Shape measuring apparatus

Details Shape measuring apparatus Shape measuring apparatus

2001-01-18

2004-02-24

Fuller, Rodney (Department: 2851)

Optics: measuring and testing

Shape or surface configuration

By focus detection

C356S601000, C356S602000

Reexamination Certificate

active

06697163

ABSTRACT:

This application is based on patent application No. 2000-14257 filed in Japan, the contents of which are hereby incorporated by references.
BACKGROUND OF THE INVENTION
This invention relates to a shape measuring apparatus for measuring an outer surface configuration of an object by reduction scanning and specifically to a technique of correcting a relative displacement between two photo-sensors provided in a photo-sensor unit of a shape measuring apparatus to enlarge a dynamic range.
As shown in
FIG. 12
, there is known a method for enlarging the dynamic range of CCDs (Charge-Coupled Devices) which uses two CCDs
100
,
101
and a semitransparent prism
102
having a large light amount splitting ratio (hereinafter, this method is referred to as a dual CCD system).
According to this system, the CCDs
100
,
101
are so arranged as to face two mutually orthogonal emerging surfaces
102
a
,
102
b
of the semitransparent prism
102
, the light amount of an incident light beam is split at a large ratio of, e.g., L1:L2=99:1 by the semitransparent prism
102
so as to focus a bright image and a dark image on the CCDs
100
,
101
, respectively, and the dynamic range is enlarged by combining the images separately sensed by the two CCDs
100
,
101
.
Since the amount of incident light is suppressed to L2/(L1+L2)=0.01 in the CCD
101
, light can be sensed even if the amount of incident light exceeds the photo-sensing sensitivity of the CCD
100
. Accordingly, the image sensed by the CCD
100
is used when the amount of incident light lies within the photo-sensing sensitivity of the CCD
100
, whereas the two sensed images are combined to use the image sensed by the CCD
101
by adjusting the brightness by multiplying it by (L1+L2)/L2, thereby enlarging the dynamic range of the CCDs when the amount of incident light exceeds the photo-sensing sensitivity of the CCD
100
.
Since the two photo-sensors (CCDs) are used in the conventional dynamic range enlarging method, photo-sensing characteristics of the two photo-sensors and the photo-sensing positions of the photo-sensors need to be adjusted. As a method for adjusting the photo-sensing positions is known the one according to which two photo-sensors are respectively provided with mechanical position adjusting mechanisms and the positions of the respective photo-sensors are adjusted using these position adjusting mechanisms. This method is a position adjusting method by hardware.
The mechanical position adjusting mechanisms are excellent in guaranteeing the positional precision of the photo-sensors by mechanical precision, but are required to have a high precision of adjustment in the order of &mgr;m and need to be complicated adjusting mechanisms adjustable in directions of six axes. This brings about problems of difficult position adjustment, larger size, and high price. To this end, a method has been proposed according to which a relative displacement of two photo-sensors actually mounted is measured at the time of production, the measured displacement is stored as a correction value in a memory, an error resulting from the displacement of the photo-sensors is corrected by the correction value when light reception data sensed by the photo-sensors are processed. This method is a position adjusting method by software.
The position adjusting method by software reduces a burden caused by the mounting construction of the photo-sensors, but calculation needs to be made for correction using the correction value every time the light reception data of the photo-sensors are processed. This increases a burden on data processing performed in a CPU (Central Processing Unit).
The shape measuring apparatus for measuring an outer surface configuration of an object to be measurement or a measurement object is provided with a photo-sensor formed of a line sensor arranged in parallel to X-direction if it is assumed that a height direction (vertical direction) of the measurement object is Y-direction, a direction of a line connecting the shape measuring apparatus and the measurement object is Z-direction, and a direction normal to Y-direction and Z-direction is X-direction. The apparatus is placed at a specified height position (specified Y-coordinate position) with respect to the measurement object, light reflected by the measurement object is sensed by the photo-sensor, distances (corresponding to Z-coordinates) from the measuring apparatus (precisely from the photo-sensor) to the outer surface of the measurement object are measured for the respective pixels (for the respective X-coordinate positions). In this way, three-dimensional data (X, Y, Z) of the outer surface of the measurement object are measured at specified measurement intervals.
Since high-precision measurement is made by reduction scanning in the shape measuring apparatus, a photo-sensor having a high sensitivity and a wide dynamic range is required. However, a desired dynamic range cannot sometimes be obtained with presently commercially available photo-sensors. In such a case, a dynamic range enlarging method as described above needs to be adopted.
In the case of adopting the dual CCD system, the displacement between the two line sensors causes a problem as described above. Particularly, a displacement in a pixel aligning direction (X-direction) of the line sensor largely influences a measurement accuracy since the respective pixel positions correspond to X-coordinates of measurement points, and light reception signals at different pixel positions are combined when the light reception signals of the two line sensors A, B are switchingly combined.
FIG. 14
is a diagram showing an influence on the measurement accuracy when the two line sensors are displaced in the pixel aligning direction. In
FIG. 14
, a black-and-white strip CH at the uppermost stage is a test chart, and two strips A, B therebelow are line sensors. It should be noted that a
1
, a
2
, . . . a
12
within the line sensor A and b
1
, b
2
, . . . b
12
within the line sensor B represent pixels, which have a sensitivity characteristic as shown in FIG.
15
. As shown in
FIG. 14
, the line sensors A, B are displaced from each other by one pixel in the pixel aligning direction. A white area (brightness level B
H
) and a black area (brightness level B
L
) of the test chart CH are sensed by the pixels a
1
to a
6
and the pixels a
7
to a
12
in the line sensor A, respectively. On the other hand, the white area and the black area are sensed by the pixels b
1
to b
5
and the pixels b
6
to b
12
in the line sensor B.
In
FIG. 14
, an upper graph shows the output level of the line sensor A, a middle graph shows the output level of the line sensor B, and a lower graph shows a combination of the output levels of the line sensors A and B. The amount of incident light on the line sensor B is 1/N of that on the line sensor A. In the lower graph, the output level of the line sensor B is combined with that of the line sensor A after being multiplied by N.
Since the brightness level B
H
of the white area exceeds a maximum output level Vmax of the line sensor A as shown in
FIG. 15
, the output levels of the pixels a
1
to a
6
of the line sensor A are saturated at the maximum output level Vmax, and the output levels of the pixels a
7
to a
12
are at V
L
corresponding to the brightness level B
L
of the black area. On the other hand, since the amount of incident light is gradually reduced to 1/N in the line sensor B, the output levels of the pixels b
1
to b
5
are at an output level V
H
/N corresponding to the brightness level B
H
/N, and the output levels of the pixels b
6
to b
12
are saturated at a minimum output level Vmin.
According to the dual CCD system, the output levels of the pixels a
1
to a
6
of the line sensor A are combined by being replaced by the output levels of the corresponding pixels b
1
to b
6
of the line sensor B since they are saturated. However, as shown in the lower graph, the output level corresponding to the pixel a
6
is substantially missing. As a result, a plurality of light rec

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