GB2054890A

GB2054890A - Diffraction grating

Info

Publication number: GB2054890A
Application number: GB7925800A
Authority: GB
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1979-07-24
Filing date: 1979-07-24
Publication date: 1981-02-18
Also published as: GB2054890B

Abstract

A reflecting grating for a Moiré fringe measuring system is formed from a member of stainless steel having a polished surface on which is formed a coating of an optically transparent material having a refractive index greater than 2. Parts of the coating are removed to form a pattern of parallel lines in which alternate lines are formed by exposed areas of the stainless steel. Those areas of the coating remaining between the exposed steel areas are of such thickness that the pattern comprises alternate lines of different phase-shifting properties to radiation of a predetermined wavelength.

Description

SPECIFICATION Measuring scales This invention relates to measuring scales, and in particular to reflecting grating scales used with More' fringe measuring systems.

Reflecting grating scales tend to be made of polished stainless steel by etching a pattern of parallel lines into the polished surface. Such etching is carried out through a photographicallyformed resist mask, which is removed after etching. This process results in a grating structure in which the contrast between the reflecting lines and the nonreflecting lines is usualiy in the range from 3 :1 to 5 :1, and may be variable. The etching process is usually applied to Ferritic and Martensitic stainless steels which are difficult to obtain with a mirror finish. On the other hand highly-reflecting Austenitic stainless steel is difficult to etch.

It is an object of the invention to provide a reflecting grating scale for a Moor6 fringe measuring system providing greater contrast than has hitherto been possible.

According to the present invention there is provided a reflecting grating scale for a Moiré fringe measuring system, which includes a member of stainless steel carrying on one surface a coating of optically transparent material having a refractive index greater than two, parts of the coating being removed to expose the surface of the stainless steel in the form of a pattern of parallel lines, the remaining parts of the coating being of such a thickness that the pattern comprises alternate lines of different phaseshifting properties to radiation of a predetermined wavelength.

Preferably the thickness of the coating is such that radiation reflected from the surface of the coating is substantially canceiled by radiation reflected from the interface between the steel and the coating.

It is known to produce a sheet of Austenitic stainless steel having a mirror finish on one side and having that surface coated with a layer of optically transparent material having a refractive index greater than 2. Such coatings are normally used for purely decorative purposes and may be of any of a range of colours depending upon the thickness of the layer, whose refractive index is about 2.2. The process used to produce such a coated stainless steel was developed and patented by International Nickel Limited. The type of steel normally used is Austenitic steel defined by the AISI 300 series.

This coated steel is particularly suitable for use in grating scales for measuring systems because of the high degree of reflectivity which may be given to Austenitic steel, of the order of 70%. The prepared coated steel is made into a grating by covering the coating with a photoresist which is suitably exposed to form a fine pattern of parallel lines of the required pitch. A subsequent etching process removes the exposed portions of the photoresist and also the coating beneath those exposed portions. After this etching, the remainder of the photoresist is also removed.

The result of this is to provide a strip of stainless steel of high reflectivity formed with a pattern of parallel lines, alternate ones of these lines being the exposed surface of the steel. The intermediate lines are areas of the steel still bearing its coating.

In general terms the upper surface of the coating may reflect some 1 5% of incident radiation, with the remainder passing into the coating. The interface between the coating and the steel reflects about 18% of the incident radiation, the remainder being absorbed. It is possible to arrange for the thickness of the coating to be such that the radiation reflected from the interface returns to the surface of the coating out of phase with the radiation reflected directly by the coating. In such a case cancellation will occur, leaving only about 3% of the original incident radiation to be reflected. Hence the contrast between the coated lines reflecting about 3% of the incident radiation, and the uncoated lines reflecting about 70% of the incident radiation, is of the order of 23 to 1.

In order to cause the necessary phase shift for cancellation, the optical thickness of the coating must be an odd number of quarter-wavelengths of the radiation in question. Purely by way of example, if the refractive index of the coating is 2, and the wavelength of the radiation is 1 At, in the infra-red range, then the desired cancellation will be provided by a coating having an actual thickness of 0.1 25,st. The same effect would be provided by coatings of thickness 0.375it, 0.625,r(, and so on.

A reflecting scale grating of the type described above may be used with most conventional radiation-sensitive detector systems in a More' fringe measuring system. However, it is sometimes preferred to provide a grating in which the line pattern is defined in terms of relative phase shift rather than relative intensity. In such a case the thickness of the coating is such that radiation reflected from the interface is in phase with radiation reflected from the top surface. This is achieved by making the optical thickness of the coating equal to an even multiple of quarterwavelengths of the radiation in question.

In either of the cases described above the phase-shift imparted to radiation reflected from the steel where the coating has been removed will differ from that resulting from the passage of the radiation through the coating.

There is a limit to the thickness of the coating which may be produced by the method referred to, and it may therefore not be possible, or desirable, to produce the thicker films. However, those which are only a quarter or a half wavelength thick are readily produced.

1. A reflecting grating for a Moor6 fringe measuring system, which includes a member of stainless steel carrying on one surface a coating of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Measuring scales This invention relates to measuring scales, and in particular to reflecting grating scales used with More' fringe measuring systems. Reflecting grating scales tend to be made of polished stainless steel by etching a pattern of parallel lines into the polished surface. Such etching is carried out through a photographicallyformed resist mask, which is removed after etching. This process results in a grating structure in which the contrast between the reflecting lines and the nonreflecting lines is usualiy in the range from 3 :1 to 5 :1, and may be variable. The etching process is usually applied to Ferritic and Martensitic stainless steels which are difficult to obtain with a mirror finish. On the other hand highly-reflecting Austenitic stainless steel is difficult to etch. It is an object of the invention to provide a reflecting grating scale for a Moor6 fringe measuring system providing greater contrast than has hitherto been possible. According to the present invention there is provided a reflecting grating scale for a Moiré fringe measuring system, which includes a member of stainless steel carrying on one surface a coating of optically transparent material having a refractive index greater than two, parts of the coating being removed to expose the surface of the stainless steel in the form of a pattern of parallel lines, the remaining parts of the coating being of such a thickness that the pattern comprises alternate lines of different phaseshifting properties to radiation of a predetermined wavelength. Preferably the thickness of the coating is such that radiation reflected from the surface of the coating is substantially canceiled by radiation reflected from the interface between the steel and the coating. It is known to produce a sheet of Austenitic stainless steel having a mirror finish on one side and having that surface coated with a layer of optically transparent material having a refractive index greater than 2. Such coatings are normally used for purely decorative purposes and may be of any of a range of colours depending upon the thickness of the layer, whose refractive index is about 2.2. The process used to produce such a coated stainless steel was developed and patented by International Nickel Limited. The type of steel normally used is Austenitic steel defined by the AISI 300 series. This coated steel is particularly suitable for use in grating scales for measuring systems because of the high degree of reflectivity which may be given to Austenitic steel, of the order of 70%. The prepared coated steel is made into a grating by covering the coating with a photoresist which is suitably exposed to form a fine pattern of parallel lines of the required pitch. A subsequent etching process removes the exposed portions of the photoresist and also the coating beneath those exposed portions. After this etching, the remainder of the photoresist is also removed. The result of this is to provide a strip of stainless steel of high reflectivity formed with a pattern of parallel lines, alternate ones of these lines being the exposed surface of the steel. The intermediate lines are areas of the steel still bearing its coating. In general terms the upper surface of the coating may reflect some 1 5% of incident radiation, with the remainder passing into the coating. The interface between the coating and the steel reflects about 18% of the incident radiation, the remainder being absorbed. It is possible to arrange for the thickness of the coating to be such that the radiation reflected from the interface returns to the surface of the coating out of phase with the radiation reflected directly by the coating. In such a case cancellation will occur, leaving only about 3% of the original incident radiation to be reflected. Hence the contrast between the coated lines reflecting about 3% of the incident radiation, and the uncoated lines reflecting about 70% of the incident radiation, is of the order of 23 to 1. In order to cause the necessary phase shift for cancellation, the optical thickness of the coating must be an odd number of quarter-wavelengths of the radiation in question. Purely by way of example, if the refractive index of the coating is 2, and the wavelength of the radiation is 1 At, in the infra-red range, then the desired cancellation will be provided by a coating having an actual thickness of 0.1 25,st. The same effect would be provided by coatings of thickness 0.375it, 0.625,r(, and so on. A reflecting scale grating of the type described above may be used with most conventional radiation-sensitive detector systems in a More' fringe measuring system. However, it is sometimes preferred to provide a grating in which the line pattern is defined in terms of relative phase shift rather than relative intensity. In such a case the thickness of the coating is such that radiation reflected from the interface is in phase with radiation reflected from the top surface. This is achieved by making the optical thickness of the coating equal to an even multiple of quarterwavelengths of the radiation in question. In either of the cases described above the phase-shift imparted to radiation reflected from the steel where the coating has been removed will differ from that resulting from the passage of the radiation through the coating. There is a limit to the thickness of the coating which may be produced by the method referred to, and it may therefore not be possible, or desirable, to produce the thicker films. However, those which are only a quarter or a half wavelength thick are readily produced. CLAIMS

1. A reflecting grating for a Moor6 fringe measuring system, which includes a member of stainless steel carrying on one surface a coating of optically transparent material having a refractive index greater than two, parts of the coating being removed to expose the surface of the stainless steel in the form of a pattern of parallel lines, the remaining parts of the coating being of such a thickness that the pattern comprises alternate lines of different phase-shifting properties to radiation of a predetermined wavelength.

2. A grating as claimed in claim 1 in which the thickness of the coating is such that radiation reflected from the surface of the coating is substantially cancelled by radiation reflected from the interface between the steel and the coating.

3. A grating as claimed in either of claims 1 or 2 in which the stainless steel member is formed from an Austenitic steel having a highly polished surface on which the coating is formed.

4. A grating as claimed in any one of claims 1 to 3 in which the pattern of parallel lines is formed by covering the coating with a photoresist material, exposing the photoresist to form the desired pattern, etching away the exposed portions of the photoresist and the underlying coating, and removing all remaining photoresist material.

5. A reflecting grating for a More' fringe measuring system substantially as herein described.