System to reduce wave shift error in spectrophotometer caused by hot spots in the light source

Abstract: An improved near infrared spectrophotometer is disclosed which reduces wave shift errors arising from hot spots in a light source. A ground quartz plate (21) uniformly diffuses radiation from the light source (12) to evenly illuminate the entrance slit (18) for a diffraction grating (20). Radiation emerges from the entrance slit with a uniform angular intensity distribution and is dispersed into a spectrum by a reflecting diffraction grating (20) towards an exit slit (26). A narrow wavelength band of radiation passes through the exit slit to illuminate a sample (28). Because the entrance slit is uniformly illuminated, without regard to variations in radiation intensity due to hot spots in the source, wave shift errors in the reflectivity measurements for the sample are reduced.

Description: Background of the Invention This invention relates to a spectrophotometer using near infrared energy to irradiate samples. Spectrophotometry determines the concentration of components in a sample by measuring transmission of radiation through or reflection of radiation from a test samples at narrow wavelength bands. Radiation generated by a light source enters the spectrophotometer through an entrance slit. The light passing through the slit illuminates, an oscillating diffraction grating which disperses the light into a spectrum. The spectrum is imaged across an exit slit and a narrow wavelength component of the spectrum passes through the exit slit and illuminates a sample after passing through exit slit optics. As the grating oscillates the wavelength of the spectral component irradiating the sample is swept through the spectrum. Accurate sample analysis hinges on the consistency of the angular position of the spectrum dispersed from the grating and errors in spectral measurements can arise due to inconsistencies in the light source. For example, the light intensity across the entrance slit can vary due to hot spots in the lamp. The resulting uneven illumination of the slit causes an apparent shift in the position of the slit and in the spectrum dispersed from the grating. Consequently, in near infrared spectrophotometric analysis, wave shift error of up, to two nanometers can sometimes occur when the lamp is changed, thus requiring recalibration of the instrument or otherwise reducing the accuracy of the analysis provided by the instrument. The prior art US-A-4 696 570 has employed fiber optics to uniformly illuminate the entrance slit, with some success. However, the fibers attenuate the radiation from the light source, reducing the intensity of radiation illuminating the sample, thus reducing the signal to noise ratio in the output signal from the spectrophotometer. In accordance with the present invention there is provided for a spectrophotometric instrument according to claim 1. The Reference DE-A-2 437 310 proposed to replace focussing optics with a quartz plate and by this expedient achieves more uniform light intensity at the specific wavelength of 2714 Å for successive discharges from a light source in which the light is generated by the discharge across two electrodes. By replacing the quartz lens with a quartz plate, the inventor was able to achieve a more uniform intensity for successive discharges then he was with the quartz lens. The inventor chosed to replace the lens with a plate even though this modification greatly reduced the light intensity passing through the entrance slit. The reason given for this improvement is that the quartz lens has spherical and chromatic defects which would not be found in the quartz plate. However, the cause of the different variations in the detected intensity may have been caused by the fact that the position of the arc varies slightly in successive discharges causing the photodetector not to be positioned properly to receive the 2714 Å wavelength from the grating for each discharge flash of light. By using a flat plate instead of a lens to focus the discharge on the entrance slit, the system may achieve more uniform illumination than would be achieved by a lens because of the lack of focussing. A further embodiment of the invention is characterized by the fact that said light source is an infrared light source and said broad spectrum light is in the near infrared wavelength band, and wherein said plate is a quartz plate. A further embodiment of the invention is characterized by the fact that said light source further comprises a reflector arranged to focus an image of said filament near said entrance slit. SUMMARY OF THE INVENTION The present invention is a system which lessens wave shift errors in spectrophotometric readings without the radiation losses related to fiber optics. The invention employs a spectrophotometer with a ground quartz plate located between a light source, such as a filament lamp, and the entrance slit of the spectrophotometer. To minimize radiation losses in the space between the source and the slit, the quartz plate is positioned immediately adjacent to the slit. Because the ground surface of the quartz plate diffuses light from the light source, the entrance slit is evenly illuminated. Notwithstanding variations in the incident radiation from the light source, the light projected from the slit is uniformly distributed through an angle of projection spanning the grating so that the grating is uniformly illuminated. The grating then produces a predictable spectrum across the exit slit and facilitates accurate measurement of the sample's reflectivity. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic drawing of a spectrophotometer according to the present invention; and Figure 2 is an enlarged fragmented view in elevation showing the quartz plate in front of the entrance slit of the spectrophotometer and taken along line 2-2 of Fig. 1. DETAILED DESCRIPTION OF THE INVENTION As shown in Figs. 1 and 2, the instrument of the invention comprises a housing 1⊘ containing a concave holographic reflection diffraction grating 2⊘, positioned to be illuminated through an entrance slit 18 by a light source 12 in the form of a linear filament infrared lamp. The lamp 12 generates light with wavelengths distributed through the near infrared spectrum. A reflector 14 in the shape of an elliptical cylinder, partially encircles the light source to reflect light radiated by the light source 12 toward the entrance slit 18. The elliptical cylindrical shape of the reflector defines a proximal focal line along which the linear filament of the light source 12 is positioned and a distal focal line located approximately along the entrance slit 18 so that an image of the filament is focused near the entrance slit. Between the light source 12 and the entrance slit 18 is an infrared light transmitting plate 21 having a light diffusing interface. The infrared light transmitting plate comprises a quartz plate adjacent to the slit 18 with a ground surface facing the slit. The ground surface is the light diffusing interface. Infrared light from the source 12 is transmitted through the plate to the ground surface where the infrared light is diffused to uniformly illuminate the entrance slit 18. The slit 18 projects the light with a uniform angular distribution toward diffraction grating 2⊘, which is oscillated at high speed by means of a direct drive motor 22. The details of the direct oscillating motor drive are disclosed in copending application Serial No. 294,679, filed January 9, 1989 invented by Phillip A. McGee, and assigned to the assignee of this application. The grating 2⊘ disperses the infrared light from the slit 18 into a spectrum reflected toward an exit slit 26. A narrow wavelength band from the spectrum passes through the exit slit 26 and through exit slit optics before illuminating sample 28. As the grating oscillates, the center wavelength of the light passing through the exit slit is scanned through the infrared spectrum. Photodetectors 29 detect the intensity of the resulting radiation reflected from the sample and generate an output signal representing this intensity. The output signal of the photodetectors 29 is amplified by an amplifier 31 and applied to an analog-to-digital converter 33. As the grating oscillates and the center wavelength of the narrow band passing through the exit slit is scanned through the spectrum, the analog to digital converter converts the output analog signal of the amplifier to digital values at incremental index points distributed through the spectrum so that a set of digital values are obtained representing the intensity of the reflected energy from the sample as the wavelength irradiating the sample varies throughout the spectrum. These digital values are received and processed by a computer 35. As shown In Figure 1, a shaft position indexing system 37 coupled to the motor 22 transmits pulses to the analog-to-digital converter 33 and to the computer 35 as the motor 22 pivots the grating 2⊘ and scans the wavelength applied to the sample through the spectrum. In response to each applied pulse, the analog-to-digital converter converts the analog signal received from the amplifier 31 to a digital value and in this manner generates digital values representing the intensity of the reflected radiation at increments distributed through the spectrum. A computer 35 counts the pulses received from the indexing system 37 and thereby monitors the angular position of the grating 2⊘ and the center wavelength of the band currently being transmitted through the exit slit. Thus, the computer 35 monitors the wavelength of the band which caused the reflected intensity represented by the digital signal currently applied to the computer by the analog to digital converter. Because the entrance slit is uniformly illuminated as a result of the ground quartz plate, the shaft position on which the grating is mounted can be accurately calibrated in terms of the wavelength being transmitted through the exit slit. Without the ground quartz plate to ensure uniform illumination of the exit slit, variations in the illumination of the entrance slit caused by hot spots in the light source would cause an apparent shift in the entrance slit and cause a corresponding shift in the spectrum reflected by the grating, so that for a given shaft position of the grating the wavelength transmitted through the exit slit might vary by as much as 2 nanometers when the lamp is replaced. With the ground quartz plate distributing the light from the light source uniformly over the entrance slit in accordance with the present invention, the shift in wavelength caused by variations in the light source is reduced to 2/1⊘⊘ of a nanometer. Thus, with the system of the present invention, the system does not need to be recalibrated when the lamp of the light source is replaced. The above description is of preferred embodiment of the invention and modification may be made thereto without departing from the scope of the invention, which is defined in the appended claims.

Claims: A spectrographic instrument comprising a housing (10) defining an entrance slit (18), an optical grating (20) within said housing positioned to be irradiated with light transmitted through said entrance slit and to disperse the received light into a spectrum, a broad spectrum light source (12) positioned external to said housing and adapted to direct broad spectrum light toward said entrance slit, a plate (21) transmissive to said broad spectrum light positioned between said light source and said entrance slit, characterized by means (14) to focus an image of said light source near said entrance slit and wherein said plate (21) has a ground surface on one side to diffuse the light transmitted from said source (12) towards said entrance slit (18) by said focusing means (14). A spectrophotometric instrument as recited in claim 1, wherein said light source (12) is an infrared light source and said broad spectrum light is in the near infrared wavelength band, and wherein said plate (21) is a quartz plate. A spectrophotometric instrument as recited in claim 1, wherein said light source (12) comprises a filament lamp having its filament arranged parallel with the long dimension of said entrance slit (18). A spectrophotometric instrument as recited in claim 3, wherein said light source (12) further comprises a reflector (14) arranged to focus an image of said filament near said entrance slit (18). A spectrophotometric instrument as recited in claim 1 or 4, wherein said plate (21) is located adjacent to said entrance slit (18). A spectrophotometric instrument as recited in claim 1 or 5, wherein said ground interface is on a surface of said plate (21) facing said entrance slit (18).

Inventors: HONIGS DAVID E, HONIGS, DAVID E.
Applicants: NIRSYSTEMS INC, NIRSYSTEMS INCORPORATED
Application year: 1990
Main classification: G01J3
IPC codes: G01J3, G01N21