metrology

• Detection: $SNR = 3$ at the detection limit — signal is just distinguishable from noise.
• Quantification: $SNR = 10$ at the quantification limit — signal is reliable for quantitative measurement.
• Improving SNR: Longer measurement time ($SNR propto sqrt{t}$), higher source intensity, better detector, or signal averaging.
• Peak-to-Peak vs. RMS: Noise can be measured as peak-to-peak (worst case) or RMS (statistical) — RMS is more common.

Why It Matters

• Measurement Quality: Higher SNR = more precise and reliable measurements — the fundamental quality metric.
• Trade-offs: Improving SNR often requires longer measurement time — throughput vs. quality trade-off.
• Semiconductor: High SNR is critical for sub-ppb contamination detection and sub-nm CD measurement.

SNR is signal quality — the ratio that determines whether the analyte signal can be reliably distinguished from measurement noise.