Description:
BACKGROUND OF THE INVENTION
This invention relates generally to spectrophotometry, and more particularly concerns advancements in emission type monochromators used primarily in fluorescence spectrophotometric instrumentation.
In general, there are two characteristically different applications for fluorescence spectrophotometers, such applications imposing quite different demands on the instrumentation. Thus, for exploratory research work, the emission monochromator should be capable of fairly good resolution, say one nm or better, when used to obtain emission spectra; however, when used to obtain excitation spectra or for routine analytical purposes, the monochromator should have the maximum bandwidth consistent with effective exclusion of the exciting radiation, i.e., at least 100 nm. At the same time, the monochromator should have only moderate width entrance and exit slits to permit convenient focusing of the optical beams external to the monochromator and entering into and emerging from it.
SUMMARY OF THE INVENTION
It is a major object of the invention to provide an emission type monochromator having good resolution for exploratory research work, and also having maximum bandwidth as needed in routine analysis.
This objective is met in a double pass monochromator characterized by:
A. MEANS FORMING ENTRANCE, INTERMEDIATE AND EXIT SLITS TO DEFINE SUCCESSIVE SECTIONS OF A DOUBLE MONOCHROMATOR OPERABLE TO ISOLATE A BAND OF WAVELENGTHS WITH HIGH AND LOW LIMITS,
B. BEAM DISPERSING MEANS IN THE BEAM PATH BETWEEN THE ENTRANCE AND INTERMEDIATE SLITS AND BETWEEN THE INTERMEDIATE AND EXIT SLITS, AND CHARACTERIZED BY OPPOSED DIRECTIONS OF DISPERSION IN THE SUCCESSIVE SECTIONS; AND
C. THE INTERMEDIATE SLIT FORMING MEANS INCLUDING A JAW WHICH IS INDEPENDENTLY ADJUSTABLE, AS BY A FIRST CONTROL, TO INCREASE AND DECREASE THE WIDTH OF SAID BAND BY CHANGING ONLY ONE OF THE LIMITS.
In this regard, only the long wavelength limit of the band is for example adjustable, as by varying the intermediate slit. The width of the latter in the case of a monochromator having collimator focal lengths of about 40 centimeters may be controllable between less than one-half and 25 millimeters. Further, the beam typically comprises scattered radiation produced by impingement of source monochromatic radiation upon a sample, in addition to fluorescent radiation, and the intermediate jaw adjustment is characterized in that excitation wavelength radiation scattered from the sample is excluded from passage through the intermediate slit throughout the adjustment.
Additional objects and advantages include the symmetrical location of the entrance and exit slit forming means, the beam dispersing means and the beam reflecting means with respect to the intermediate slit to define a double pass monochromator characterized by subtractive dispersion; the provision of the beam dispersing means either in the form of separate gratings at opposite sides of a plane defined by the intermediate slit, or a single grating for dispersing the beam both before and after beam passage through the intermediate slit; and the employment, as beam reflecting means, of a pair of diagonal mirrors between which the beam is reflected for passage through the intermediate slit, and an optionally usable beam chopper movable in the beam path between the diagonal mirrors.
These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following description and drawings, in which:
DRAWING DESCRIPTION
FIG. 1 is a side elevation showing one form monochromator embodying the invention;
FIG. 2 is a top plan view of the FIG. 1 monochromator;
FIG. 3 is an end elevation view of the FIG. 1 monochromator;
FIG. 4 is diagrammatic showing of a multiple slit width control;
FIGS. 5a to 5d are diagrammatic showing of jaw apertures;
FIGS. 6 and 7 are band pass diagrams; and
FIG. 8 is another monochromator configuration.
DETAILED DESCRIPTION
Referring first to FIGS. 1-3, jaw means includes jaws 10 and 11 defining an entrance slit S 1 , jaws 12 and 13 defining intermediate slit S 2 , and jaws 14 and 15 defining exit slit S 3 . Such structure may be considered as defining successive sections of a double monochromator operable to isolate a band of wavelengths with high and low limits.
The illlustrated monochromator further includes beam reflecting means as for example may take the form of spherical collimating mirror 16, in the beam path between the entrance and exit slits. Also beam dispersing means, as for example single plane grating 17, extends in the beam path between the entrance and intermediate slits, and in the beam path between the intermediate and exit slits. Further, that grating is characterized by opposed directions of dispersion in the successive monochromator sections, the first of which is associated with the beam path between S 1 and S 2 , and the second of which is associated with the beam path between S 2 and S 3 . The complete beam path is shown in principle ray form as including ray 15a passing from S 1 to the mirror for reflection at 18, ray 15b passing from the mirror to the grating for dispersion, ray 15c passing from the grating to the mirror 16 for reflection at 19, ray 15d extending from the mirror to the diagonal Newtonian mirror N 1 for reflection at 20, ray 15e extending from mirror N 1 through slit S 2 and to diagonal Newtonian mirror N 2 for reflection at 21, ray 15f extending from mirror N 2 to the mirror 16 for reflection as ray 15g returning to the grating 17 and ray 15h extending from the grating to the mirror for reflection as ray 15i passing through slit S 3 . In this regard, rays 15a -- 15d, and a part of ray 15e extending from mirror N 1 to the plane of the intermediate slit S 2 , may be considered as first pass rays, while rays which include the remainder of 15e and 15f -- 15i may be considered as second pass rays. Such a double pass monochromator is further characterized by symmetry and cancellation of optical aberations.
FIG. 3 shows schematically a ray 15' emanating as fluorescence radiation from a liquid specimen 24 on which light 25 is incident. The latter orginates at source 26 (which may be a laser) and passes through optical elements 27 which may include a quarter wave retarder and an electrooptic modulator, as described in copending application Ser. No. 192,815 by Ahmad Abu-Shumays and Jack J. Duffield and entitled Linear Polarization Apparatus For Use In Circular Dichroism Polarimetry. The radiation exiting from the monochromator and shown schematically as ray 15j passes to a photodetector and associated electronics and recording mechanism indicated at 28, and described in U.S. Pat. No. 3,013,194 to H.H. Cary. A rotating beam chopper 70 proximate slit S 2 may be used to prevent detection of light scattered toward the exit slit on the first pass through the monochromator, by providing for A.C. detection, as described in U.S. Pat. No. 2,652,742 to A. Walsh.
In accordance with an important aspect of the invention, one jaw of the intermediate slit S 2 is made independently adjustable to increase and decrease the width of the transmitted radiation band; and, typically only the long wavelength limit of the band is so adjustable. Referring to FIG.4, control means 29 is operable to simultaneously adjust jaws of all the slits S 1 , S 2 and S 3 , whereas control means 30 (graduated in nanometers bandwidth) is operable to independently adjust only one jaw of slit S 2 . Thus, as seen in FIGS. 5a and 5c, when control 29 is operated, all the slits may be simultaneously widened, with both jaws of each slit moving away from a center line defining a nominal wavelength. Such center lines are indicated at 31-33. In the other hand, when control 30 is operated, and as appears from comparison of FIGS. 5a and 5b, only jaw 13 is moved away from nominal center line 32, all other jaws remaining unmoved. Jaw 12 remains unomved by control 30 so as to exclude exciting radiation throughout the adjustment of jaw 13. The same functioning appears from comparison of FIGS.5c and 5d.
In FIG. 6, the FIG. 5a condition of equal slit widths S 1 = S 2 = S 3 is further represented by the narrow pass band 32 of scattered fluorescence radiation from a sample, the wavelength of the exciting radiation appearing at 33. When S 2 is increased in relation to S 1 and S 3 as in FIG. 5b, the widened pass band for S 2 appears as at 34 in FIG. 6. Similarly, the FIG. 5c condition of equal and widened slit widths is represented by the band 35; whereas, when S 2 is increased as in FIG. 5d, the widened pass band for S 2 appears as at 36, in FIG. 7.
In one representative monochromator embodying the concepts of FIGS. 1-3, the width of the intermediate slit S 2 is variable from less than one-half mm to about 25mm. The Newtonian mirrors are large enough to accept the 25mm wide beam. To avoid vignetting, it may be desirable to provide a field lens shown as 65 in FIG. 8 at the intermediate slit, and large enough to accept the full beam when the slit is 25 mm wide. The field lens renders the aperture stops in the successive sections of the monchromator optically conjugate. In the example chosen, the monochromator has 4 nm/mm reciprocal dispersion when the intermediate slit matches the entrance and exit slits in width, or is narrower. Widening the intermediate slit S 2 increases the bandwidth of the monochromator unsymmetrically with respect to the nominal wavelength, without changing the proportion of stray to accepted light. This assumes a narrow band fluorescence excitation source, and that the fluorescence is "white," or of constant energy, over the bandwidth.
In the modified form of the instrument seen in FIG. 8, separate gratings 40 and 41 are employed in two entirely separate sections of a double monochromator, and at opposite sides of a plane 42 defined by the intermediate slit S 2 ', and usually provided with a mask at or near this plane to prevent radiation from passing from one monochromator section unto the other by any means other than through the slit S 2 '. Jaw 43 of the intermediate slit forming means is movable relative to jaw 44, by the control as seen at 30 in FIG. 4. Jaws of all slits S 1 ', S 2 ' and S 3 ' are simultaneously movable as by the control means 29 of FIG. 4. Note that a beam entering S 1 ' is defined by a principle ray 46 between S 1 ' and mirror 47, ray 48 between mirror 47 and grating 41, ray 49 between grating 41 and mirror 50, ray 51 reflected from mirror 50 to Newtonian mirror 52, ray 53 reflected from mirror 52 to Newtonian mirror 54 via slit S 2 ', ray 55 reflected from mirror 54 to mirror 56, ray 57 reflected to grating 40, ray 58 transmitted from the grating to mirror 59, and ray 60 reflected to and through exit slit S 3 '.
In FIGS. 1-3 and 8, the gratings may be rotated about an axis located approximately in the "face" of the grating, and normal to the planes of each of FIGS. 2 and 8, as by the schematically illustrated actuators 61, and 61a and 61b, to control the wavelength of radiation transmitted via the exit slits. The gratings 40 and 41 may, for example, have 600 lines per millimeter, be oriented to the substractive dispersion configuration, and be mounted on separate, counter-rotating tables.
A field lens 65 proximate slit S 2 ' accepts the full beam when slit S 2 ' has maximum width, as described previously.
In this double monochromator configuration it is ordinarily not necessary to locate a beam chopper at the intermediate slit, the mask at or near center line 42 being effective to prevent light scattered toward slit S 3 ' on passage through the first monochromator section from reaching S 3 ' except such small proportion as may unavoidably accompany beam 53 through slit S 2 '.