WO1983001999A1

WO1983001999A1 - Method and apparatus for measuring the radii of spherical bodies

Info

Publication number: WO1983001999A1
Application number: PCT/SE1981/000347
Authority: WO
Inventors: Ab Eloptricon
Original assignee: Stenberg, Lars
Priority date: 1981-11-30
Filing date: 1981-11-30
Publication date: 1983-06-09

Abstract

In a method of measuring the radii of spherical bodies by means of a spherometer it has been found extremely valuable to accurately determine and take into consideration the radii of curvature of the measuring edges (13), by means of nomograph-like or similar storage said values being utilized in measurements on spheres whose radii are to be determined. A preferred measuring probe (1) electrically connected to a control and display circuit (2) has a measuring member (11) with a rigidly mounted measuring transducer (12). The transducer, provided with an adjustable measuring tip (14), and the measuring member are glidably arranged in a handle (16), against the bias of a spring (18) for applying the annular measuring edge (13) arranged on said member to a spherical body, the radius of which is to be measured.

Description

DESCRIPTION

Method and Apparatus for Measuring the Radii of Spherical Bodies Technical Field

The present inventions relates to measuring the radii of spherical surfaces, more specifically to measuring radii on optical lenses with the aid of a spherometer, having a transducer, which is settable against the object to be measured, and which is situated between measuring edges placed against the surface of the object which is to be measured. Background Art

The principle of using a spherometer for determining the radius of curvature of optical lenses and the like is known from a plurality of patents describing different variations and arrangements of dial indicators for determining the radius of curvature of different types of optical lenso Everything from optical lenses made from glass to contact lenses comprising plastics material, where the entire measurement must take place under water in order to obtain a realistic result compared with when the plastics lens is positioned in the eye, have been the objects of inventors attempts to solve the difficult problems associated with such measurements. Examples of known methods are provided inter alia, in the Swiss Patent 574 098, the German Offenlegungsschriften 29 34 243 and 29 12 640, the German Patent 1 698360, the U.S. Patent 3 848 339, the British Patent 1 427 038 and the British Published Specification 2 006 436. Common to all these measuring methods is that the chord theorem is utilized, where a² = h. (2R - h), which may be written .......(1)

where R is the radius of the object to be measured, and the remining denotations are apparent from the appended Figure 2.

However, the known methods and apparatus have not been of assistance in solving the troublesome problems occurring when it is desired to achieve great measuring accuracy with simple means in measuring objects of highly varying size, i.e. with greatly varying spherical radii. In the known spherometers, it has always been attempted to make the measuring edges as sharp as possible, and varying, sophisticated grinding and polishing methods have been resorted to in order to make these edges as sharp as possible. These manufacturing methods make the spherometer considerably more expensive, and also make it sensitive to mechanical injury, i.e. it must be handled with great care so that the measuring edges are not damaged.

In practice, it has of course been found that one can never manufacture a spherometer with measuring edges having a radius of curvature which is infinitely small. For example, in check measurement with a spherometer, having measuring edgeswith a radius of curvature of 10 μm and where a = 5 mm and h = 0.600 mm, R will be 21.133 mm if formula (1) is utilized. For the case where a = 5.010 mm and h = 0.600 mm, R will be 21.217 mm. A difference of Δa = 0.01 thus gives an error of 84 μm in the determination of R according to this known method. Disclosure of Invention

The present invention is based on taking into consideration that the radius of curvature of the measuring edges is a finite number and that each measuring apparatus supplied is compensated for this indispensxble radius of curvature by measuring a for different such radii, and thus measuring the exact profile of the measuring edges. This profile can then be adapted to a mathematical expression, which has a somewhat different appearance depending on whether the object to be measured has a positive or negative radius. Por a positive radius it is solely an inner edge of the support which comes into contact with the object, and for a negative radius it is solely an outer edge which comes into contact with the object, the radius of which is to be determined.

The properties which remove all the drawbacks mentioned above with regard to methods in the prior art for measuring the radii of spherical bodies are obtained by a method characterized by the following steps: a) The radii of curvature of the measuring edges are measured to determine their exact profile, b) the values of measured radii of curvature are stored, c) the measuring edges are placed on a flat surface for calibrating the measuring transducer relative the edges, and d) the edges are put on a surface, the radius of which is to be determined, the measuring transducer being adjusted so that a measurement is obtained, which is compensated for the exact profile of the edges by taking into account the stored values of their radii of curvatureo

This method can be achieved by an apparatus having the characterizing feature that the profile of the edges has a radius of curvature, the value of which is a maximum of 50 μm and where the edges are kept against the surface of the object to be measured by means of a force predetermined by the bias of a spring. Preferred Embodiment

The invention will now be described in detail while referring to the appended drawings which illustrate a preferred embodiment of the invention, and where

Figure 1 illustrates a preferred embodiment of a measur ing probe associated with the measuring apparatus, the probe being connected to a control and display circuit,

Figure 2 illustrates the principle for utilizing the measuring probe, and

Figure 3 is a detail of the measuring edge of the measuring probe.

In Figure 1 there is shown a preferred embodiment of the measuring apparatus in accordance with the invention, which includes a measuring probe 1 and a control and display circuit 2. The embodiment of the measuring probe 1, illustrated in cross section, includes a member 11 which is rigidly connected to a measurement transducer 12. The member 11 has an annular measuring edge 13 of hard carbide steel, which surrounds a measuring tip 14 adjustably arranged on the measuring transducer 12. The transducer 12 is retained in the member 11 by means of a set screw 15. The member 11 and transducer 12 are glidably arranged in a handle 16 provided with a lid 17 through which the transducer 12 projects. Movements between the member 11 and handle 16 are counteracted by a spring 18. The handle 16 is formed from a heatinsulating material, e.g. plastics, so that the transducer 12 will not be acted on by exterior temperature changes, e.g. such as occur when the operator heats the handle 16 with the warmth of his hand. There are gliding surfaces 19 between the parts which move relative each other. The transducer 12 of the measuring probe 1 is of the electronic type which gives an electric measuring signal in response to the stroke of the measuring tip 14. The transducer 12 is electrically connected to the control and display circuit 2 via the cable K. Similar to the measuring probe 1, said circuit 2 is only schematically depicted. The circuit 2 includes an actuable microcomputer and has a measuring value display window 21, preferably arranged for a digital display, the number of digits being selectable in respect of the number of decimals, all according to measuring range and accuracy requirements. The circuit 2 also has a plurality of control means, indicated by the illustrated keys 20 - 25 and is arranged for switching the computor to different test sequences. The key 22 is a switch for switching off and on the curret source (indicated by the mains connection 26) which is intended to power the control and display circuit 2. The key 23 is intended for depression when calibrating the measuring apparatus, when the measuring probe 1 is placed on a surface plate (not shown). The key 24 is intended for depression when carrying out a test where the probe 1 is placed on different reference objects (not shown) during a sequence of measurements. Finally, the key 25 is intended for normal measurement of the radius of curvature of the different bodies with spherical radii which are to be measured. All the depressable keys 23 - 25 can be of the type with return spring, and on depression of one of them the display means 21 may be illuminated to show the current measured value. The principle of utilizing the measuring probe 1 is illustrated in Figure 2. The curve 3, drawn in with a full line, indicates a lens with a positive radius, and the dashed curve 4 indicates a lens with a negative radius. As will be seen from Figure 2, the measuring edges 13 have a width such that different edges A, B must be taken into account, depending on whether the measurement relates to a lens with a positive (3) or negative (4) radius. The measuring method will however be the same for both types of lens. Figure 3 is an enlargement of one face A of the measuring edge 13, encircled by the ring III in Figure 2. From this it will be seen that for the method in accordance with the present invention to have a practical value, the radius of curvature of the faces A and B of the measuring edge must not be too great, since it then requires a very large number of measuring points to determine the exact profile of the measuring edges, and after several experiments it has been possible to determine that the radius of curvature r should not exceed 50 μm. Radii in the order of magnitude of preferably 5 - 10 μm have been found to be particularly favourable.

When a measurement is taken with the measuring instrument in accordance with the invention, the measuring probe 1 is first pressed against a flat surface, the measuring transducer 12 then being zeroed. The probe 1 is then pressed against an object, the radius of which is to be determined. The signals are electronically transferred to the micro-computor which begins by ascertaining whether the h is positive or negative (positive h for positive radii and negative h for negative radii). For positive radii 3, the h value function, stored in the microcomputor memory after test measurements of reference objects, is recalled, and subsequent thereto determination is effected of the value of a associated with the h value in question. In a corresponding manner, the a value for the h value in question is determined for a possible negative radius 4. By the measuring equipment being constructed such that consideration is taken to the faces A and B of the measuring edges not having a negligible radius of curvature, great accuracy is achieved in determining R, and finally it is the error in determining h which puts a limit on the accuracy.

Thus, the prototype which has been constructed has given measuring results with the class of accuracy of + 1 μm . for radii in the order of magnitude 6 - 20 mm and + 10 μm for radii between 50 and 100 mm. From the description given hereinbefore it will be seen that different modifications can be carried out on the measuring apparatus, without departing from the inventive concept. The embodiment described, and illustrated on the drawings, is therefore not to be regarded as restricting the scope of the invention, and that the latter is defined solely by the following claims.

Claims

1. A method of measuring radii, preferably those of optical lenses, by means of a spherometer having a measuring transducer and measuring edges which are placed during measurement on a surface, the radius of which is to be determined, characterized by the steps: a) the radii of curvature of the measuring edges are measured to determine the exact profile thereof, b) the values of measured said radii are stored, c) the measuring edges are placed on a flat surface for zeroing the measuring transducer relative said edges, and d) said edges are placed on a surface, the radius of which is to be determined, the measuring transducer being adjusted so that a measuring value is obtained which is compensated for the exact profile of the measuring edges by taking into consideration the stored values of the radii of curvature.

2. A method as claimed in claim 1, characterized in that steps a) and b) comprise a sequence of measurements on reference objects with known reference radii of different magnitudes, a reference measurement value being obtained for each reference object by the measuring edges being placed on the respective reference object and the measuring transducer adjusted for this, the reference measurement values thus stored subsequently describing a diagram where the reference measuring value is an independent variable and the reference radius is a dependent variable.

3. Apparatus for measuring radii, preferably of optical lenses by means of a spherometer which has a measuring transducer adjustable to the surface of an object to be measured, and situated between measuring edges adapted for engagement against the surface of the measured object during the measuring process, characterized in that the profile of the measuring edges have a radius of curvature with a maximum value of 50 μm and are applied to the surface of the object to be measured by means of a force predetermined by a spring.

4. Apparatus as claimed in claim 3, characterized in that the measuring edges constitute a hard carbide steel ring attached to a measuring member centrally pierced by the measuring transducer, which transmits electrical measured value signals in a mode known per se from a measuring tip the measuring member being actuable via the spring which is arranged between the member and a handle.

5. Apparatus as claimed in claim 4, characterized in that the measuring member and transducer are rigidly connected to each other for being glidably movable in the handle, the measuring tip being adjustably displaceable on the measuring transducer.

6. Apparatus as claimed in claim 4 or 5, characterized in that the measuring transducer is arranged in a heat-insulating air-cooled chamber inside the handle.

7. Apparatus as claimed in claim 4, 5 or 6, characterized in that the handle of the measuring apparatus is of heat- insulating material, preferably plastics.

8. Apparatus as claimed in claim 3, characterized in that the radius of curvature associated with the measuring edges preferably has a value of 5 - 10 μm.