The present invention relates to an instrument for measuring in relatively short periods of time concentrations of constituents in optically dense materials using the spectra
near infrared radiation transmitted through thick samples of the material while the material is either stationary or flowing. The invention uses a
broadband incandescent intensity stabilized
light source combined with collimating
optics to transmit a
parallel beam of light through the
material under test. The light transmitted through the material is then collected by a focusing lens and imaged onto a rectangular entrance slit of a special purpose
spectrometer. This
spectrometer has no
moving parts and employs a fixed
diffraction grating to physically spread the image of the entrance slit into a continuous range of wavelengths. A portion of the diffracted slit images covering the selected portion of the near
infrared range is focused onto an array of individual rectangular photodiodes. By using relatively large area photodiodes and a relatively small number of photodiodes, high sensitivity is achieved and low intensity
radiation levels can be measured quickly. By using a relatively narrow spectral range,
medium resolution can be achieved. The outputs of each
photodiode, or the outputs of a selected number of the photodiodes, are fed into current to
voltage converters: either resistive (instantaneous) or preferred capacitive (integrating). Thus the outputs of all photodiodes are measured in parallel, which reduces the time to acquire the spectra. The
gain of these current to
voltage converters is programmable so that both
high intensity and low intensity
near infrared radiation levels can be measured without reducing the intensity of the
radiation incident on the
material under test and thereby eliminating the need for
moving parts in the
spectrometer. After the spectra are acquired, they are operated on by models developed to predict the percentages of various constituents in the material. These models are pre-calibrated using spectra obtained from materials of known concentrations and developed using chemometric, neural net, and / or genetic algorithms.