Research On The Key Technologies Of Hadamard Transform Near-Infrared Spectrometer Based On DMD

Near infrared technology is widely applied into qualitative and quantitative analysis for its fast,non-destructive and accurate characteriscs.Near Infrared spectrometer is used to the vibrational spectrum,and its classification mainly includes dispersive grating instrument and Fourier transform instrument.Fourier transform instrument combining the characteristics of high signal to noise ration(SNR)and high wavelength resolution is more suitable for laboratory.Dispersive optical-based instruments can be devided into two kinds.The fist one consists of one-point detector and scaning grating,and the other one consists of fixed grating and sensor array.The scanning speed of the first kind spectrometer is low,and it is suitable for low speed analysis,such as agicultural-product constituent analysis.The instrument consisting of sensor array acquires the spectrum more quickly and is suitable for process control.Since Digital Micro-mirror Device(DMD)apeared,a new kind of spectrometer has been proposed.This instrument consists of one-point detector,fixed grating and DMD.With Hadamard multiplexing technology,this kind of spectrometer can reach high SNR.The key technologies of this spectrometer are studied in this dissertation.Firstly,noise in the instrument is classified into two kinds,the pre-encoding noise and the post-encoding noise.Different scaling factors of noise suppression to the two kinds of noise with different Hadamard masks are deduced in detail.The MATLAB simulation is carried out to verify the correctness of the results.The results are compared to other scholars' results,and the consistency and inconsistency are analyzed.Secondly,DLP4500 DMD based NIR spectrometer is designed,including optical design,circuit design and software design.Five prototypes are manufactured and key indicators are tested.It is difficult to calibrate the absolute value of the wavelength of the spectrometer.A wavelength calibration method based on the correlation of absorbance curves is proposed.It can make the wavelength repeatability between different prototypes less than 0.2nm,which meets the consistence requirement of clibation transfer.Thirdly,according to the parameters of the designed spectrometer,the noise of the spectrometer is calculated and tested in details.The circuit noise and the photon noise are calculated,and the light power fluctuation and background noise are tested.The photon noise can be neglected for this spectrometer.The method to deal with noise caused by fluctuation of light source is proposed.Finally,the relationship between the noise of light source and the noise of detector circuit is given when Hadamard matrix is applied to the Hadamard transform spectrometer to improve SNR.Three kinds of Hadamard transform methods based on H-complementary matrix,S matrix and S-complementary matrix are tested respectively,under strong light condition and low light condition.It is pointed out that the S matrix has the best performance,but the performance under strong light condition is not as good as the scanning method.Considering the characteristics of DMD in spectrum amplitude modulation,a constant power S-matrix Hadamard mask is proposed,which can improve the SNR of the instrument under both of strong light and low light conditions.