Detection and display device

A device for detecting and displaying the relative position of a generally horizontal reference plane of light. even when partial shading of the device occures, includes a photodetector means having including first and second interdigitated photodetector elements positioned adjacent each other on the device. The first and second interdigitated photodetector element provide first and second detection signals, respectively. A circuit means, responsive to the photodetector means, for determines the relative levels of the first and second detection signals such that the position of the reference plane of light with respect to the detector device is determined. A display means, responsive to the circuit means, for provides an indication of the position of the referece plane of light with respect to the detector device.

BACKGROUND OF THE INVENTION 
The present invention relates to equipment of the type used in surveying 
and construction and, more particularly, to a detector device for such an 
application which detects the position or level of a reference plane 
defined by a rotating laser beam or defined by a stationary plane of laser 
light. 
Laser systems have been employed in surveying and construction in which a 
laser beam is rotated in either a horizontal or a graded plane. U.S. Pat. 
No. 4,062,634, issued Dec. 13, 1977, to Rando, illustrates a laser 
projector device which provides such a rotating reference beam. The 
rotating beam defines a plane, and various measurements can be made using 
the plane as a reference. For example, the elevation of a point remote 
from the laser beam projector device may be measured by means of a rod on 
which a laser beam detector is mounted. The bottom of the rod rests on the 
ground at the point where the measurement is to be made, and the operator 
moves the detector along the rod to a position where it intercepts the 
laser beam, as indicated by a meter or other display on the detector 
device. One such detector device is shown in U.S. Pat. No. 4,240,208, 
issued Dec. 23, 1980, to Pehrson. 
A similar surveying system is shown in U.S. Pat. No. 4,732,471, issued Mar. 
22, 1988, to Cain et al. In the Cain et al system, a rotating beam is not 
used. Rather, a laser transmitter produces an alignment field by 
projecting laser energy in a non-planar, stationary reference cone. When 
the transmitter is level, this reference cone declines from the horizontal 
in a amount sufficient so that some compensation is provided for 
positional errors which occur due to the curvature of the earth. The Cain 
et al patent discloses a small, hand-held device which includes a display 
and a photodetector module. 
Both of the detector devices shown in the Cain et al and Petersen patents 
include a pair of adjacent, triangularly shaped photodetector elements. 
The orientation of the photodetector elements is such that the changes in 
the signal outputs from the elements which occur due to movement of the 
light are inversely related. As the reference light moves upward, the 
signal output from one of the elements increases while the signal output 
from the other of the elements decreases. The opposite signal changes 
occur when the position of the reference light moves downward. Naturally, 
the signal levels are also affected by the intensity of the light source 
and the distance of the photodetector elements from the light source. By 
comparing the relative signal output levels from the two photodetector 
elements, however, it is possible to obtain an indication of the position 
of the reference light without regard to the absolute intensity of the 
light. 
While such a detector arrangement provides accurate operation under varying 
conditions, it has been found to be subject to error in instances when the 
detector device is improperly oriented. The photodetector elements are 
positioned behind an aperture in the case of the detector device. If the 
detector device is misaligned by rotation about a vertical axis such that 
the light does not strike the elements at normal incidence, shading of the 
elements by the case at the edge of the aperture can occur. If the angle 
of incidence is large, the photodetector elements may be entirely shaded 
and the device will not function. This is an inconvenience for the 
operator, but is simply corrected by realignment of the device. More 
serious, however, is the situation in which the photodetector elements are 
only partially shaded. In this event, the two elements are not shaded 
equally, thereby producing a shift in the relative signal levels from the 
elements, and a corresponding error in the detected position of the 
reference light. Since the device provides an indication of light 
position, the operator may be unaware of the misalignment of the device 
and the resulting error in its operation. 
Accordingly, it is seen that there is a need for an improved device for 
detecting and displaying the relative position of reference light which is 
less sensitive to misalignment of the device. 
SUMMARY OF THE INVENTION 
This need is met by a device according to the present invention for 
detecting and displaying the relative position of a generally horizontal 
reference plane of light, even when partial shading of the device occurs. 
The device includes photodetector means having first and second 
interdigitated photodetector elements positioned adjacent each other on 
the device. The first and second interdigitated photodetector elements 
provide first and second detection signals, respectively. The device 
includes circuit means, responsive to the photodetector means, for 
determining the relative levels of the first and second detection signals 
such that the position of the reference plane of light with respect to the 
detector device is determined. Finally, the device includes display means, 
responsive to the circuit means, for providing an indication of the 
position of the reference plane of light with respect to the detector 
device. 
The first photodetector element comprises a plurality of sections arranged 
in a generally vertically oriented row. The second photodetector element 
comprises a plurality of sections, each section of the second 
photodetector element being positioned between adjacent sections of the 
first photodetector element. The heights of the sections of the first 
photodetector element increase from the bottom to the top of the row, and 
the heights of the sections of the second photodetector element decrease 
from the bottom of the top of the row. 
The sum of the height of a section of the first photodetector element and 
the height of the section of the second photodetector element directly 
therebeneath remains constant along the row. The sum of the height of a 
section of the first photodetector element and the height of the section 
of the second photodetector element directly therebeneath is preferably no 
greater than approximately one-half the vertical thickness of the 
reference plane of light. 
A device for detecting the relative position of a reference plane of light, 
the operation of the device being unaffected by partial shading of the 
device, comprises a first photodetector element including a plurality of 
sections arranged in a row which is generally perpendicular to the 
reference plane of light; a second photodetector element including a 
plurality of sections arranged along the row, each section of the second 
photodetector element being positioned between adjacent sections of the 
first photodetector element; and circuit means, responsive to the first 
and second photodetector elements, for determining the relative levels of 
the first and second detection signals such that the position of the 
reference plane of light with respect to the detector device is 
determined. 
The dimensions in a direction parallel to the row of the sections of the 
first photodetector element increase from a first end to a second end of 
the row, and the dimensions in a direction parallel to the row of the 
sections of the second photodetector element decrease from the first end 
to the second end of the row. The sum of the dimension in a direction 
parallel to the row of a section of the first photodetector element and 
the dimension in a direction parallel to the row of the section of the 
second photodetector element directly thereadjacent remains constant along 
the row. The sum of the dimension in a direction parallel to the row of a 
section of the first photodetector element and the dimension in a 
direction parallel to the row of the section of the second photodetector 
element directly thereadjacent is no greater than approximately one-half 
the thickness of the reference plane of light. 
Accordingly, it is an object of the present invention to prvoide an 
improved device for detecting and displaying the position of a plane of 
light; to provide such a device in which the partial shading of the device 
does not adversely affect the position detected and displayed; and to 
provide such a device in which a photodetector means includes a pair of 
interdigitated photodetector elements. 
Other objects and advantages of the invention will be apparent from the 
following description, the accompanying drawings and the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference is made to FIGS. 1 and 4, which generally illustrate a detection 
and display device 10 according to the present invention. The device 
detects reference light and provides an indication of the location of the 
reference light with respect to the device. It is to be understood that 
the device 10 may be used with transmitters which provide either a 
stationary plane or field of light, or a rotating beam of light. 
Additionally, the light may be projected in a true plane, or in a conical 
shape or other shape to define a reference. Reference light may, for 
example, be produced by a device such as shown in U.S. Pat. No. 4,062,634, 
issued Dec. 13, 1977, to Rando et al, or by a device such as shown in U.S. 
Pat. No. 4,732,471, issued Mar. 22, 1988, to Cain et al, both of which are 
discussed above. 
The detector device 10 includes a display 12, and a photodetector means 14 
positioned in a window or aperture 16 defined by case 18. The detector 
device 10 is switched on by control knob 20 and then positioned at the 
approximate height of the reference light. The light strikes the 
photodetector means 14, and a circuit means including detection circuitry 
22, provides an indication on display 12 of the position of the reference 
plane of light with respect to the detector device. Specifically, the 
display 12 indicates whether the light is above, below, or within a 
reference band which is centered between the top and bottom of the 
photodetector means 14. (Although this discussion may make reference to 
the top or bottom of the device or a component, or to a horizontal 
orientation or a vertical orientation, it will be appreciated that these 
terms are presented as relative to each other, for ease of explanation and 
understanding, and that they are in no way intended to be limited to 
absolute orientations.) 
The detection circuitry 22 is responsive to the photodetector means 14 for 
determining the relative levels of the first and second detection signals 
on lines 23 and 24, respectively, such that the position of the reference 
light is determined. The display 12, which may for example be an LCD 
display, includes a pair of arrows 25 and 26, and a bar 28. Arrow 25 and 
arrow 26 are displayed if the beam is below or above the reference band. 
Bar 28 is displayed if the beam is within the band. Additionally, the 
display may provide an indication as to whether the light is above, below, 
or within a second, larger reference band, which is aligned with the first 
reference band. The details of the detection circuitry 22 by which this 
may be accomplished are described in the above identified Cain et al 
patent. 
As discussed above, prior art detection devices have utilized a pair of 
detection elements, each configured in a triangular shape, and positioned 
such that movement of the reference light vertically causes an increase in 
the output signal from one of the detection elements while at the same 
time causing an increase in the output signal from the other of the 
detection elements. This arrangement is illustrated in FIG. 2. 
The elements 30 and 32 are positioned to intercept a plane of light 34. It 
will be appreciated that the level of the output signal from an element is 
directly related to the area of the element which is illuminated. As a 
consequence, movement of the light 34 upward increases the output signal 
from element 30 on line 23' and decreases the output signal from element 
32 on line 24', while movement of the light 34 downward decreases the 
output signal from element 30 and increases the output signal from element 
32. 
When the detection device is misaligned by rotation about its vertical 
axis, a portion of the photodetector elements may be shaded from the light 
34 by the case 18 at the side of the aperture 16. This results in light 
being blocked from hatched area 36. It will be apparent that element 30 
will provide a higher level output signal in this situation than element 
32, since a larger area of element 30 remains illuminated. As a 
consequence, the light 34 will be indicated as being higher than is 
actually the case. Since the device does provide an output, however, the 
operator may be unaware of this error. 
This problem is eliminated by the photodetector means 14 of the present 
invention, illustrated in FIG. 3. The photodetector means 14 includes 
first and second interdigitated photodetector elements 38 and 40, 
positioned adjacent each other on the device. The first and second 
interdigitated photodetector elements 38 and 40 provide first and second 
detection signals on lines 23 and 24, respectively. The first 
photodetector element 38 is made up of a plurality of separate sections 
which are arranged in a generally vertically oriented row and electrically 
conencted to line 23. Similarly, the second photodetector element 40 is 
made up of a plurality of sections which are electrically connected to 
line 24. Each section of the second photodetector element 40 is positioned 
between adjacent sections of the first photodetector element 38. 
As is apparent from FIG. 3, the heights of the sections of the first 
photodetector element 38 increase from the bottom to the top of the row, 
and the heights of the sections of the second photodetector element 40 
decrease from the bottom to the top of the row. This arrangement is 
provided so that the sum h of the height of a section of the first 
photodetector element 38 and the height of the section of the second 
photodetector element 40 directly therebeneath remains constant along the 
row. 
As a consequence, it will be appreciated that relative levels of the output 
signals on lines 23 and 24 will provide an indication of the area which is 
illuminated along the row of element sections. If the light strikes the 
photodetector elements near the middle of the row, the two signal levels 
on lines 23 and 24 will be approximately equal. Illumination of the 
elements at the upper part of the row produces a higher level signal on 
lines 23, whereas illumination of the elements at the lower part of the 
row produces a higher level signal on line 24. If the device is misaligned 
by rotation about a vertical axis to a degree sufficient to produce 
shading of the type illustrated in FIG. 2, both the first and second 
detection signals on lines 23 and 24 are reduced in amplitude. The 
relationship between these signals remains the same, however, since the 
relationship between the illuminated areas of the two interdigitated 
photodetector elements is unchanged. 
Preferably, the sum h of the height of a section of the first photodetector 
element 38 and the height of the section of the second photodetector 
element 40 directly therebeneath is no greater than approximately one-half 
the vertical thickness of the reference plane of light. It will be 
appreciated that this ensures that enough of the sections of both elements 
are illuminated that the desired relationship between the first and second 
detection signals is obtained. 
Having described the invention in detail and by reference to the preferred 
embodiment thereof, it will be apparent that other modifications and 
variations are possible without departing from the scope of the invention 
defined in the appended claims.