Continuously rotating grating rapid-scan spectrophotometer

• US 4,804,266 A
• Filed: 01/29/1987
• Issued: 02/14/1989
• Est. Priority Date: 07/26/1985
• Status: Expired due to Fees

• Current Assignee: Barspec Limited
• Original Assignee: Barspec Limited
• Inventors: Barshad, Yoav
• Primary Examiner(s): Evans, F. L.
• Application Number:
  US07/008,544
• Time in Patent Office: 747 Days
• Field of Search: 356/308, 356/309, 356/319, 356/326, 356/328, 356/331, 356/332, 356/334, 364/498
• US Class Current: 356/308
• CPC Class Codes: G01J 2001/444 Compensating; Calibrating, e.g. dark current, temperature drift, noise reduction or baseline correction; Adjusting
• CPC Class Codes: G01J 2003/062 motor-driven
• CPC Class Codes: G01J 2003/066 Microprocessor control of functions, e.g. slit, scan, bandwidth during scan
• CPC Class Codes: G01J 2003/2866 Markers; Calibrating of scan
• CPC Class Codes: G01J 3/06 Scanning arrangements arrangements for order-selection
• CPC Class Codes: G01J 3/1838 Holographic gratings
• CPC Class Codes: G01J 3/2889 Rapid scan spectrometers; Time resolved spectrometry

Abstract

A system for rapid-scan spectral analysis comprising a concave holographic diffraction grating continuously rotated at a substantially constant angular velocity to provide a rapid scanning monochromator (a monochromator is used to transfer nominal regions of wavelengths out of the continuous light source). The unique sampling circuitry uses an optical shaft encoder. The angular velocity and angular acceleration of the grating are calculated from time measurements, just before the first wavelength of interest falls on the detector. This information is used to control the Analog to Digital converter sampling rate across the region of interest. The samples as a function of time are stored in a memory buffer so that each data point corresponds to a wavelength.

Claims

1. A monochromator comprising:

a source of polychromatic radiation and means to direct the radiation to an optical grating,

an optical grating located in the path of said radiation, said grating being mounted to a shaft for rotation,

a turntable of relatively high mass mounted on the shaft for rotation with the shaft,

means for continuously rotating the shaft and grating at an approximately constant uninterrupted angular velocity,

an exit slit positioned to intercept radiation reflected from said optical grating,

a shaft encoder assembly mounted on and adjacent said shaft to deliver information representing specific angular positions of the shaft, and

an electronic logic circuit delivering pulses in response to information received from said shaft encoder assembly whenever a specific wavelength is to appear at the exit slit.

2. The monochromator as claimed in claim 1 wherein the grating is rotated about its optical axis.

3. The monochromator as claimed in claim 1 wherein said logic circuit defines the position at which said specific wavelength appears at the exit slit by sensing the angular velocity and acceleration of the grating.

4. The monochromator as claimed in claim 3 wherein said shaft encoder assembly produces index pulses and said logic circuit delivers a pulse whenever a specific wavelength appears at the exit slit by interpolating the time interval between two successive index pulses delivered by the shaft encoder assembly.

5. A monochromator as claimed in claim 1 wherein the means of rotating the grating include a motor and a motor control to set the motor speed.

6. The monochromator as claimed in any of claims 1 to 5 inclusive additionally comprising a filter wheel synchronized with the rotation of the grating.

7. A spectrophotometer incorporating a monochromator comprising:

a source of polychromatic radiation and means to direct the radiation to an optical grating,

an optical grating located in the path of said radiaiton, said grating being mounted to a shaft for rotation, whereby the grating is rotated about its optical axis,

a turntable of relatively high mass mounted on the shaft for rotation with the shaft,

a motor and a motor control to set the motor speed for continuously rotating the shaft and grating at an approximately constant uninterrupted angular velocity,

an exit slit positioned to intercept radiation reflected from said optical grating,

a shaft encoder assembly mounted on and adjacent said shaft to deliver information representing specific angular positions of the shaft,

an electronic logic circuit delivering pulses in response to information received from said shaft encoder assemlby whenever a specific wavelength is to appear at the exit slit,

wherein said logic circuit defines the position at which said specific wavelength appears at the exit slit by sensing the angular velocity and acceleration of the grating, and

wherein said shaft encoder assembly produces index pulses and said logic circuit delivers a pulse whenever a specific wavelength appears at the exit slit by interpolating the time interval between two successive index pulses delivered by the shaft encoder assembly, and

additionally comprising a sample cell through which the monochromatic radiation delivered by the monochromator passes and a detector intercepting the radiation creating an electrical signal representing the intensity of the radiation passing through said sample cell.

8. The spectrophotometer as claimed in claim 7 additionally comprising a filter wheel synchronized with the rotation of the grating.

9. The spectrophotometer as claimed in claim 7 including a non-reflecting back surface of the grating, and additionally including continuous dark current compensation obtained by measuring the electrical dark current signal produced by the detector when the non-reflecting back surface of the grating intercepts the radiation and means for correcting the signal values representing spectral data with the dark current signal value.

10. A spectrophotometer comprising:

a source of polychromatic radiation and means to direct the radiation to an optical grating,

an optical grating located in the path of said radiation,

a rotatable shaft attached to said grating,

a turntable of relatively high mass attached to said shaft,

means for continuously rotating the shaft, turntable and grating at an approximately constant uninterrupted angular velocity,

an angular shaft encoder assembly mounted on and adjacent said shaft to deliver information representing specific annular positions of the shaft,

an electronic logic circuit delivering pulses in response to information received from said shaft encoder assembly,

a sample cell located in the path of monochromatic radiation from the rotating grating and a detector positioned to intercept the radiation exiting the sample cell, said detector creating an electrical signal representing the intensity of the radiation passing through said sample cell, and

a sampling circuit receiving said pulses from the electronic logic circuit and continuously receiving the electrical signal from the detector to thereby sample the continuous electrical signal in response to pulses received and produce a spectrum of radiation intensity values relative to specific wavelengths.

11. The spectrophotometer as claimed in claim 10 wherein said sampled radiation intensity values are presented in digitized form.

12. A spectrophotometer as claimed in claim 10 incorporating a trigger circuit initiating the sampling cycle before the first wavelength of interest intercepts the detector.

13. The spectrophotometer as claimed in claim 12 wherein said angular shaft encoder assembly delivers an index pulse for each revolution and said trigger circuit starts the sampling cycle in response to said index pulse.

14. The spectrophotometer as claimed in claim 13 wherein the detector senses the zero order spectrum (polychromatic radiation reflected by the grating) and produces a signal by which the angular position of the shaft is verified by the relation of this signal to the index pulse.

15. The spectrophotometer as claimed in claim 11 including associated circuitry comprising a scratch-pad memory buffer receiving digitized data from said sampling circuit in real time and adapted to transmit said data to subsequent processing unit.

16. The spectrophotometer as claimed in claim 15 wherein the data from said scratch-pad memory buffer are transmitted to vacate the buffer for a burst of fresh data from the following scanning cycle during the next revolution of the grating.

17. The spectrophotometer as claimed in claim 15 including subsequent processing units comprising direct processing means, permanent storage and means for retrieving information from said permanent storage for processing in conjunction with said direct processing means.

18. The spectrophotometer as claimed in any of claims 10, 14, 15, 16 or 17 including a non-reflecting back surface of the grating, and additionally including continuous dark current compensation obtained by measuring the electrical dark current signal produced by the detector when the non-reflecting back surface of the grating intercepts the radiation and means for correcting the signal values representing spectral data with the dark current signal value.

19. A spectrophotometer comprising:

a source of polychromatic radiation and means to direct said radiation to an optical grating,

an optical grating located in the path of said radiation, said grating being mounted to a shaft for rotation about the optical axis of the grating at an approximately constant uninterrupted angular velocity,

a turntable of relatively high mass mounted for rotation with said grating,

a sample cell through which the radiation diffracted by the grating passes,

detector means to intercept said radiation and deliver a continuous electrical signal representing the intensity of radiation impinging on said detector means after passing thorugh said sample cell,

means for sensing the actual angular position of said grating,

means for producing signal representing the selected wavelengths at which the spectral data are to be measured, and

means for sampling the continous electrical signal produced by said detector means in response to said signals representing said selected wavelengths.

20. The spectrophotometer as claimed in claim 19 wherein the means for sensing the actual position of the grating includes a pulse producing shaft encoder assembly mounted on and adjacent the shaft about which said grating is rotated.

21. The spectrophotometer as claimed in claim 20 wherein said means for producing signals representing said selected wavelengths include an electronic logic circuit producing signal pulses in response to pulses produced by the shaft encoder assembly, said electronic logic circuit sensing the actual angular rotational velocity and acceleration of the grating and interpolating to produce the signal pulses representing said selected wavelengths between two successive index pulses produced by the angular shaft encoder assembly.

22. The spectrophotometer as claimed in claims 20 or 21 wherein the shaft encoder assembly produces an index pulse for each revolution and the detector means senses the zero order spectrum impinging on the detector means and produces a zero order signal, the relative angular position of the shaft, grating and detector means being verified by the relation between said index pulse and said zero order signal to thereby continuously calibrate the shaft and grating to the detector means.

23. The spectrophotometer as claimed in claim 19 wherin the grating is rotated by means of a motor and motor control to set motor speed.

24. The spectrophotometer as claimed in claim 19 additionally comprising an optical filter wheel rotated in synchronization with the grating.

25. The spectrophotometer as claimed in claims 19 or 21 including a non-reflecting back surface of the grating, and additionally including continuous dark current compensation obtained by measuring the electrical dark current signal produced by the detector means when the non-reflecting back surface of the grating intercepts the radiation and means for correcting the signal values representing spectral data with the dark current signal value.