Variable angle attachment for infrared reflection spectroscopy

SWEDEN

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PATENTS AND REGISTRATION OFFICE

EXPLANATORY STATEMENT No 314 535 intci G 02 b 5/14 κι. 42 h 20/01

Patent Application. No. 8807/66 Received on 28 VI 1966

Validity Day, 28 VI 1966

Ans. generally available on 1 Jan 1968

Ans. published and the pamphlet published on 8 IX 1969

Priority requested from 30 VI 1965 (United States, 468,283)

NV PHILIPS 'GLOBILAMPENPAERIEKEN, EINDHOVEN, NETHERLANDS

Inventor NJ NJ Harrick

Agents: G Emerot

Device for changing the angle of incidence of a beam to a cell for internal reflection by infrared spectroscopy

The present invention relates to a device for changing the angle of incidence of a radiation beam to a cell with internal reflection by infrared spectroscopy.

Spectroscopy using internal reflection was widely used, among other things. in the analysis of substances by infrared absorption spectrum. In infrared spectroscopy, as is known, the radiation is directed through a cell or a light conductor at such an angle that it falls on a surface for examination at an angle greater than the boundary angle, so that total reflection occurs, with the exception

Dupl. kl. 42 lu38 when prevented by the presence of an absorbent material on the surface for examination. The choice of angle determines the depth of radiation penetration into the material on the interface and by changing the angle of incidence the material can be analyzed to a selected depth. In addition, at a multiple reflection cell of a specified length, a change in angle of incidence allows the number of reflections to be regulated, thereby also regulating the degree of absorption that the bundle undergoes in its interaction with the unknown substance intended for analysis.

Attempts have already been made to adapt this technology to conventional infrared spectrometers, one of the difficulties encountered being the limited space available for a cell in a conventional apparatus. To overcome this difficulty, an optical system has been used to bring the bundle out of the normal path so that it passes through the cell outside the confined examination room and then returns to the apparatus. It is clear that a mechanism by which the angle of incidence of the bundle on the cell surface can be changed also easily detaches the bundle from the desired web. In addition, any change in the wavelength of the bundle can change the focus of the optical system. Therefore, within the infrared spectroscopy, a device is needed with which the angle of incidence of the bundle can be varied through the cell and at the same time enables the bundle to be returned to the correct path, whereby the loss of focus can be small or easily corrected.

The object of the invention is to provide a device which solves this problem in a simple manner, especially when using a focused instead of a collimated light beam. A device according to the invention contains a first and a second reflecting surface which intersect at almost a right angle, the first surface being arranged to intercept an incident radiation beam and the second surface to deflect the bundle in a certain

- 3 direction, a retro director located at some distance from the first reflecting surface and having a receiving portion for receiving the bundle reflected at the first surface and, furthermore, a directional portion for directing the bundle to the second surface, a cell with internal reflection , which cooperates with the retro-director and has a surface for examination, against which the bundle is reflected at a fixed angle in an area in which also the boundary angle lies, and means for rotating the retro director to change the angle of incidence of the bundle to the surface of the cell for examination. In this way, the distance between the retro director and the perpendicularly intersecting, reflecting surfaces remains unchanged, while the angle of incidence is changed by rotation of the retro director. In this case, the particular optical properties of the retro-director are thus fully utilized, in which the retro-director namely the incident bundle is always parallel to the exit bundle regardless of the orientation, while the distance between the bundles and thus also the path traveled by the bundle remains the same.

In this way, when using a collimated bundle, no displacement of the outgoing bundle occurs, no matter which angle of incidence is selected, and in this system no other setting is required other than such an arrangement of the cell to intercept the bundle. In a focused bundle, where it is desirable to use a multiple reflection cell, through which the bundle passes twice, an adjustment is evidently required to maintain the bundle's focus on the incident surface of the cell. However, it has been found that with very little to maintain foku314535

The adjustment required of the angle of incidence is variable within a wide range and that only a slight displacement of the entire device is required to return the bundle to the desired optical path.

The invention will now be described in more detail with reference to the accompanying drawing, in which Fig. 1 is a schematic side view of the optical system of a device according to the invention; Fig. 2 is a simplified side view of the optical system of Fig. 1; according to the invention with an optical system according to fig. 1, fig. 4 shows a schematic side view of the optical system of another embodiment according to the invention.

Fig. 1 schematically shows the examination room of a conventional infrared spectrometer. This apparatus, as known, has a source of infrared radiation sent through a cell containing the material intended for analysis. The leaving bundle, the intensity of which has been modulated in dependence on the wavelength by the absorbent properties of the sample to be examined, enters through an entrance slot again in the spectrometer and is passed through a monochromator which divides the bundle into different portions with respect to the waveguide, whereupon it is broken apart. an infrared radiation display device, usually a thermocouple whose output is coupled to a recording device. This, in turn, is linked to a radiation analyzer. The recording device writes a curve indicating the radiation intensity depending on the wavelength of the infrared radiation so that the conventional and well known infrared absorption spectra are obtained. The drawing shows, schematically, how the bundle from the radiation source in the form of a focused

A radiation bundle 11 emerges from an opening 10. On the opposite side of the cell examination chamber 12, an entry slot 13 for the return bundle is mounted. The monochromator, the radiation syndication device and the recording device are not manufactured, since they are well known elements.

According to the invention, there is a right angled silver prism 15, which has a first reflective surface 16, which deflects the bundle from the examination room, and a second reflecting surface 17, which is perpendicular to the surface 16 and directs the return bundle through the entrance slot 13 · The focused bundle, which after its reflection on the surface 16 has been designated 18, falls against a device 20, which is basically a two-dimensional angular reflector or retrodirector (reversing system). This retro director 20 can pivot or rotate about an axis 21, the position of which is fixed relative to the axis 22, in which the reflecting surfaces 16 and 17 of the prism intersect. The retrodirector 21 has a receiving surface or first surface 23 to which a cell 25 for internal reflection is mounted. In the fabricated embodiment, this cell consists of a thin infrared pervious plate 26 in which a sample for analysis is applied to one of the main surfaces 27 and 28 and which at the combined entry and exit end is provided with an infrared transmissible half cylinder 29 , which allows the bundle to occur at different angles in the cell 25 ·

As shown in the drawing, the bundle 18, immediately before it falls on the half-cylinder 29, is focused at a point 31, so that on its way through the half-cylinder and the cell 25 it is polymerized and, after exiting the half-cylinder 29, is refocused to a point t

at 32. This bundle, which, after exiting the half-cylinder 29, is designated 33, falls on a second reflecting surface 3® +

- 6 of the retro-conductor 20. As can be seen from Fig. 1, the optical paths thus obtained are the same as would be obtained if the cell 25 had been replaced by a plane mirror which extended in the plane 23 perpendicular to the reflecting surface 34. The bundle 33 is reflected on the other reflecting surface 3 3 as a bundle 35, which is then reflected on the second reflective surface 17 of prism 15 and then forming? a bunch 36? which is directed through the instrument entry slot 11.

The angle of incidence of the bundle against the surfaces 27 and 28 of the cell 25 for examination can be changed by rotating the retro-director 20 around the shaft 21. As is apparent from FIG. 2, which shows a simplified side view of the optical system of the device according to FIG. which, for simplicity's sake, a non-focused bundle was used, while the corresponding parts are provided with the same reference numerals as Fig. 1. The position of the retro-conductor 20, which corresponds to the position of Fig. 1, is made in solid lines. The broken lines indicate the change that occurs in the bundle's path if the retro director is rotated to a new position 20 'and the cell 25 has been brought to a new position 25'. The bundle 18 now falls against the surfaces of the cell for examination at an angle θ / , which is smaller than the angle θ ^, below which the bundle falls on these surfaces at the solid line position of the cell. An out of cell Exiting bundle now follows a new path 33 'instead of the path 33, which it follows when the retro director took the original position. However, it is also seen that it knipoe. which is now deflected from the second reflecting surface 34 ', finally coincides with the position of the bundle which it assumed when the reflector.-ί j λ;&gt; 35

- 7 was in its original position so that the bundle 36 entering the spectrometer column 13 does not deviate from the original path. The selected geometry causes the bundles 18 and 35 to remain parallel and to run at a fixed distance 38 from one another regardless of the orientation of the retrodirector 20, unless the distance between the shafts 21 and 22 remains the same. Furthermore, the optical path length remains unchanged in this way, in the case of a collimetre bundle, the angle of incidence of the bundle to the cell is changed by rotating the retro-conductor 20 without displacing the outgoing bundle and without requiring any new setting in the system.

Therefore, the device shown in the optical system of Fig. 2 can be used with a collimated bundle on single or multiple reflection cells, which do not exhibit any refraction at the entry and exit surfaces. However, in multiple reflection cells of the type prepared in Figure 1, at which a focused bundle is desired in the examination room, a new setting is required to equalize the cell's displacement if the retrodirector is rotated relative to the focal point 31 (of course, an additional optical system is required to again focus the bundle at the monochromat entry slot). This offset can be corrected very easily in a number of ways. :

Figure 1 shows a first embodiment in which the cell performs a translational movement on the receiving surface 23 of the retro-director, whereby the whole device is assigned a horizontal translation movement to restore the cell in the correct position relative to the focal point 31. The new position in Figure 1 is indicated by broken lines. . As shown in the figure, cell 25 has been displaced outwardly a distance of 4-0

8 relative to shaft 21, which is greater than the original distance 4l between the cell and the shaft. The entire device is now moved to the right a distance 42. This causes the focal point 31 to be displaced upwardly a distance similar to the distance 42, so that the bundle is now well focused with respect to the offset cell 20. Since the prism 15 is fixed relative to the retro director. 20, this also ends up in a new position 15. As a result, after leaving the second surface 17, the exit bundle 35 is brought back to the original web.

Fig. 4 shows a second embodiment in which the position of the cell 25 on the receiving surface 23 of the retro-conductor 20 is fixed. Now that the retro director 20 is rotated to a new position 50j, in order to achieve proper focusing, it is necessary to displace the whole device horizontally - βη stretch 51 and vertically a distance 52, whereby the bundle returns to the correct path. It has been found that when changing the angle of incidence within a wide range, for example between 15 ° -75 ° in the embodiment of Fig. 4, all distances by which a new adjustment must be made in the vertical and horizontal direction are not greater than 6 mm. It is also clear that suitable mechanisms can be introduced to interconnect the required movements so that in the embodiment of Fig. 1, a rotation of the retro director 20 automatically causes a translational movement of the cell on the receiving surface 23 and at the same time the whole device is assigned such a large translation movement that the focus is restored. . In the embodiment of Fig. 4, a rotation of the retro director 20 automatically causes the entire device to move vertically and horizontally a distance necessary for resetting the focus.

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9 Fig. 3 shows in perspective an embodiment in which the optical systems of Figs. 1 and 4 have been combined. The prism 15 and the retro director 20 are mounted on a common support surface 60, which in turn is supported by a support plate 6l, which is provided with micrometric screws 62 and 63, with the aid of which the entire support 60 is vertically and horizontally a desired distance. The retro director 20 can be rotated by a button 65 ·

The retrodirector 20 contains a spindle 66, which at 67 is stored in the carrier surface 60 and at the other end of an arm 68, which is arranged on the bark 69 affixed to the carrier surface 60. The retrodirector 20 is connected to a scale 70 on which the angle of incidence Θ can be read. On the spindle 66 is a carrier 71 for a mirror 72, which corresponds to the second reflecting surface 3. A carrier 73 extends perpendicular to the mirror 72. On this carrier 73 is mounted a cell holder consisting of a groove 74 provided on which a carrier 75 in the form of up and down U is arranged with a clamping block 76. Between the clamping block and the support 74, the cell 26 is mounted. On the entry surface of the cell, the half-cylinder 29 is fastened. By means of a non-manufactured adjusting screw on the underside of the spring 74, the cell device 26, 29, 74, 75 can be displaced along the arm 73 ·

The prism 15 is arranged on an adjustable slide 78, by which, at the beginning of the examination, the distance between the prism 15 and the spindle 66 can be determined. Once this distance has been set, it remains unchanged during the investigation.

The device 20, which has been termed retro-director, differs from the conventional element of this name, which is constituted by a right-angled mirror. However, the use of this term has been justified by the entry area of ​​cell 25,

- designated by the designation 80 in Fig. 4, lies in a plane 23 perpendicular to the plane of the mirror reflector 34. Since the beam of light emanating from the cell 25 encloses with a normal to the plane 23 an angle equal to the angle of incidence of the cut, For example, the device 20 can be practically regarded as the optical equivalent of a reflector extending in the plane 23. If a simple reflection cell were to be used, it would lie on the site of the half-cylinder 29, whereby the material for analysis would be applied to the flat surface of the planet 23. In a simple reflection cell, the retrodirector 20 and the cell could form a unit in shape of a prism whose hypotenuse is in the vicinity of the hypotenuse of the prism 15 of Fig. 1 and approximately extends in the same direction, while the position of the side surfaces corresponded to that of the surfaces 23 and 34.

It can be seen from the foregoing that with the aid of the improved angle change device for cells with internal reflection, the angle of incidence can be changed within a wide range, so that any defocusing of the source imaging at the entrance gap is negligible, without displacement of the light beam at the spectrometer's entrance gap and with a negligible change of the optical wavelength. Said device is highly adaptable and can be used for both single and multiple reflection cells. The device can be changed with existing instruments with small changes and allows a change of angle of incidence within a wide area &lt;whereby the depth of penetration of the material test for examination is adjustable, so that various surface investigations can be carried out

- 11 while, even the number of reflections can be regulated.

ARMS