GB2084758A - Optical slit apparatus - Google Patents

Abstract

A pair of adjacent relatively slidable members 12, 14 are formed with pairs of slits 28, 30 and 30, 34 respectively. The width of the effective slits through the pair of members depends on the degree of overlap between corresponding slits, this being controlled by relative sliding of the members 12, 14 under the control of a rotary shaft 58 carrying a contact bar 56 which engages protrusions 48 and 50 on the respective members, which are biased into contact with it by a spring 26. The shaft 58 is driven by a stepper motor through a drive arm. <IMAGE>

Description

SPECIFICATION Optical slit apparatus This invention relates to optical slit apparatus for use in spectrophotometry.

Many modern analytical instruments employ light radiation as means for determining elemental characteristic information. In order for such information to be detected, light radiation representative of that information must be separated from light radiation not of interest. This detection usually involves isolating a relatively narrow spectral band contained in the relatively broad spectral band of conventional sources. One such mechanism for performing this isolation is commonly known as a monochromator.

Conventional monochromators include an optical entrance slit or aperture, and an optical exit slit, or aperture. The width of the entrance slit defines the area of the radiation source being used by the monochromator and that of the exit slit defines the spectral band being passed to a detector.

The relative widths of the slits are usually controlled by various camming mechanisms. A feature common to most conventional slit mechanisms is the presence of a fixed rigid central member having knife-like optical edges on either side and movable side members for controlling the width of the slits.

One major drawback of such a mechanism is the required accuracy of the control mechanism. For example, the arc travelled by the cam is extremely critical and the length of the associated cam arms involved must be precisely determined and maintained. Because of the complexity of the components involved, such mechanisms are usually quite sensitive as well as expensive.

According to the present invention optical slit apparatus comprises a pair of slit members each having a number of slits splaced apart on the same predetermined centres, the two slit members being adjacent to and in movable relationship with one another and means for sliding the slit members with respect to each other to vary the degree of overlap of corresponding slits. Such apparatus is less complex than previously but nevertheless quite precise.

An example of apparatus in accordance with the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a front view, partially in section; Figure 2 is a cross sectional view of the apparatus shown in Figure 1 taken along the line 2-2; Figure 3 is an illustration of a slit drive arrange ment for the apparatus of Figures 1 and 2; and Figure 4 is a cross sectional view of the slit drive arrangement shown in Figure 3 taken along the line 4-4.

As shown in Figures 1 and 2 an optical slit apparatus indicated generally by the numeral 10 includes a pair of optical slit members 12 and 14, top and bottom containment members 16 and 18 respectively, side retention members 20 and 22, a drive mechanism 24 and a slit member positon biasing spring 26.

The optical slit members 12 and 14 are generally rectangular in shape, each including a rectangular first aperture 28 and 30 respectively, and a rectangular second aperture 32 and 34 respectively. For reasons discussed below, the rectangles defining the apertures 28, 30, 32 and 34 preferably have optical knife-like edges. Each slit member 12, 14 includes a protrusion 36,38 respectively, extending from a first face 40,42 respectively, which can be used as a position indicator or as a means for preventing light straying from an entrance slit. The slit members 12 and 14 are preferably fabricated from plastic and can be formed by known plastic moulding techniques.

The slit members 12 and 14 are arranged adjacent each other in back-to-back fashion and are held in this position by the top and bottom containment members 16 and 18 respectively. The containment members 16, 18 maintain the slit members 12 and 14 adjacent each other, but allow them to slide laterally with respect to each other within the confines of the side retention members 20 and 22.

As shown in Figures 1 and 2 the bottom containment member 18 is secured to a base 44 via screws, rivets or the like. The side retention members 20 and 22 are also rigidly affixed to and protrude from the base 44. The slit members 12 and 14 are located within the bottom containment member 18 and held therein by the top containment member 16 which is secured in place via nuts 46 affixed over the side retention members 20 and 22.

Each slit member 12, 14 includes a side protrusion 48,50 respectively extending from the ends thereof which serve as drive contact means. The opposite ends extend beyond the side retention member 22 and contact the biasing spring 26, which, as shown, is in the form of a spring clip roughly in the shape of an S having one end 52 secured between the bottom containment member 18 and the base 4. The clip is shaped so that contact is made between the other end 54 and the members 12 and 14.

The sliding movement of the slit members 12 and 14 is controlled by the drive mechanism 24 which includes a slit member contact bar 56 rigidly affixed to and movable with a rotatable shaft 58 having a drive arm 60 affixed thereto and extending therefrom as seen in Figures 3 and 4. The position of the drive arm 60 is controlled by a drive pin 62 driven by a motor 64.

The drive mechanism 24 is preferably adjusted so that when the slit member contact bar 56 is perpen dicularto the plane of the slit members 12 and 14, the first and second apertures thereof, 28 and 30 and 32 and 34 respectively are overlapping with half of the total slit opening available to pass light radiation therethrough.

The shaft 58 extends through and beyond the base 44 and is surrounded by V-shaped bushings 66A and 66B at the ends of an aperture 68 in the base 44. The bushing 66B at the lower end of the shaft 58 is retained in place, with respect to the shaft 58, via a washer 70 and spring clip arrangement 72.

As described above the drive arm 60 as shown in Figures 3 and 4, is rigidly affixed to the rotatable shaft 58 and extends in a direction which is perpen dicularto the axis of the shaft 58. The arm 60 is biased by a spring 74 towards a fixed datum point 76 and is contacted by and responsive to the drive pin 62. The datum can be translated to indicate the position of the slit members 12 and 14.

As shown in Figure 4, the drive pin 62 is perpendicularto the drive arm 60 when the latter is in its datum position and has an externally threaded shoulder 78 at its lower end. The shoulder 78 of the drive pin 62 rides in an internally threaded shaft 80 which is rigidly connected to the drive shaft 82 of the control motor 64. In practice, the control motor 64 is a stepper motor of the type effectively having ninety six equal steps or increments, per revolution.

In one specific embodiment, each aperture 28,30, 32 or 34 has a width of about 3 millimetres and a height of about 12.7 millimetres. In addition, the internal and external threads of the shaft 80 and the shoulder 78 respectively are 10/32. Such a thread results in a lateral movement of about 0.76 millimetres of the drive pin for each complete revolution of the drive shaft 82 of the stepper motor 64.

Further, in this example, the effective length of the drive arm 60 is selected to be about 8.1 centimetres.

Thus by use of elementary mathematics and trigonometry, it can be shown that the lateral movement of each slit member 12 and 14for each revolution of the motor 64 is about 76 micrometres. Since the slit members 12 and 14 are equally responsive to the rotational movement of the contact bar 56, the total effective movement of the slit members 12 and 14 is about 152 micrometres per revolution of the drive shaft 82 of the motor 64. This results in a movement of the order of about 1.8 micrometres per step of the stepper motor 64.

The error in correlating each motor step to a linear distance moved by the slit members 12 and 14 is of the order of about 1%. This error is introduced by the cosine effect between the linear movement of the drive pin 62 and the angular rotation of the rotatable shaft 58. However, since the maximum arcuate distance travelled by the drive arm 60, i.e. where the apertures would effectively be completely overlapping and thus closed, is about + 6 and, since the COS 6" = 0.995, the error thus introduced is less than 1%.

It will be recognised that the position of both drive arm 60 and the shaft 80 can readily be continuously monitored by well known methods.

Thus, the above described optical slit apparatus 10 provides a quite accurate mechanism for use as the entrance and exit apertures of a monochromator. It can also be seen that the apparatus 10 can be easily and inexpensively manufactured without the usually complex cam mechanism.

Claims (9)

1. An optical slit apparatus for use in a spectrophotometer and comprising a pair of slit members each having a number of slits spaced apart on the same predetermined centres, the two slit members being adjacent to and in movable relationship with one another and means for sliding the slit members with respect to each other to vary the degree of overlap of corresponding slits.

2. Apparatus as claimed in claim 1 wherein there are two slits in each member.

3. Apparatus as claimed in claim 1 or claim 2 wherein the members are moulded in plastics material.

4. Apparatus as claimed in any one of the preceding claims wherein the members are laterally slidablewith respectto each other.

5. Apparatus as claimed in any one of the preceding claims wherein the relative movement of the slit members is perpendicular to the length of the slits.

6. Apparatus as claimed in any one of the preceding claims wherein the sliding means includes a rotary drive and means for translating rotational to linear motion.

7. Apparatus as claimed in claim 6 wherein the translation means comprises a contact bar mounted at the end of a shaft connected to the rotary drive and extending perpendicularly to the planes of the slit members when in a datum position, the slit members being resiliently biased into engagement with the contact bar so as to slide in opposite directions when the bar is turned by movement of the shaft.

8. Apparatus as claimed in claim 6 or claim 7 wherein the rotary drive is a stepper motor.

9. Optical slit apparatus substantially as described and as illustrated with reference to the accompanying drawings.