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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a three-dimensional shape measurement apparatus
which directs a laser beam at an object, picks up the light reflected back
from the object, photoelectrically converts the light and subjects it to
signal processing to obtain three-dimensional information about the
object.
2. Description of the Prior Art
There are many non-contact, optical methods of measuring the
three-dimensional shape or profile of an object ( See, for example,
Applied Optics, Vol.21(1982) page 3200, Vol. 23(1984) page 3837 or Vol.
25(1986) page 1630). One medical application of an optical
three-dimensional measuring method that has been attracting attention
recently is an apparatus for obtaining three-dimensional information about
the human eye fundus.
As well as its use in ophthalmology, examination of the eye fundus is also
employed in internal medicine for diagnosing hypertension, diabetes,
diseases of the cerebral nerves and other such disorders. For this, one
method in widespread use is that of using a fundus camera to conduct a
photographic examination. However, because quantitative measurement,
particularly of the degree of depression of the optic papilla of the eye
fundus is useful in the early detection of glaucoma, and as such is
directly related to the prevention of loss of vision, recently there have
also been attempts to acquire three-dimensional information about the
fundus, in addition to the two-dimensional information provided by the
usual fundus camera.
One such three-dimensional fundus measuring method consists of projecting a
fixed stripe or grid pattern on the fundus and observing it at a
prescribed angle from a separate direction in order to measure the
deviation of the stripe or grid image from a straight line. The principle
of triangulation is then used to convert the amount of deviation in the
depth direction and thereby quantitatively evaluate the state of fundus
depression (see, for example, U.S. Pat. No. 4,423,931).
In another method, based on the principle of stereoscopic photography, a
fundus camera or the like is used to take two photographs of the fundus at
different angles of pupil incidence. By then analyzing the images of the
two fundus photographs, depth information can be extracted and quantified.
Apparatuses have also been developed in which the photographic film of the
fundus camera is replaced by two equivalent television cameras positioned
at different observation angles and linked to a computer so as to provide
three-dimensional information automatically (see, for example, U.S. Pat.
No. 4,715,703).
However, with each of these methods still suffering from low spatial
resolution capabilities and difficulties relating to accuracy and the
degree of reproducibility, their clinical feasibility remains uncertain.
One reason for this relates to physical restrictions inherent in the
fundus of the human eye; i.e., observation of the inner surface of a
relatively large spherical object via the limited window of a small pupil.
In the case of both triangulation and stereophotography, it is not
possible to increase the angular difference of the observations. Another
reason is the very low reflectivity and contrast of the fundus, which is a
product of reflection characteristics specific to the object, i.e., the
abrupt changes in the reflection intensity of the optic papilla of the eye
fundus. It is mainly for these two reasons that, in the case of measuring
three-dimensional shapes, it is difficult to improve spatial resolution
capabilities, accuracy and reproducibility.
Another problem with methods of measurement using the conventional
apparatuses is that none of them are methods of directly measuring depth
information, and as such, in each case processing time is required. The
method employing the principle of triangulation requires time for the
calculations involved in the conversion of the amount of stripe or grid
image deviation to depth information, and the stereophotographic-based
method requires time for analyzing the image information in the two
photographs to elucidate depth information. Also, there is a tradeoff
between shortening of the processing time and improvement in the spatial
resolution. Computer advances can reduce the time needed for the
processing, but not enough for clinical applications.
One electronic ophthalmic examination apparatus for human eye fundus
applications that employs laser scanning was developed by the Retina
Foundation of the U.S.(See U.S. Pat. No. 4,213,678 and Japanese Laid-open
Patent Publication 62-117524.) This apparatus has attracted attention for
the many features it possesses, such as the ability to display in
real-time on a monitor television a high-contrast video image of the eye
fundus, using a low level of illumination. In a paper (Applied Optics,
vol. 19 (1980) page 2991), the Retina Foundation mentioned that it was
feasible to use this laser-beam scanning method to ascertain the
three-dimensional configuration of the eye fundus. However, the method is
based on the same principle of stereoscopic photography explained above,
in which different angles of pupil incidence are used, so there are
problems concerning resolution and accuracy. As such, regarding the direct
determination of the three-dimensional shape of the fundus, the method is
not practicable.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to provide a new type of
practical, three-dimensional shape measurement apparatus that can be
applied to objects, such as the fundus of the human eye, where it is
impossible to obtain a large angular difference such as is needed for
triangulation-based observations, and which exhibit abrupt changes in
reflection intensity.
To attain this object, the present invention comprises a three-dimensional
shape measurement apparatus which directs a laser beam at an object,
detects the light reflected back from the object, photoelectrically
converts the light and subjects it to signal processing to obtain
three-dimensional information about the object, comprising: a laser light
source that generates a laser beam; a first optical deflector for scanning
the laser beam in one direction at a predetermined frequency; a second
optical deflector for scanning the laser beam in a direction that is
perpendicular to the above said direction at a predetermined frequency; an
optical system for projecting the laser beam scanned two-dimensionally by
the first and second optical deflectors at the object; optical means for
scanning light reflected from the object and guiding it so that scanning
is stationary fixed at least with respect to the direction of scanning by
the second optical deflector; detection means for acquiring information
relating to the shape characteristics of the object in a direction of an
optical axis that is perpendicular to the direction of the scanning by the
first and second optical deflectors, depending on displacement of the
focal point of the light reflected from the object; and signal processing
means for removing from the output signal of the detection means the
influence of the optical reflection characteristics of the object.
The three-dimensional shape measurement apparatus of this invention whereby
a laser beam is directed at an object, the light reflected back from the
object is picked up, photoelectrically converted and subjected to signal
processing to obtain three-dimensional information about the object also
comprises: a laser light source that generates a laser beam; an optical
deflector for scanning the laser beam at a predetermined frequency; an
optical system for projecting the laser beam scanned by the optical
deflector at the object; two detection slits oriented in a direction
parallel to the direction in which light reflected from the object is
scanned and facing the object along an optical axis that is perpendicular
to the direction of the scanning by the optical deflector, said detection
slits being mutually displaced by a prescribed distance in the vicinity of
a position that is optically conjugate with the object; two photosensors
for detecting light passing through the respective detection slits; and
signal processing means for cancelling out from the output signal of the
photosensors optical reflection characteristic information of the object
for acquiring information relating to the shape characteristics of the
object in a direction of an optical axis toward the object that is
perpendicular to the direction of scanning by the optical deflector.
The three-dimensional shape measurement apparatus which directs a laser
beam at an object, detects the light reflected back from the object,
photoelectrically converts the light and subjects it to signal processing
to obtain three-dimensional information about the object further
comprises: a laser light source that generates a laser beam; an optical
system for projecting the laser beam from the laser light source at the
object; two detection apertures mutually displaced by a prescribed
distance in the vicinity of a position that is optically conjugate with
the object; two photosensors for detecting light reflected from the object
passing through the respective detection apertures; a divider circuit for
dividing the output of one photosensor by that of the other; and a
correction means for correcting prescribed non-linearity in the output of
the divider.
With this arrangement in which the light reflected from the object is
scanned and an optical means is used for guidance purposes to fix the
scanning, on the detection side the scanning of the reflected light is
fixed at least in one dimension, enabling information to be detected in
the direction of the optical axis, by using a detection slit, for example,
at the focal point position. This enables it to be applied to objects such
as the fundus of the human eye, where it is impossible to obtain a large
angular difference such as is needed for observations for triangulation.
As the apparatus also has a means that uses a divider to cancel out
optical reflection characteristic information of the object, even when the
object exhibit abrupt changes in reflectivity, the effect is nullified, so
the accuracy and reproducibility are high. Further, using optical
deflectors as the scanning means makes it possible to achieve high spatial
resolution in the measurements by employing many measuring points. And, as
information in the direction of the optical axis is detected directly, the
time needed for processing the measurements can be made relatively short.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of the overall configuration of the
optical system of the apparatus according to the present invention;
FIG. 2 shows in detail part of the optical system shown in FIG. 1;
FIG. 3 is a graph showing the characteristic of the output signal of the
photosensor of FIG. 2;
FIG. 4 is a graph showing the characteristic of the computed output signal
of the photosensor; and
FIG. 5 is a block diagram showing the electrical configuration of the
apparatus of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The embodiments of the present invention will now be described in the
following with reference to FIGS. 1 to 5.
FIG. 1 is a general schematic of the optical system arrangement of the
three-dimensional shape measurement apparatus according to the present
invention. In FIG. 1, reference numeral 1 denotes a laser light source of
helium-neon (He-Ne) or argon (Ar.sup.+), for example. A laser beam 2
produced by the laser light source 1 is expanded to a specific size by a
beam expander 3 and is then reflected by a mirror 4 to impinge on a lens
5. The lens 5 is for shaping the laser beam for the rectangular aperture
of a following acousto-optical deflector (AOD) 6, and incorporates a
multiplicity of cylindrical lenses. The AOD 6 is bracketed by a pair of
prisms 7 and 8 which compensate for the wavelength dependency of the angle
of incidence and angle of emergence of the laser beam with respect to the
AOD 6. The laser beam deflected in one dimension (horizontally) by the AOD
6 is reformed from the rectangular (elliptical) shape to its original
circular shape by a lens 9 which is constituted analogously to the lens 5,
following which the beam passes through a lens 10 and a slit 11. The slit
11 is for blocking zero-order light (not shown) from the AOD 6 so as to
utilize only first-order diffraction light. First-order diffraction light
12 from the slit 11 is scanned one-dimensionally with the central portion
of a mirror 15 disposed at a position optically conjugate with the pupil
of the eye 14 being examined as the pivot point of deflection.
A laser beam scanning frequency of 15.75 KHz corresponding to the ordinary
NTSC standard television horizontal scan rate is selected for the AOD 6.
The prisms 7 and 8 may not be required if only a laser beam of one
wavelength is used.
The laser beam reflected by the mirror 15 is reflected onto a mirror 17 by
a concave mirror 16 which possesses the function of a lens. The mirror 17
is attached to a galvanometer 18 for vertical scanning of the laser beam,
and is referred to as an oscillating mirror or a galvanometer mirror. The
laser beam scanned two-dimensionally by the mirror 17 is passed through an
objective lens 19 and is thereby projected onto the eye fundus via the
central portion of the pupil of the eye 14 being examined. The light
reflected from the fundus, which in FIG. 1 is indicated by a dashed line,
is guided via the objective lens 19 to be reflected again by the mirror 17
and the concave mirror 16. The light reflected from the eye fundus is in a
two-dimensionally scanned state, but after being directed onto the concave
mirror 16 by the mirror 17 it is fixed in a vertical scanning state by the
deflective action of the mirror 17; i.e., it becomes reflected light that
is scanned only one-dimensionally.
A vertical scanning frequency of 60 Hz corresponding to the ordinary
television vertical scanning frequency is selected for the oscillating
mirror 17.
Light reflected from the eye fundus and then reflected by the concave
mirror 16 passes around the periphery of the mirror 15, which is the point
at which it is separated from the projected laser light. This
one-dimensionally scanned reflected light from the fundus that passes
around the periphery of the mirror 15 passes through a lens 20. Half of
this reflected light is then reflected by a half mirror 21, passes through
a detection slit 22 and a lens 23 and is detected by a photosensor 24.
Disposed in front of the photosensor 24 is a filter 25 which corresponds
to the wavelength of the laser beam. Also, between the lens 20 and the
half mirror 21 is a black spot 26 for eliminating the effect of light
reflecting from the surface of the objective lens 19.
The other half of the light reflected from the eye fundus is transmitted by
the half mirror 21 and, similarly to the reflected half, passes through a
detection slit 27, a lens 28 and a filter 29 to a photosensor 30. The
filter 29 has the same characteristics as the filter 25, corresponding to
the wavelength of the laser beam. As is apparent from FIG. 1, each of the
detection slits 22 and 27 has been situated slightly away from the
respective positions 22a and 27a that are optical conjugates of the fundus
of the eye 14. In addition, viewed along the optical axis in the direction
of the eye 14, one of the detection slits is situated slightly to the
front, and the other slightly to the rear, of the positions that are the
optical conjugates of the fundus of the eye 14.
FIG. 2 illustrates part of the optical system of FIG. 1, particularly the
portion for guiding the light reflected from the fundus, in a
configuration that is closer to an actual arrangement. In FIG. 2, the
laser beam 12 (first-order diffraction light) deflected one-dimensionally
(in the horizontal direction) by the AOD 6 is guided via the lens 13 to be
scanned with the mirror 15 as the pivot point of deflection, said mirror
15 being disposed at a position that is optically conjugate with the pupil
of the eye being examined.
In FIG. 2 the scanning direction of the first-order diffraction light 12 is
perpendicular to the drawing sheet, so the laser beam is therefore
depicted as following the central axis of the optical system. The laser
beam reflected by the mirror 15 and scanned in the horizontal direction
(vertically with reference to the FIG. 2 drawing sheet) is reflected by
the concave mirror 16, is again reflected by the oscillating mirror 17
attached to the galvanometer 18 and is also scanned vertically
(horizontally with respect to the drawing sheet). The laser beam scanned
two-dimensionally (horizontally and vertically) by the oscillating mirror
17 is projected onto the fundus of the eye 14 by the objective lens 19,
and the light reflected from the eye fundus, depicted by a dashed line in
FIG. 2, is returned through the same optical system elements 19, 17 and 16
as the incident beam.
The light reflected from the eye fundus that has been separated from the
incident beam at the mirror 15 passes through the lens 20 and impinges on
the half mirror 21 which splits the optical path by reflecting one half of
the light and transmitting the other half. The light reflected by the
mirror 21 passes through the detection slit 22, lens 23 and filter 25 and
is detected by the photosensor 24. The portion of the light transmitted by
the mirror 21 passes through the detection slit 27, lens 28 and filter 29
and is detected by the photosensor 30. In FIG. 2, the reflected light from
the eye fundus, represented by the dashed lines from the oscillating
mirror 17 to the photosensors 24 and 30, is only being scanned
horizontally (vertically with reference to the FIG. 2 drawing sheet). This
being the case, the laser beam is depicted as following the central axis
of the optical system as if it is not being scanned.
As is apparent from the drawing, the two detection slits are oriented so
that the reflected light is scanned in a direction that is parallel to the
slits, and in a direction that is perpendicular to the slits the scanning
is stationary, in addition to which they are oriented along the optical
axis toward the eye so that one is displaced slightly to the front, and
the other slightly to the rear, of the positions 27a and 22a that are the
optical conjugates of the fundus.
The electrical signal obtained as a result of detection and optoelectronic
conversion by the photosensors 24 and 30 reflects the reflection
characteristics of the fundus of the eye 14. Also, as the frequencies of
the horizontal and vertical scanning of the laser beam by the AOD 6 and
the oscillating mirror 17 correspond to the scanning rates of an ordinary
television, by appropriately amplifying the output signals from the
photosensors 24 and 30 for feeding to a television monitor,
two-dimensional images of the fundus can be displayed on the screen. The
image shown on the monitor will mirror the optical reflection
characteristics of the layer of fundus tissue corresponding to the
wavelength of the laser beam.
However, because in this case the detection slits 22 and 27 are slightly
displaced from the position that is optically conjugate with the fundus,
the output signals from the two photosensors that detect the reflected
light from the fundus that passes through the two detection slits will
differ in intensity depending on the state of the optical conjugate
relations between the detection slits and the fundus. That is, assuming
the two slits are equal, when the positions that are optically conjugate
with the fundus, i.e., the positions of the focal points of the fundus,
are an equal distance from the detection slits 22 and 27 the output
signals from the two photosensors will be about equal in intensity.
However, unevenness in the fundus will cause a slight displacement in the
said conjugate positions, giving rise to a difference in intensity between
the signals output by the two photosensors.
FIG. 3 shows a typical characteristic waveform of the signal output by the
two photosensors 24 and 30. The horizontal axis represents a distance in
the direction along an optical axis Z perpendicular to both the horizontal
x and vertical y directions in which the laser beam is scanned, and the
vertical axis represents the intensity I of the photosensor output
signals. The two waveforms I.sub.1, I.sub.2 show the changes in the
respective output signals of the photosensors 24 and 30, such as when a
depression, for example, in the fundus causes the location of the position
of optical conjugate with the fundus to shift along the optical axis Z. As
is apparent from FIG. 2, when the position 22a that is an optical
conjugate of the fundus coincides with the position of the detection slit
22 (indicated in FIG. 3 by A) the intensity I.sub.l of the output signal
of the photosensor 24 is at its peak, and on each side thereof there is a
smooth fall-off in signal intensity I.sub.1. On the other hand, when the
position 27a that is an optical conjugate of the fundus coincides with the
position of the detection slit 27 (indicated in FIG. 3 by B) the intensity
I.sub.2 of the output signal of the photosensor 30 is at its peak, falling
off to each side thereof in the same way as I.sub.1. What has to be noted
here is that the signal intensity is completely dependent on the
reflectivity of fundus. This means that when the laser beam is scanning a
part of the fundus that exhibits high reflectivity the waveform of FIG. 3
shows a proportional increase in size and, conversely, decreases in size
when reflectivity is lower.
FIG. 4 shows a typical waveform characteristic obtained by computing the
output signals of the photosensors 24 and 30 having the type of
characteristic shown in FIG. 3. The horizontal axis represents a distance
along an optical axis Z and the vertical axis represents the signal
intensity I' obtained by computation. The two waveforms were obtained by
calculating I.sub.l -I.sub.2 and I.sub.2 /I.sub.1, respectively. The
I.sub.l -I.sub.2 type of differential characteristic is featured by the
intensity value having a zero point and being partially proportional to Z,
and because of this it is used in video-disc systems for detection of the
position of the focal point in the Z direction.
However, as is clear from the above explanation, because I.sub.l -I.sub.2
depends entirely on the reflectivity of the fundus, unless the intensity
of the reflected light from the fundus is known, the Z-direction value
cannot be determined from the value I.sub.l -I.sub.2. Regarding I.sub.2
/I.sub.1, even if the intensity of I.sub.l and I.sub.2 depend on the
reflectivity of the fundus, each can cancel out the other by denominator
and numerator to make them independent of the fundus reflectivity, and
thus making it possible to determine the value in the Z direction from
I.sub.2 /I.sub.1. Expressed mathematically, if I.sub.0 (x, y) is the
intensity of the reflected light from the fundus, when
I.sub.1 =f.sub.1 (Z).I.sub.0 (x, y)
I.sub.2 =f.sub.2 (Z).I.sub.0 (x, y)
as I.sub.2 /I.sub.1 =f.sub.2 (Z)/f.sub.1 (Z), I.sub.2 /I.sub.1 becomes a
prescribed function that depends only on the value of Z, not on the
reflectivity, so it is clear that the Z value can be obtained from I.sub.2
/I.sub.1. As such, by finding the value of I.sub.2 /I.sub.l by calculation
it is possible to quantify the degree of unevenness three-dimensionally,
the amount of displacement in the location of the optically conjugate
position along the optical axis Z, i.e., the displacement in the Z
direction of each location in the fundus as it is scanned by the laser
beam in the x and y directions.
The range of values that can be set along Z, based on such characteristics,
becomes a constant measurement range that falls inside the points A and B
shown in FIG. 4. Also, as the value of I.sub.2 /I.sub.l is not
proportional to the Z value but is instead rather non-linear, when it used
with an actual measurement apparatus a means of correcting this
non-linearity is required.
Moreover, it is, for example, also possible to combine the addition,
subtraction, and division to cancel the intensity of the reflected light
according to the following equation.
##EQU1##
In this case, the calculated value possesses the different non-linearity
from that obtained using the value I.sub.2 /I.sub.1.
FIG. 5 is a block diagram illustrating the electrical configuration of the
three-dimensional shape measurement apparatus according to the present
invention. The laser beam produced by the laser light source 1 is
deflected horizontally and vertically by means of the AOD 6 and the
oscillating mirror 17, and is beamed at the eye 14 being examined. A
driver 31 is connected to the AOD 6, the driver 31 being controlled by a
sawtooth signal generated by a sawtooth waveform generator 32. Connected
to the galvanometer 18 which drives the mirror 17 is a driver 33 which is
controlled by a sawtooth signal generated by a sawtooth waveform generator
34.
The light reflected from the eye 14 is detected by the photosensors 24 and
30, and the output signals therefrom are amplified to the required level
by amplifiers 35 and 36. The output signals I.sub.l and I.sub.2 from the
amplifiers are input to a divider 37, the divisional operation I.sub.2
/I.sub.l is performed, and the result is input to a function generator 38.
As was explained above, the calculated outputs of photosensors 24 and 30
have the type of intensity characteristics shown in FIG. 4. The purpose of
the function generator 38 is to cancel the nonlinearity of the value
I.sub.2 /I.sub.l seen in FIG. 4. The intensity of the output signal of the
function generator is proportional to the displacement of the fundus in
the direction of the optical axis Z, i.e., to the degree of unevenness of
the fundus. The divider 37 and the function generator 38 may be
constituted entirely of analog integrated circuitry, or it may be
comprised of A/D, D/A converters and digital operation circuitry.
After the output signals of the function generator 38 and of the amplifiers
35 and 36 are input to a signal processor 39, the information is selected
and the prescribed processing carried out, an output device 40 such as a
display monitor is used to display the output image. When the signal
selected is from the amplifier 35 or 36, the image output is an ordinary
two-dimensional depiction of the reflection characteristics of the fundus,
while when the function generator 38 signal is selected the image displays
unevenness of the fundus surface with different shade levels or colors.
Also, microprocessors and software provided in the signal processor 39 can
be used to create three-dimensional graphic patterns based on the output
signals of the function generator 38, enabling, for example, an
observation of the fundus taken at an angle to be displayed on the output
device 40 as a three-dimensional bird's-eye view.
The control system of the two-dimensional scanning of the laser beam and
the output system that detects and processes the reflected light from the
fundus are synchronized by a horizontal synchronizing signal 41a and
vertical synchronizing signal 41b from a synchronizing signal generator
41, thereby enabling time-based control of the overall system.
Moreover, although in this embodiment the scanning is fixed(stationary) in
the vertical direction at the photosensors, the light reflected from the
object (fundus) being scanned by the oscillating mirror only, and two
detection slits are therefore used to detect displacement of the focal
point of the reflected light, a rotating multi-faced mirror, for example,
could of course be used as the horizontal deflector for scanning the
reflected light, and when the horizontal scanning is also fixed the
detection slit could be replaced by a round aperture. That is, the present
invention encompasses use of two deflectors, and the reflected light from
the object is scanned with respect to one direction and the scanning is
stationary in at least direction. Also encompassed is the arrangement in
which the scanning of the reflected light is performed with respect to
both directions and the scanning is completely stationary in both
directions at the detection aperture, if the arrangement is one that
performs detection of information related to shape characteristics of the
object in the direction of an optical axis perpendicular to the direction
of scanning by the two deflectors, based on the displacement of the focal
point of the light reflected from the object.
While the invention has been described with reference to a preferred
embodiment in which the object is an eye fundus, it will be understood
that the invention is not limited thereto but may be utilized to ascertain
three-dimensional shapes such as the features of integrated circuit
patterns, or of microorganisms, cells and the like by applying this to,
for example, a laser scanning microscope.
As can be seen from the above description, the three-dimensional shape
measurement apparatus according to the present invention is new and
eminently practical, applicable as it is to objects, such as the fundus of
the human eye, which exhibit abrupt changes in reflectivity and where it
is impossible to obtain the type of large angular difference needed by the
triangulation method. It can also be used to carry out measurements on a
television monitor at a resolution as high as the screen resolution; and
it has good accuracy and reproducibility. In addition to this, the time
required for processing the measurements can be shortened: by using
different shades to represent the unevenness in the contours of the
object, it is possible to conduct the measurements on a fully real-time
basis and at the same time obtain ordinary two-dimensional information
relating to the reflection characteristics of an object.
* * * * *
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