The Importance of Dynamic Range and Signal to Noise Ratio in Spectrometers

Spectroscopy is a complex technique that needs to consider many changes and nuances, usually framed by terms that are unfamiliar to users or terms that can be explained in different ways. In this context, we have provided some practical definitions of dynamic range and signal-to-noise ratio (SNR), which are generally considered to be common measures of spectrometer performance.

Dynamic Range

In spectroscopy, dynamic range is the ratio between the maximum peak height signal intensities and readout noise ( no peak area ) that a spectrometer can detect. More specifically, dynamic range is the maximum detectable signal (i.e., near saturation) divided by the minimum detectable signal. The minimum detectable signal is defined as the signal with an average equal to the baseline noise.

For our spectrometers, Optosky reports dynamic range in terms of a single acquisition, which is defined as the shortest integration time giving the highest possible dynamic range. The dynamic range specification of the system as a whole is defined as the product of the ratio of maximum to minimum signal at the longest integration time and the ratio of the maximum to minimum integration time.

Signal to Noise Ratio (SNR) 

Signal to noise ratio (SNR) is defined as the signal intensity divided by the noise intensity at a certain signal level, which means it can vary from measurement to measurement. Since system noise typically increases as a function of signal due to photon noise, the SNR function is a plot of individual SNR values versus the signal at which they were obtained. The value of a spectrometer’s SNR reported by Optosky is the maximum possible SNR value (obtained at detector saturation). The SNR response curve for each pixel is assumed to be the same.

The SNR measurement is performed as follows: Set the integration time of the spectrometer according to the light intensity to make the spectrum reach 80% of the full scale position; set the accumulation times to 1, and the sampling interval to 0.1s. Collect the spectrum signal 10 times continuously. Calculate the signal-to-noise ratio of the spectrometer according to formulas (6) and (7):

Where:
SNR---signal-to-noise ratio;
S_λ-the standard deviation of n measurements at the wavelength λ;
A_λi-the value of the i-th measurement at the wavelength λ;
^A_λ-the average value of n measurements at the wavelength λ;
n—Number of measurements.