US2236379A - Monochromator - Google Patents

Description

0. W. PiNEO MONOCHROMATOR March 25, 194-1.

2 Sheets-Sheet 1 Original Filed Sept. 2, 1938 March 25, 1941. O w PINEO 2,236,379

' MONOCHROMATOR Original Filed Sept. 2, 1938 2 Sheets-Sheet 2 MILLIM 0N5 MILLIMET'ERs FIG.2.

INVENTOR. 05PM Wlfim/vp/wia' ATTORNEY.

Patented Mar; 25, 1941 lTE STATES MONOCHROMATOR Orrin Weston Pineo, Milo, Maine, assignor, by mesnc assignments, to American Cyanamid Company, New York, N. Y., a corporation of Maine Original application September 2, 1938, Serial No.

228,127. Divided and this application September 26, 1940, Serial No. 358,570

5 Claims.

This invention relatesto monochromators and more particularly, to monochromators for'use in spectrophotometers. This application is a division of my copending application, Serial No. 228,127 filed September 2, 1938 now Pat. No. 2,227,510 issued January 7, 1941, which application in turn is a continuation in part of my earlier application, Serial No. 54,829 filed December 17, 1935, which application in turn was a division of my earlier application, Serial No. 11,600 filed March 18, 1935 issued as U S. Pat. No.2,l26,410 on August 9, 1938.

In the past, difliculties have been encountered with mohochrom'ators to be used through a wide band of the spectrum, particularly in conjunction with spectrophotometers, due to the fact that the band of light defined by the slits in the monochromator, and particularly by the exit slit, varies in wave length or frequency range in difierentparts ofthe spectrum if the slit width is maintained constant. As a result, a broader band of wave length or frequency is transmitted at one part of the spectrum than at another which is undesirable for spectrophotometric use. It has been proposed to provide a drive, for example by cams, which varies the slit width at difierent parts of the spectrum to maintain a constant range of wave length or frequency in the band of light transmitted by the slit. Such a device represents an improvement and permits maintaining a constant range of wave length or frequency in the transmitted band. However, this range cannot be changed except by dismantling a portion of the device and inserting cams of different shape.

Monochromators according to the present in- I vention are provided with means for maintaining bands of constant wave length or frequency throughout the spectrum and for varying the width of the band by adjustable mechanism so that bands of any desired range of frequency or wave length can be chosen andthe range thus chosen maintained throughout the spectrum.

Essentially, the present invention is directed to a drive linkage between a portion of the monochromator which selects successive bands throughout the spectrum and adjustable linear drive mechanisms, the variable ratio drive members being so constructed that themovement of the linear drive portion is in pro-portion to the logarithm of the slit width so that a displacement of the adjustable linear drive will vary the width of the slit by an amount depending on the amount of displacement. The drive linkage may consist of two or more portions, for example, the variable ratio drive portion may be divided into two sets of cams with an adjustable linear drive connection between them.

, The monochromator may have one or more slits controlled by the present invention. In some cases, it is sufiicient to control only the exit slit. However, where a double monochromator is used with a. recording photoelectric spectrophotometer, more accurate results are obtained by providing all three slits with drives according to the present invention. The

The invention will be described in detail in conjunction with the drawings which show a general type of monochromator slit drive in conjunction with a double monochromator suitable for use with a photoelectric spectraphotometer.

Fig. 1 is a diagrammatic plan view of a monochromator;

Fig. 2 is a side view, partly broken away, of a slit adjusting mechanism and one connecting slit;

Fig, 3 is a vertical section along the line 33 of Fig. 2;

Fig. 4 is an enlarged detail section along the line d---& of Fig. 3;

Fig. 5 is a plan view of the top of the adjusting mechanism of Fig. 2- on an enlarged scale; Fig. 6 is an elevation of the cam drive for slit width adjustment.

In the monochromator shown diagrammatically in Fig. 1, the table 28 carrying the record chart of the recording spectrophotometer and moving on balls 29 (Fig. 2) is moved by a motor or other suitable drive producing a slow movement as is conventional with recording spectrophotometers. The positions of the various lines in the spectrum produced by a prism monochromator are non-linearly related to the wavelengths due to the fact that dispersion is not uniform through the spectrum. The variation in dispersion requires the interposition of a drive of varying ratio between a table moving linearly with wavelength and the movable portion of the monodhromator which shifts the wavelength selector slit through the spectrum. This driveof varying ratio is shown as a cam 30 (Fig. 3) which is driven by the table 28 and in turn moves a bell crank 31 through the roller 33. other arm carries an adjusting screw 34 and flex- The bell crank is pivoted at 32 and on its' ible portion 35 which connects it to the rod 35. This rod in turn moves a carriage M on which is mounted the mirror with the knife edge at defining the wavelength selector slit.

The operation of the monochromator is as follows: light from the light source 47 passes through condensing lenses 48, entrance slit 49 and a second lens 50 to the prism 5|. The 'spectrum formed by this prism passes through lenses 52 and 53, strikes the mirror 55 in which it is reflected, passes through the lens 5d, the second prism 55 and lens 56 to the exit slit 51. Move-' ment of the carriage 44 moves the'slit, defined by the knife edge (it and its image in the mirror 45, through the spectrum produced by the first prism and therefore determines the location in the spectrum of the band which passes out through the exit slit 51 after a further removal of unwanted .wavelengths by the second prism 55. The cam 30 transforms a uniform motion of the table 28 into a non-uniform motion of the carriage 4 so that a given displacement of the table 28 gives always the same change of wavelength of the band of light leaving the monochromator. Therefore, the surface of the table or a record paper carried by it can be uniformly graduated in terms of wavelength of the band leaving the monochromator. If it is desired to have a record bearing a scale uniform in frequency of light rather than wavelength, the shape of the cam 30 will have to be changed accordingly.

Since the width of slit corresponding to a band of given wavelength range will vary in different parts of the spectrum, the present invention provides for changing the width of the three slits of the monochromator'; i. e., 49, 51 and the selector slit between knife edge lit and its mirror image so that their change in width will exactly compensate for the change in width of a band of constant wave length (or frequency) range. This is effected by interposing a drive of varying ratio between the bell crank 3| and the members defining the slits. This is shown in Figs. 1 and 2 as a cam gear composed of the cams 39 and to (detailed in Fig. 6) and a cam 31 carried by the bell crank 3| (Fig. 3). The cam 3'! drives a roller 38 which is mounted in the sector arm 2'! journaled on the shaft 2|, the sector arm being pulled to the left of Fig. 3 by the tension of a spring (not shown). This arm is provided with a slot in its upper end through which passes a clamp screw 25 mounted in a framework 26 and provided with a collar. A second sector arm 22, beside the arm 21, is fastened to the shaft 2| by the pin 23. This arm is provided with an enlarged threaded portion opposite the slot in arm 21 which threaded portion takes the threaded end of the clamp screw 25 (Fig. 4). Both sector arms move in a framework 24 which serves as a guide and limits theirmotion. By the clamp screw 25, the two arms 22 and 21 can be clamped together in any desired relative positions, in which case the motion imparted to arm 21 by the cam 31 is transmitted to the arm 22 and by it to the shaft 2|.

The shaft 2| rotates the two cams 39 in the case of slits 49 and 57. They, in turn, drive the cams 40 which are keyed to the shafts 4|. These latter shafts are provided with two threaded portions 42 and 43 having right and left hand threads respectively engaging with the slit defining members 58 and 59 respectively. Rotation of the shafts 4| serves to move the slit defining members together or apart depending on the direction. The profiles of the cams 31, 39 and ll) are so chosen that the movement of the slit defining members is such as to keep constant the wavelength range (or frequency range) of the beam of monochromated light leaving the exit slit 51. The selector slit defined by the knife edge as and its image in the mirror 45 is actuated in a similar manner which is shown on Fig. l. Shaft 2| carries equal beveled gears 60 which turn the shaft 6| at the same rate as the shaft 2|. The end of shaft 6| is provided with an enlarged portion having an internal slot in which slides a spline on shaft 62. This shaft 62 rotates the cam 63 which in turn rotates the cam 64 keyed to the shaft 65 whose threaded end movesthe knife edge Mi. Cams 63 and 64 are similar respectively to cams 39 and M. The thread on shaft 65 has a pitch substantially equal to that of the threaded portions 42 of shafts 4i increased by the factor cosecant of the angle of light incidence and reflection at the mirror 45. As the carriage 44 moves, the splined shaft 62 slides in the end of shaft Bi. and is maintained at all times in positive driving relation thereto.

i It will be apparent that there is interposed between the drive of the carriage 44 and the slit defining members a linkage consisting of two variable ratio drives (cam 31 and cams 39 and ll] or alternatively, cam 31 and earns 63 and 64) between which there is an adjustable linear drive embodied in the adjustable clamping of the sectors 21 and 22. When it is desired to change the'relative wave length range of the band of monochromated light, this can be effected by adjustment of the linear drive portion of the linkage. At any adjustment, the first varying ratio portion of the drive will take care of maintaining the wave length range of the band constant throughout the spectrum. Adjustment of the linear portion of the linkage is simply effected by screwing out the clamp 25 so that arms 21 and 22 are no longer connected together. These arms may then be moved relative to each other within the limits defined by the slot in 21 and clamped in their new relative positions. This results in a selection of a different wave length range of the band for the monochromator and the same wave length range is'then automatically maintained through the spectrum by the first portion of the linkage, cam 3|;which is a varying ratio drive. For convenience scales are mounted on the top surfaces of the two sec- . tor arms 22 and 21. This is shown in Fig. 5 where one of the arms carries a scale marked for purposes of example in milli-microns and the other carries an index cooperating therewith.

In the linkage shown in the drawings, the cams 39 and 40 and 63 and 64 are exponential cams so that a given displacement in the ad justable linear drive portion will result in varying the slit width by an amount depending on the amount of angular displacement of the shafts 2| and 6| with respect to sector arm 21 resulting from the adjustment. The cams are pref erably designed according to the well-known principle of circular gears to insure rolling contact.

While I have described my invention in detail with reference to its embodiment in a particular type of monochromator and further with that monochromator used for the particular purposes of a recording spectrophotometer, nevertheless after understanding my invention, it will be obvious to those skilled in the art that changes and modifications may be made, without departing from the spirit or scope of the invention,

whereby the invention may be applied to other types of. monochromators and to monochromators used for other purposes and whereby further the invention may be employed in a single monochromator to produce more than one variation of its slits; for instance, a single monochromator might transmit, as selected, either intervals of wavelength, intervals of frequency, or amounts of energy from an associated light source, which intervals, or amounts are maintained constant at a selected value throughout the range of wavelength setting of the monochromator.

What I claim is:

1. In a double monochromator having in optical alignment at light source, a..variable width entrance slit, a spectrum forming prism, an intermediate slit consisting of a mirror and slit defining member both mounted on a movable carriage, a second prism and a variable width exit slit and driving means for moving the carriage so as to cause the exit slit progressively to receive bands of light through the spectrum, the improvement which comprises means actuated by a part of said driving means to vary the width of the slits so that they transmit in all portions of the spectrumlight bands of constant range of a function of wavelength, said means containing a linkage inter-connecting the drive means and the slit defining members; the linkage including a cam actuated by said drive means and having a profile proportional to the logarithm of the slit width for constant range, the cam driving an adjustable linear portion of the linkage which in turn drives an exponential cam drive to the slit defining members, the exponential cam drive having a ratio variation such that the motion in the linear portion of thelinkageis proportional to the logarithm of the movement of the slit defining members, and means for adjusting the adjustable linear portion of the linkage to effect a relative movement of one cam with respect to the other.

2. A monochromator according to claim 1 in which the exit slit defining members are knife edges mounted in frames threaded onto a threaded shaft with right and left hand threads respectively, and the said shaft is driven by the exponential cam drive.

3. In a monochromator having a light source, means for forming a prismatic spectrum from the source having spectral bands of equal wavelength range which vary in width in different portions of the spectrum, members movable relative to each other defining a variable width exit slit, means for. selecting for transmission through the slit light from a desired portion of the spectrum and driving means for actuating said selecting means, the improvement which comprises a drive linkage interconnecting v said driving means and slit defining members, said linkage consisting of a cam driven by the drive means and having a profile which gives a drive proportional to the logarithm of the slit width for transmitting a light band of constant range, a sectorarm journaled on a shaft and moved by said cam, a second sector arm keyed to said shaft and provided with means for looking it to the journaled sector arm, an exponential cam drive driven by the said shaft and driving the movable slit defining members, the ratio means for forming a prismatic spectrum from the source having spectral bands of equal wave length range which vary in width in different portions of the spectrum, members movable relative to each other defining a variable width exit slit, means for selecting for transmission through the slit light from a desired portion of the spectrum and driving means for actuating said selecting means, the improvement which comprises a drive linkage interconnecting said driving means and slit defining members, the linkage having three portions, the first being a variable ratio driv'e actuated by the driving means and having a ratio variation corresponding to the variation in physical width of the slit for constant band range throughout the spectrum, a second adjustable linear drive portion driven by the first portion and a third portion interconnecting the linear drive portion and the slit defining members including a variable ratio drive which transforms linear movement into exponential movement so that the movement of the linear portion of the linkage is proportional to the logarithm of the movement of the slit defining members, and adjusting means in the adjustable linear portion of the linkage for causing movement of the second variable ratio drive with respect to the first and means for locking the adjustment in any adjusted position.

5. In a monochromator having a light source, means for forming a prismatic spectrum from the source having spectral bands of equal wave length range which vary in width in different portions of the spectrum, members movable relative to each other defining a variable width exit slit, means for selecting for transmission through the slit light from a desired portion of the spectrum and driving means for actuating said selecting means, the improvement which comprises a drive linkage interconnecting said driving means and slit defining members, the linkage having three portions, the first portion being a cam actuated by the driving means and having a profile proportional to the logarithm of the slit width for constant band range throughout the spectrum, the second portion being a linear. portion driven by the cam which in turn drives a third portion comprising an exponential cam actuating the slit defining members, the exponential cam drive having a ratio variation such that the motion in the linear portion of the linkage is proportional to the logarithm of the movement of the slit defining members, and ad'- justing means in the linear portion of the linkage for causing movement of the exponential cam with respect to the first cam and means for locking the adjustment in any adjusted positlon.

ORRIN WESTON PINEO.

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