Meter for Testing Progressive Multifocus Lenses

1. Field of the Invention

The present invention relates to a lens meter for measuring the optical characteristics of a lens including spherical refractive power, cylindrical refractive power, axial angle and prism quantity and, more particularly, to a lens meter for measuring the additive diopter of a lens.

2. Description of the Prior Art

A known lens meter, such as "Auto Lens Meter LM-870" proposed by the applicant of the present patent application, determines the optical characteristics of a lens including spherical refractive power, cylindrical refractive power, axial angle and prism quantity on the basis of results of measurement of the locus of measuring light transmitted through the lens by a photo-detecting device. This known automated lens meter is capable of operating in an additive diopter measuring mode and of measuring the additive diopter of a progressive lens. When measuring the additive diopter of a lens by this known lens meter, the lens meter is set for the additive diopter measuring mode, the far viewing section of the lens is measured, the measured data of the far viewing section is stored in a memory, the lens is shifted to position the near viewing section of the lens at the measuring position, the data of the near viewing section is measured when the operator decides that the near viewing section is positioned at the measuring position, and the difference between the measured data of the far viewing section and that of the near viewing section is calculated to determine the additive diopter.

The respective positions of the far viewing section and the near viewing section in the lens are determined subjectively by the operator and the correctness of the determination is dependent on the perception and experience of the operator. Generally, a lens as manufactured is provided with marks indicating the respective positions of the far viewing section and the near viewing section. Therefore, the respective positions of the far viewing section and near viewing section of the lens as manufactured can be fairly accurately determined for measuring the characteristics of the lens. However, these marks are easily erasable, the marks are erased when the framed lens is wiped for cleaning and concealed marks are difficult to recognize visually. Therefore, the accurate measurement of the lens requires a skillful expert operator. Furthermore, any objective data securing correct measurement is not available.

The present invention has been made in view of the foregoing problems and it is therefore an object of the present invention to provide a lens meter capable of obtaining highly reliable measured data including additive diopter of a lens.

A lens meter in one aspect of the present invention, which project measuring light on a lens, detects the locus of the measuring light transmitted through the lens by a photo-detecting device and determines the optical characteristics of the lens on the basis of the locus of the measuring light detected by the photo-detecting device, comprises: an additive diopter measuring mode selector switch; a control means for continuously measuring the lens at specified intervals to position measuring points on the lens successively on the optical axis of the measuring light; a position detecting means for detecting the deviation of each measuring point on the lens from a specified reference point; and a display means for displaying for displaying the relation between the deviations determined by the position detecting means and the variation of the spherical refractive power.

The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in connection with the accompanying drawings, in which:

FIG. 1 is a front view of a lens meter in a preferred embodiment according to the present invention;

FIG. 2 is a diagrammatic view of a measuring optical system included in the lens meter of FIG. 1;

FIG. 3 is a front view of a target plate shown in FIG. 2, showing the shape of a slit formed in the target plate;

FIG. 4 is a view of a slit image formed on a photo-detecting device;

FIG. 5 is a plan view of a position detecting system included in the lens meter of FIG. 1;

FIG. 6 is a sectional view taken on line 6--6 in FIG. 5;

FIG. 7 is a block diagram of the lens meter of FIG. 1; and

FIG. 8 is a diagram of a graph displayed on a display in an additive diopter measuring mode.

FIG. 9 is a flow chart explaining a series of steps of an additive diopter measuring procedure.

Referring to FIG. 1 showing a lens meter in a preferred embodiment according to the present invention, a CRT display 1 displays a reticle indicating the optical axis of a measuring optical system, a positioning cross target and measured results when the lens meter is set for a measuring mode. In FIG. 1, a graph obtained in an additive diopter measuring mode, which will be described later, is displayed on the CRT display 1. The lens meter is provided with a print button 2, which is pushed to print out measured data, lens selector buttons 3, read button 4, which is pushed to read measured data, an additive diopter measuring mode selector button 5, a lens holder 6, a nose piece 7 for supporting a lens L, a positioning plate 8 for determining the longitudinal position of the lens L, and a guide pin 9 for determining the lateral position of the lens L. The lens L is positioned by the positioning plate 8 and the guide pin 9 for the measurement of additive diopter and pupil distance.

A refractive power measuring system included in the lens meter will be described hereinafter.

Referring to FIG. 2, four light emitting diodes (LEDs) 11 (11a, 11b, 11c and 11d) are disposed in a plane perpendicular to the optical axis of the refractive power measuring system and including the focal point of an objective lens 12. When the lens L is set on the nose piece 7, a microcomputer controls a LED driver to turn on the four LEDs 11a, 11b, 11c and 11d sequentially. A measuring target plate 13 is provided with a crossing slits and disposed fixedly or movably near the respective focal points of the objective lens 12 and a collimator lens 14. The nose piece 7 is disposed near the respective focal points of the collimator lens 14 and a focusing lens 15. Shown also in FIG. 2 are a half prism 16, and two linear image sensors 17 each comprising a plurality of photodiodes arranged in a linear array, and disposed perpendicularly to each other and to the optical axis.

Light emitted by the LED 11 refracted by the objective lens 12, collimated by the collimator lens 14, refracted by the lens L and focused by the focusing lens 15 on the two image sensors 17. Thus, the image of the crossing slits of the target plate 13 as shown in FIG. 4 are formed on the image sensors 17.

A method of calculating the refractive power of the lens L on the basis of measured data obtained by measuring the lens L by the measuring optical system will be briefly described.

The four LEDs 11 are turned on individually to illuminate the target plate 13. If the lens L is not set on the nose piece 7, images of the crossing slits of the target plate 13 (hereinafter referred to as "target images") formed on the image sensors 17 by the light emitted by the LEDs 11a, 11b, 11c and 11d overlap each other.

If the lens L has only spherical refractive power, the target image is shifted on the imaging device by a distance corresponding to the spherical refractive power; the target image formed by the light emitted by the LED 11a or 11c is shifted along the line linking LED 11a to LED 11c, and the target image formed by the light emitted by the LED 11b or 11d is shifted along the line linking LED 11b to LED 11d.

If the lens L has only cylindrical refractive power, the lens L refracts the incident light in a direction perpendicular to the principal meridian (or parallel to the principal meridian). The cylindrical refractive power of the lens L can be calculated on the basis of the distance of shifting of the target image.

If the lens L has both spherical refractive power and cylindrical refractive power, the target image is shifted on the image sensors 17 by distances according to the spherical refractive power and cylindrical refractive power of the lens L.

Suppose that the respective coordinates of the respective centers of the target images formed respectively by the light emitted by the LEDs 11a, 11b, 11c and 11d are A(x.sub.a, y.sub.a), B(x.sub.b, y.sub.b), C(x.sub.c, y.sub.c) and D(x.sub.d, y.sub.d). Then, ##EQU1## where S is the spherical refractive power, C is the cylindrical refractive power, .theta. is the axial angle and PQ is the prism quantity.

A microcomputer 35 calculates the spherical refractive power, the cylindrical refractive power, the axial angle and the prism quantity of the lens L by using the coordinates of the target images formed by the light emitted by the LEDs 11a, 11b, 11c and 11d, and the expressions (1) to (8). The calculated data is displayed.

The four target images are blurred and dislocated by a distance proportional to the refractive power of the lens L, causing errors in the measurement. Accordingly, it is desirable to shift the target plate 13 to diminish the dislocation of the target images and to calculate the optical characteristics of the lens L on the basis of the distance of shifting of the target and the dislocation of the target images.

A position detecting system for positioning the lens L and detecting the position of the lens L will be described hereinafter.

Referring to FIGS. 5 and 6, a rack 20 is supported in a horizontal position within the positioning plate 8 for lateral movement and the guide pin 9 is fixed to the rack 20. The guide pin 9 is biased toward the left, as viewed in FIG. 5, by coil springs 29. The positioning plate 8 is fixed to a rack 21 supported for longitudinal movement and biased continuously toward the front by a spring 22. A pinion 24 fixedly mounted on one end of a rotating shaft 23 engages the rack 20 and is moved longitudinally together with the rack 20 by the rack 21. A gear 25 is mounted fixedly on the other end of the rotating shaft 23. The revolution of the gear 25 is detected by a potentiometer 26 to detect the distance of lateral movement of the guide pin 9. The revolution of a pinion 27 engaging the rack 21 is detected by a potentiometer 28 to detect the distance of longitudinal movement of the positioning plate 8. The lens L is held in contact with the positioning plate 8 and the guide pin 9, so that the distance of longitudinal movement and that of lateral movement of the lens L can be detected through the detection of the revolution of the gear 25 and that of the pinion 27.

Referring to FIG. 7, signals provided by the two image sensors 17 are transferred through a CCD driving circuit 31 to a comparator 32 and a peak hold circuit 33. A peak voltage detected by the peak hold circuit 33 is converted into a corresponding digital peak signal by an A/D converter 34, and the digital peak signal is given to the microcomputer 35. The digital peak signal corresponding to the peak voltage detected by the peak hold circuit 33 is transferred through the microcomputer 35 to a D/A converter 36, the D/A converter 36 converts the digital peak signal into a half voltage signal corresponding to half the peak voltage, and then the half voltage signal is given to the comparator 32. The comparator 32 compares the half voltage signal with a signal given directly thereto by the CCD driving circuit 31 and provides a strobe signal. When the strobe signal is provided by the comparator 32, a signal provided by a counter 37 is given to a latch 38. The position of a shading edge is determined from the waveform, the microcomputer 35 determines the coordinates, and then the microcomputer 35 calculates the optical characteristics of the lens L on the basis of the coordinates.

The position data provided by the potentiometers 26 and 28 is transferred through the A/D converters 39 and 40 to the microcomputer 35. Then, the microcomputer 35 processes the position data, and then a display control circuit 41 controls the CRT display 1 to display the processed position data together with data stored in the memory in characters and graphs.

Operation of the lens meter in a measuring mode for measuring the optical characteristics of a single-focus lens will be described.

When measuring the spherical refractive power, astigmatic degree and astigmatic angle of a lens, a reticle is displayed on the CRT display 1. The LEDs 11 of the measuring optical system are turned on at predetermined intervals and the lens is mounted on the nose piece 7. Then, the refractive power on the measuring optical axis is calculated and displayed on the CRT display 1. The dislocation of the lens is calculated on the basis of the prism quantity and the target image is superposed on the reticle on the CRT display 1 at a position corresponding to the dislocation. Measured data obtained when the reticle and the target image meet a predetermined positional relation represents the optical characteristics of the lens.

The operation of the lens meter in a measuring mode for measuring the optical characteristics of framed progressive multifocus lenses will be described hereinafter.

Either the right lens selector button 3 or the left lens selector button 3 is depressed to specify a lens to be tested. A selected lens is put on the nose piece 7, and then the positioning plate 8 and the guide pin 9 are brought into contact with the lens. The reticle is displayed on the CRT display 1. The position of the lens is adjusted so that a section of the lens which is supposedly the far viewing section (a section specified by the lens maker) of the framed lens is positioned on the measuring optical axis. The additive diopter measuring mode selector button 5 is depressed to set the lens meter for operation in an additive diopter measuring mode, and then data representing the refractive power of the far viewing section of the lens is stored in the memory. A graph showing the relation between the spherical refractive power (equivalent spherical value SE is used for the sake of convenience) measured on the horizontal axis and the distance from a starting point from which the focal length starts varying progressively measured on the vertical axis is displayed on the CRT display 1.

Then, the position of the lens is adjusted so that the near viewing section is positioned on the measuring optical axis. Generally, the near viewing section is dislocated laterally relative to the far viewing section. In most lenses, the distance between the center of the far viewing section and that of the near viewing section is in the range of 2 to 2.5 mm. The distance detected by the potentiometer 26 is displayed on the CRT display 1. Since the optical strain (=.vertline.(measured cylindrical refractive power)-(cylindrical refractive power of the far viewing section).vertline.) of the far viewing section--the progressive focus section--the near viewing section is zero, an optical strain calculated by the microcomputer 35, for example, "0.25" is displayed on the CRT display as a guide message. The lens is moved so that the optical strain is reduced to the least extent. From the view point of operating facility, a suitable allowable optical strain is on the order of 0.25. The longitudinal and lateral distances by which the lens is moved to position the lens at the measuring position are detected by the potentiometers 26 and 28. The longitudinal and lateral distances, the optical strain and the refractive power (or the difference between the measured spherical value and the spherical value of the far viewing section) are stored in the memory. Upon the completion of the measurement, the additive diopter measuring mode selector button 5 is depressed again.

A graph as shown in FIG. 8 is completed by using the stored measured data and the graph is displayed on the CRT display 1. Data corresponding to the lateral dislocation, for example, 3 mm, and the optical strain, for example, 0.25 D, exceeding predetermined values may be omitted to enhance the reliability of the graph. The additive diopter may be read from the graph. The near viewing section 50 may be specified by operating a switch for scanning and a position may be specified by the distance from the far viewing section.

FIG. 9 is a flow chart showing a series of steps of an additive diopter measuring procedure. Referring to FIG. 9, the spherical refractive power SPH, cylindrical refractive power CYL and axial angle AXIS of the lens positioned at an initial position are measured in step S1. Then, the initial position is determined from signals provided by the potentiometers 26 and 28 in step S2. Then, query is made in step S3 to see if the lens mounted on the nose piece 7 has been moved longitudinally by a predetermined distance. If the response in step S3 is affirmative, optical strain (=.vertline.(measured cylindrical refractive power)-(cylindrical refractive power of the far viewing section).vertline.) is calculated in step S4. Then, a query is made in step S5 to see if the calculated optical strain is less than 0.25 D. If the response in step S5 is affirmative, the distance of lateral movement (=.vertline.(position for measuring the far viewing section)-(initial position).vertline.) is calculated in step S6. A query is made in step S7 to see if the calculated distance is less than 3 mm. If the response in step S7 is affirmative, step S8 is executed to store the measured data in the memory and to display the same on the CRT display 1, and to store the data of the initial position in the memory for reference in calculating the distance of longitudinal movement. Then, the routine returns to step S1. The additive diopter measuring procedure is executed every time the lens mounted on the nose piece 7 is shifted. Thus, a graph showing the relation between the distance of the progressive focus band from the starting point of the progressive focus section and the diopter of the progressive focus band is displayed on the CRT display 1 on the basis of the measured data. If the measuring point is positioned outside the progressive focus band, the response in step S5 is negative, namely, the measured optical strain is not less than 0.25 D. In such a case, information that the measuring point is positioned outside the progressive focus band is given to the operator, data obtained at the measuring point is omitted and the measuring position is not changed in step S9. Then the routine returns to step S1 to repeat the additive diopter measuring procedure. If the measuring point is positioned outside the progressive focus band, the response in step S7 also is negative, namely, the distance of lateral movement is not less than 3 mm. In this case also, step S9 is executed and the routine returns to step S1.

Although the invention has been described in its preferred form with a certain degree of particularity, obviously many changes and variations are possible therein. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein without departing from the scope and spirit thereof.