Infrared analysis instrument with offset probe for particulate sample

#This invention relates to near infrared analysis instruments and more particularly to instruments of rugged design for field use for analysis of particulate material.

#A powerful technology for analyzing or identifying material involves measuring the reflectance from or the transmission through the material to be analyzed at narrow wavelength bands in the near infrared range, known as NIR. A typical analyzing instrument comprises a spectrophotometer employing a grating to disperse the infrared light into a spectrum to enable the absorption of the matter to be measured at narrow wavelength bands. In one type of instrument, broad band NIR is dispersed into a spectrum by an oscillating grating. As the grating oscillates, the wavelength of the dispersed light passing through an exit slit is scanned through the spectrum. The infrared light passing though the exit slit irradiates a sample of material to be analyzed. The light transmitted through the sample or reflected from the sample is detected to obtain a measurement of the absorption spectrum of the sample. Alternatively the broad band NIR may irradiate the sample, in which case the light reflected from the sample or transmitted through the sample is directed to the grating to be dispersed to obtain the absorption spectrum measurements. In another type of spectrophotometer, a fixed grating is used to disperse the infrared light into a spectrum, which is detected by an array of photodetectors each positioned and shaped to detect a narrow band of the dispersed spectrum. In this kind of instrument, the sample is irradiated with broad band NIR and the light transmitted through or reflected from the sample is dispersed by the fixed grating into the spectrum detected by the array of photodetectors.

#An instrument with a fixed grating array would be preferred for field use because it is more rugged than an instrument employing an oscillating grating and also it is less expensive than an oscillating grating instrument. In an instrument with a fixed grating, it is preferable to use silicon photodetectors in the detector array in order to achieve the desired resolution in the detection. However, silicon detectors are effective to detect light only below 1100 nm and accordingly the array silicon photodetectors should be arranged to detect this portion of the spectrum dispersed by the fixed grating.

#FIG. 1 is a schematic illustration of an instrument in accordance with the present invention.

#FIG. 2 is a block diagram illustrating the electronics of the instrument of the present invention.

#FIG. 3 is an axial sectional view of the probe employed in the instrument of the present invention, showing the probe inserted into a particulate sample

#In the instrument of the present invention, a near infrared light source 11 introduces broad band near infrared light, including the range 500 nm to 1100 nm, into an optical cable 13 designed to transmit the near infrared light. The optical fibers of the cable 13 transmit the near infrared light to a probe 15, which is designed to be inserted into a particulate sample. The probe 15 has both fibers for transmitting light into the sample and fibers for receiving light reflected from the sample. The optical fibers receiving light reflected from the sample are also designed to transmit NIR. The NIR is carried by the optic fibers through a cable 17 to an entrance slit 19 in the wall of a housing 21 of a spectrophotometer, in which fixed optical grating 23 is mounted. The entrance slit 19 is defined by the shape of the set of proximal ends of the receiving optic fibers which extend through the wall of the housing 21. The infrared light received from the sample and carried by the receiving fibers, through the cable 17 is emitted from the proximal ends of the fibers at the entrance slit 19 to irradiate the grating 23. The grating 23 disperses the incident light into an infrared spectrum directed toward an array of abutting silicon photodetectors 25 mounted on the wall of the housing 21. The silicon detectors 25 are positioned on the housing wall to detect the infrared spectrum throughout the range of 500 nm to 1100 nm. Each silicon detector is sized to detect a portion of the spectrum 2 nanometers in width. Accordingly the array includes 300 photodetectors.

#As shown in FIG. 2, the signals generated by the photodetectors 25 in response to the intensities of the narrow bandwidth light detected by each photodetector are transmitted to an NIR spectral analysis data processor 27. The processor 27 analyzes the received signals to identify the sample material, or to determine the percentage composition of its constituents, or to determine the value of a functional characteristic of the material by methods well known in the art. When the material of the sample is being identified, the absorbance spectrum obtained from the unknown sample is matched against spectrum from a library of known samples to determine the identity of the sample such as by the method disclosed in U.S. Pat. No. 5,822,219, issued Oct. 13, 1998, invented by Xiaolin Chen et al. If the sample is to have its percentage constituents determined or to have a functional value determined, the coefficients of an equation expressing the quantity or value being determined as a function of NIR absorbances, are computed from the spectra obtained from samples for which the quantity or value is known by means of a statistical technique such as multiple regression or by least squares. These coefficients are then used with the measurements obtained from the unknown material in the equation to determine the value of the functional characteristic or a percentage constituent of the unknown material.

#As shown in FIG. 2, the probe 15 of the invention comprises an offset design wherein a central set of fibers 29 consists of the fibers in the cable 13 carrying light transmitted from the infrared source 11. The set of fibers 29 project from the distal end of the probe 15 and terminate in a face plate 31. A second set of fibers 33 are formed in a ring around the fiber 29 and consist of the fibers in a cable 17 which carry light reflected from the sample to the entrance slit 19. The distal ends of the optic fiber set 33 terminate in face plate 35 which is set back from the face plate 31. Thus the distal ends of the optic fibers 33 receiving reflected light from the sample are set back from the distal ends of the optic fibers 29 transmitting light into the sample. A sheath 36, which encloses the set of fibers 33, has its distal end terminate at the faceplate 35.