In order to improve the stability of the portable near infrared (NIR) spectrometer, a measurement method in NIR spectroscopy (e.g. reference correction with the homologous optical beam) was proposed. Based on the measurement method, the contrast sampling mechanism was designed to obtain the reference beam from light source to ensure that the reference beam and the light source drift in the same trend. The reference beam can be used to eliminate the drift of the measured spectral data due to the drift of the light source. Then, by using the contrast sampling mechanism, the NIR spectrometer for reference correction with the homologous optical beams was developed. Using the newly designed spectrometer, the experiment to compare the performance of the single optical path detection method and dual optical path detection method was conducted. According to experimental results, the stability of the newly designed spectrometer achieved substantial improvement. The baseline drift of the reflectance spectra of gilding aluminium plate was decreased from 0.37% (with single optical path detection) to 0.074%, while the baseline drift of the reflectance spectra of kaolin was decreased from 2.92% to 0.16%.

With the evolution of digital technology, the digital demodulation method has become widely employed in near-infrared (NIR) spectrometers due to its advantages of reducing the volume, enhancing the reliability, and increasing the flexibility of instruments. However, digital demodulation has a lower signal-to-noise ratio (SNR), which can be improved by lowering the measurement precision, speed, and stability. To improve the SNR without sacrificing the measurement speed or stability, an improved digital demodulation method was developed based on multichannel combined acquisition. To validate the proposed method, a multichannel combined acquisition digital lock-in amplifier demodulation system was designed for portable raster scanning NIR spectrometers. In this system, six ADC channels were used in conjunction to acquire the modulated sampling signal. Afterwards, comparative experiments were conducted to compare the conventional and novel methods. The experimental results revealed that the SNR increased from 2499 to 3007 with a measurement time of 16.75 s and 17.08 s, respectively. Furthermore, scanning the reference reflector resulted in a baseline drift of 0.064% and 0.060% with a scanning time of 128.27 s and 128.21 s. Scanning the kaolin sample resulted in a baseline drift of 0.27% and 0.23% with a scanning time of 128.44 s and 128.53 s. In brief, the multichannel combined acquisition method can improve the SNR of spectrometers without compromising their stability or measurement time.