Because of the substrate back reflectance phenomena, the reflectance of optical elements on a transparent
substrate is totally different from that of on an opaque substrate. In this paper, the differences between
normal and quasi-normal substrate back reflectance are analyzed for the first time. Then a detailed study
of the influences of the substrate thickness, substrate material, measuring distance and other parameters
in the measurement are performed, the computational expressions of fitting surface reflectance are
derived, and a compensation model of the substrate back reflectance measurement is proposed.
Simulation and experimental results prove the correctness of the theoretical model and show this
compensation model can be used in the substrate back reflectance measurement.
Traditional dispersive spectrographic systems based on single slits cannot achieve dual advantages simultaneously and
statically. A novel dual advantages system model for dispersive spectrographic system appears and the traditional entrance
slit is replaced by a coded aperture based on orthogonal independent column codes. The extended aperture lets more energy
enter the system and the independence of the codes helps to avoid degradation of the spectral resolution. On the basis of the
derivation of system model, this paper emphasizes on system tolerance analysis, to find out the key factors which affect the
realization and performance of the system model. First, the influences of inaccurate system adjustment and calibrations on
system performance were studied. The standard methods for system adjustment and calibrations were represented. Then,
simulations were carried out to study the influence of several abnormal situations of the coded aperture on system
performance, which includes the existing of error fabrication and imperfect illumination of the coded aperture. The
analysis result shows that the system can still function normally when the coded aperture has a fabrication error rate of 30%
or severe imperfect illumination of the coded aperture.
Scanning and interference imaging spectrometer are widely used in earth observing, microscope imaging, etc. Motion
mechanisms are utilized in all these simple imaging spectrometers, which make the system big, expensive and unstable.
A novel static non-imaging spectrometer is introduced specifically. 2D (two-dimension) static multi-channel spectral
measurement is realized in this spectrometer, utilizing 2D hadamard matrix code mask and 2D detector. The hadamard
matrix is different from traditional S circular code. The orthogonal property of hadamard matrix is utilized, and with
special transform negative codes in hadamard matrix are also realized. Basing on this non-imaging spectrometer static
imaging spectrometer is developed. First 2D imaging spectrometer can be divided into many 1D ones which work
independently. The 1D imaging spectrometer is obtained with position information added into the hadamard coding
system in dispersive direction. To get 2D imaging spectrometer, extend the coded aperture: put some masks together in
non-dispersive direction. According to this model, the spectrum of an image instead of a slit can be estimated at one
time. The optical system is designed to simulate the imaging spectrometer. Compared to the simple scanning imaging
spectrometer the simulated result shows the model's resolution can be compared to simple scanning spectrometer, but
the scanning time of simple scanning imaging spectrometer is about 3 times of our simulation. At last some
implementation issues are mentioned.
Traditional dispersive spectrographic systems usually base on slit imaging. The entrance slit can ensure a remarkable
spectral resolution but a loss of system throughput and vice versa, thus this kind of instruments can't achieve Jacquinot
advantage and Fellgett advantage simultaneously and statically. To derive a high gain model for the stationary dispersive
systems with these two advantages, traditional single-slit and the multi-slit spectrographic systems were studied. A
method for single channel spectrum abstraction in a multi-slit spectrographic system was derived. This method
demonstrates that if each individual slit in a multi-slit system possesses a transformation form of orthogonal independent
column codes, the spectrum of each individual slit can be abstracted from superposed spectrum, spectral resolution of
equivalent single-slit system is obtained and the system achieves dual advantages. This high gain computational system
model is applicable for almost all kinds of existing dispersive spectrographic systems. Furthermore individual coded slits
can be integrated as a coded aperture to increase system's integration level and reduce off-axis aberration. Based on the
derivation of theoretical model, two kinds of coding forms' mathematical models were studied. To verify the derived
theory, a testing system with a specially designed flat-field holographic concave grating and a coded aperture of order 16
Hadamard matrix form was set up. The experiment indicated that although optical aberration and other system noise
were involved, this high gain system model could still achieve a high spectral resolution of 0.4nm as single-slit system,
while remarkable system etendue (8X) and SNR (4X) were also obtained, which proved the correctness of the theoretical
derivation.
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