Improvements in or relating to surface curvature measurement

Abstract: The apparatus measure the radius of curvature of a reflective or partially reflective surface 18, and comprises first and second lasers 2, 4 which generate two parallel beams a known distance apart. A lens 12 focuses the beams, which are passed via a beam splitter 14 and transform lens 16 to the surface 18. The beams are reflected back via the transform lens 16 and the beam splitter 14 onto a detector 20. A processor 22 calculates the radius of curvature of the surface in dependence of the known distance X separating the beams and the positions of the beams on the detector 20.

Description: The invention relates to apparatus for the measurement of the radius of curvature of at least a partially reflective surface. More especially but not exclusively it relates to apparatus for providing a simple, but accurate, 'statement' of whether the surface radius of curvature of a manufactured product or of a product being manufactured is within specification, together with an indication of how far it is from a required radius of curvature specification. Apparatus for effecting surface radius of curvature measurement is known and in this connection attention is directed to GB-B-2 265 215 for background information. US-A-5 067 817 discloses a method and device for non-contacting self-referencing measurement of surface curvature and profile by simultaneous measurement of slope at two closely spaced points on a test surface. A pair of parallel light beams which are slightly displaced relative to one another are directed onto the test surface and the reflected beams are focussed onto position sensitive detectors. The sensed slopes at the points at which the beams intersect the test surface are used to determined the local curvature of the test surface. Scanning of the device across the test surface provides a profile of the curvature of the whole test surface. US-A-4 735 508 discloses a method and apparatus for measuring surface curvature of a reflective surface. A pair of beams generated by a laser are directed onto the surface and the separation of the light beam images reflected from the surface is measured to provide an indication of the curvature of the surface. An aim of the present invention is to provide apparatus which effectively measures the radius of curvature of the surface of at least a partially reflective surface. According to the present invention there is provided apparatus for measuring the radius of curvature of a surface which is at least partially reflective, the apparatus comprising;- source for generating at least first and second beams of electromagnetic radiation, the beams being parallel to each other and spaced a known distance apart; beam splitter means located between the source and the surface for directing the beams onto the surface and for receiving reflected beams from the surface; first lens means located between the source and the beam splitter means for focussing the beams and directing them onto the beam splitter means; second lens means located between the beam splitter means and the surface for directing the beams from the beam splitter means onto the surface and for directing the reflected beams from the surface onto the beam splitter means; electromagnetic radiation detector means arranged to receive the reflected beams from the beam splitter means; and processing means for calculating the radius of curvature of the surface in dependence on the positions of the reflected beams on the detector means, characterised in being adapted to receive beam separation means positioned between the second lens means and the surface for increasing or decreasing the beam separation. Various embodiments of the invention will now be described with reference to the accompanying drawings, in which:- FIGURE 1 shows a block diagram of apparatus for measuring the radius of curvature of a partially reflective surface; FIGURE 2 shows the relationship between the angles measured and the distance between beam 1 and beam 2; FIGURE 3 shows a block diagram of a multibeam variant of the apparatus of FIGURE 1; and, FIGURE 4 shows the apparatus of FIGURE 1 including a beam separation device. In Figure 1, the apparatus shown may be hand-held, but it will be appreciated that the apparatus may be fixed to a lathe stock or robot arm, for example. Referring to Figure 1, the hand-held apparatus comprises first and second sources of electromagnetic radiation 2, 4, which are lasers. The lasers are driven by laser drivers 6, which are controlled by a controller and processor circuit 8 having a man machine interface input 10. The lasers 2, 4 each produce a beam of electromagnetic radiation, which is passed through a focusing lens 12 into a beam splitter 14. The beams are parallel to each other and are a known distance apart. The beams pass through the beam splitter and into a transform lens 16, which directs beams towards a surface 18 to be measured. This surface may be a lens but could be any reflective or partially reflective surface which requires the radius of curvature to be measured. The surface reflects the beams 1, 2 back towards the transform lens, which in turn pass into the beam splitter and are directed onto the surface of a detector 20. These beams are identified as X1, X2. A detector processor 22 is arranged to calculate the radius of curvature of the surface based upon the positions of the reflected beams X1, X2 on the surface of the detector and upon the separation of the two beams at the surface 18. The detector 22 is controlled by the controller and processor circuit 8. The lasers are diode lasers 2, 4 which emit in the visible range of wavelengths, and will thereby provide an indication to an operator of where the measurements are actually being made. The lasers emit collimated beams which are directed to a surface to be measured by various lenses and prisms as shown in Figure 1. The beams are parallel with each other, and are a known distance apart. The measurement method used is non-contact and potentially more accurate than a method using a spherometer, which is a contact instrument. In this embodiment, as shown in Figure 1, two parallel beams, beam 1 and beam 2, a known distance apart impinge on a surface. When the beam returns are incident on the detector 20, the positions are stored in a memory in the detector processor 22 and used to calculate the radius of curvature. The diagram, as shown in FIGURE 2, and related equations (discussed later) show that there is a simple relationship between the two measured angles, the distance between the beams and the required radius of curvature of the surface 18. In order to distinguish between the returns from the two input beams it is necessary to activate each beam separately such that the returns are uniquely identified, when using a position sensing diode in the detector 20. Another way to accomplish this would be to use polarised light and separate detectors for each polarisation. It should be noted that for widely spaced beams this method will only be accurate for spherical surfaces. However, if the beams are close to each other, so as to afford a sufficiently high sampling frequency, then the method will work for any smooth surface profile. Referring to Figure 3, a variation of the device shown in Figure 1 will now be discussed and shows a multibeam variant. Similar elements in Figure 3 have been given the same designation as that of Figure 1 and their functions are the same. The difference between Figure 1 and Figure 3 is that the electromagnetic source 2, 4 in Figure 1, which were lasers, have been replaced by a discrete beam generator 24 which is driven by a laser and a laser driver 26. The discrete beam generator generates a plurality of beams, beam 1 to beam N, which are all parallel and where the distance X is known between any two beams. The beams are passed through the focusing lens 12 and thereafter are treated in exactly in the same manner as that discussed in relation to Figure 1. In this case a probe beam is scanned across the surface by having, for example, a scanning mirror/prism and appropriate optics, or an LCD light shutter, located in a discrete beam generator 24 in Figure 3, thus providing continuous, or variable, sample intervals for providing a profile of the surface. FIGURE 4 is a block diagram of apparatus similar to Figure 1, having an additional device 28 positioned between the surface to be measured 18 and the transform lens 16. Identical elements bear the same designations as in Figure 1 and their operation is the same. The device 28 is effective to increase, or decrease, beam separation by having add-on optics which is attached to the transform lens 16, for example. The device 28 permits different sizes of lenses and different radii of curvature to be determined with accuracy. The measurements could be accomplished in real time such that as a surface is machined the output from the unit is used to control a cutting tool, thus providing an automatic system which may be used to modify the machining as required, or stop when the required radius of curvature, or profile, is accomplished. Although the units shown are eminently suitable for a hand held design, when the measurements are made the unit could, for example, alternatively be affixed to a lathe stock or temporarily affixed near a surface to be tested. The following set of mathematical equations show how the radius of curvature, r, for two parallel beams a known distance, x, apart, as shown in Figure 2 can be determined. r sin(90 + ) = r - y sin(90 - ) r(sin(90 - )) sin(90 + ) = r - y ⇒ y = r - r(sin(90 - )) sin(90 + )) y = r - r cos  cos  x + δx sin( - ) = r sin(90 + ) = r cos  ⇒ r = (x + δx) sin( - ) (cos ) r = x cos  sin( - ) - sin (cos  - cos ) r = x cos  sin  cos  - cos  sin  - sin  cos  + sin  cos  r = x cos  sin  cos  - sin  cos  r = x sin  - sin  where: r = required radius curvature y = distance from intersect of perpendicular from upper radius normal with lower radius normal x = beam separation ∂x = extra distance between x and intersect with lower radius normal  = upper measured angle of surface normal  = lower measured angle of surface normal In addition, it is possible to derive the height of the surface from a plane surface by the following: Δh = y cos  r = x sin  - sin  therefore: where: Δh = height between impact points of the incident beams. By using wavelength diversity for each of the lasers one can isolate the lasers by wavelength filters and use linear detectors for each wavelength. Using this technique it is possible to have the beams very close together, (i.e. x is small), and still have resolution between the returns. This can be of benefit to a mechanically scanned measuring unit whereby improved resolution of the radius of the curvature is obtained. It will be appreciated by those skilled in the art that the beam splitter described is intended to include any means for causing beam separation, for example, partially reflective mirrors.

Claims: Apparatus for measuring the radius of curvature of a surface (18) which is at least partially reflective, the apparatus comprising:- a source (2, 4, 6; 24, 26) for generating at least first and second beams of electromagnetic radiation, the beams being parallel to each other and spaced a known distance apart; beam splitter means (14) located between the source (2, 4, 6; 24, 26) and the surface (18) for directing the beams onto the surface (18) and for receiving reflected beams from the surface (18); first lens means (12) located between the source (2, 4, 6; 24, 26) and the beam splitter means (14) for focussing the beams and directing them onto the beam splitter means (14); second lens means (16) located between the beam splitter means (14) and the surface (18) for directing the beams from the beam splitter means (14) onto the surface (18) and for directing the reflected beams from the surface (18) onto the beam splitter means (14); electromagnetic radiation detector means (20) arranged to receive the reflected beams from the beam splitter means (14); and processing means (22) for calculating the radius of curvature of the surface (18) in dependence on the positions of the reflected beams on the detector means, characterised by beam separation means (28) positioned between the second lens means (16) and the surface (18) for increasing or decreasing the beam separation. Apparatus as claimed in claim 1, wherein the processing means (22) comprises control means for controlling the detector means (20) and the source (2, ,4, 6; 24, 26) of electromagnetic radiation. Apparatus as claimed in claim 2, wherein the source (2, ,4, 6) comprises first and second lasers (2, 4) and the control means controls laser driver means (6) for the first and second lasers (2, 4). Apparatus as claimed in claim 1 or 2, wherein the source (24, 26) comprises beam generating means (24) which generates a plurality of discrete beams which are all parallel to one another and the distance between any two beams is known. Apparatus as claimed in claim 4, wherein the beam generating means (24) is driven by a single laser source (26). Apparatus as claimed in claim 4 or 5, wherein the beam generating means (24) comprises means for scanning a probe beam across the surface. Beam separation means (28) positioned between the second lens means (16) and the surface (18) for increasing or decreasing the beam separation. Apparatus as claimed in any one of the preceding claims, wherein the surface (18) comprises the surface of a lens. Apparatus as claimed in any one of the preceding claims, wherein the apparatus is hand-held. A method to measure the radius of curvature of a surface which is at least partially reflective comprising: generating at least first and second beams of electromagnetic radiation parallel to each other and a known distance apart; locating beam splitter means between the source and the surface for directing the beams onto the surface and for receiving reflected beams onto the surface; locating a first lens means between the source and the beam splitter means to focus the beams and directing them onto the beam splitter means; locating a second lens means between the beam splitter means and the surface for directing the beams from the beam splitter means onto the surface and for directing the reflected beams from the surface onto the beam splitter means; arranging electromagnetic radiation means to receive the reflected beams from the beam splitter; calculating the radius of curvature in dependence on the positions of the reflected beams on the detector surface; characterised in further arranging beam separation means between the second lens means and the surface to vary the beam separation.

Inventors: MANNING KEVIN ROSS, MANNING, KEVIN ROSS
Applicants: ROKE MANOR RESEARCH, ROKE MANOR RESEARCH LIMITED
Application year: 1996
Main classification: G01B11
IPC codes: G01M11, G01B11, G01B21