Abstract:
The present invention provides an apparatus and method for projecting a reference plane. The invention features emitting a light beam, splitting the light bean into two or more resulting beams, and converting the resulting beams into respective projected lines. The projected lines can be used to indicate a reference plane that substantially spans greater than 90 degrees from a reference point. The invention also features at least two lasers configured and arranged to emit a laser beam. An optical element corresponds to each laser to convert each laser beam into an output line that are projected to form only a single reference plane.

Description:
TECHNICAL FIELD 
   This invention relates to laser alignment devices. 
   BACKGROUND 
   Portable devices capable of indicating a reference plane have been useful in the construction and carpentry trades, as well as in other applications, for some time. The advent of laser technology has seen the use of portable laser emitting devices capable of indicating a level 360-degree reference plane. For example, in the construction industry, narrow beams of collimated light, in the form of laser beams are being increasingly used in connection with establishing and marking long, straight, level lines, such as those required for markings for walls, ceilings, and/or floors. Also, laser beam projectors and receivers are often used in the construction industry and in agricultural land leveling applications to ensure that a target area is graded in the proper or desired slope or grade. The laser beam projector is placed in a known position and one or more sensors are placed in the targeted area to sense the impingement of the laser beam. 
   A lase emitting device that can generate a 360-degree reference plane typically includes a laser source for generating a beam of collimated light and a rotating mechanism for rotating the beam of light about an axis to generate a plane of light. 
   In order to provide a substantially level plane, it is necessary to have a known orientation for the laser plane with respect to the true earth reference. Typically, the laser plane is oriented perpendicular to the earth&#39;s gravitational field, such as by manually leveling the mechanism, or by a self-leveling mechanism though a corresponding pair of servo motors and inclinometer sensors. Additionally, some laser plane generators are operable to orient the laser plane at an angle with respect to the level orientation by rotating each or both of the axes according to the desire slope. While such laser projectors facilitate generating a laser plane at a desired grade and orientation, the higher number of moving parts, including the rotating mechanism, within the laser projector may raise reliability concerns over a prolonged period of time. Also, when sensors are not used to sense the laser beam, the reference plane generated by the rotated laser beam can be difficult to see with the human eye as the beam of light is intermittently flashed along a surface. 
   In some prior art tools, a 360-degree reference plane can be generated without a rotating mechanism, a laser beam is directed at a conical surface, such as a right angle cone whose optical surface is aligned with the axis of the laser beam. 
   SUMMARY 
   The present invention provides apparatus and a method for projecting a reference plane. In general in one aspect, the invention features an apparatus to project a reference plane, including a laser to emit a light beam, a beam splitter to split the light beam into two or more resulting beams, and an optical element corresponding to each of the resulting beams to convert the resulting beams into respective projected lines. The projected lines can be used to indicate a reference plane that substantially spans greater than 90 degrees from a reference point. 
   Implementations of the invention may include one or more of the following. The reference plane can span approximately 180 degrees or 360 degrees from the reference point. The projected lines can overlap or be adjacent to each other to form a continuous reference plane. The laser can be a laser diode. The laser diode can be pulsed to screen out ambient light. The optical element can include a rod lens, a cylindrical lens or a hybrid lens. A collimator to collimate the light beam before it is split by the beam splitter can be included in the apparatus. The collimator can be a focus lens or an aspheric lens. 
   In general, in another aspect, the invention features an apparatus to form a reference plane, including at least two lasers within a housing, each laser configured and arranged to emit a light beam. A collimator to collimate each light beam, and an optical element corresponds to each laser to convert each of the collimated light beams into an output line. The output lines are projected from the housing in different directions along the same plane to only form a single reference plane that substantially spans approximately 140 degrees from a reference point. 
   In general, in yet another aspect, the invention features an apparatus to project a reference plane, including a housing and a projection unit within the housing. The projection unit includes five lasers each configured and arranged to emit a light beam and an optical element corresponding to each of the lasers to convert each light beam into an output line. The output lines are projected from the housing in different directions along the same plane to only form a single 360 degree reference plane. 
   Implementations of the invention may include one or more of the following. The projection unit can be pendulously suspended in the housing. A damper to dampen pendulous motion of the projection unit can be included in the apparatus. The damper can include a magnet mounted in the housing below the projection unit and a damping plate rigidly suspended from the projection unit, such that the damping plate is positioned above the magnet with a gap maintained therebetween sufficiently small that eddy currents are generated in the damping plate by motion thereof above the magnet. An error indicator to indicate when the housing is inclined such that the accuracy of the reference plane is compromised can be included in the apparatus. 
   In general, in still another aspect, the invention features an apparatus to form a reference plane, including at least one laser means to emit a light beam and means to convert the light beam into at least two output lines that are projected in different directions to only form a single reference plane that substantially spans greater than 90 degrees from a reference point. 
   In general, in another aspect, the invention features an apparatus to form a reference plane, including a projection unit pendulously suspended in a housing. The projection unit includes at least two lasers. Each laser is configured and arranged to emit a light beam, and a collimator is provided to collimate each light beam. An optical element corresponds to each laser to convert each of the collimated light beams into an output line, and a damper dampens pendular motion of the projection unit. The output lines are projected from the housing in different directions along the same plane to only form a single reference plane that substantially spans approximately 140 degrees from a reference point. 
   In general in yet another aspect, the invention features an apparatus to form a reference plane, including a housing and a projection unit pendulously suspended within the housing. The projection unit includes five lasers each configured and arranged to emit a light beam and an optical element corresponding to each of the lasers to convert each light beam into an output line. The output lines being projected from the housing in different directions along the same plane to only form a single 360 degree reference plane. The apparatus further includes a damper to dampen pendular motion of the projection unit. 
   In general, in another aspect, the invention features a method of forming a reference plane, including forming a light beam, splitting the light beam into at least two resulting beams, and converting the resulting beams into projected lines that indicate a reference plane that substantially spans greater than 90 degrees from a reference point. 
   Implementations of the invention may include one or more of the following. The reference plane can span approximately 180 degrees or 360 degrees from the reference point. The projected lines can be formed such that they overlap or are adjacent to each other to form a continuous reference plane. The light beam can be collimated prior to being split. 
   In general, in still another aspect, the invention features a method of forming a reference plane, including forming at least two light beams, collimating each light beam, and converting each collimated light beam into an output line that is projected from a housing in different directions along the same plane to only form a single reference plane that substantially spans approximately 140 degrees from a reference point. 
   In general, in another aspect, the invention features a method of forming a reference plane, including forming five laser beams and converting each laser beam into an output line that is projected from a housing in different directions along the same plane to only form a single 360 degree reference plane. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1  is a laser-based tool and a schematic representation of five output beams. 
       FIG. 2A  shows a plan view of the laser-based tool and projected reference planes that are overlapped. 
       FIG. 2B  is a plan view of the laser-based tool and projected reference planes that are adjacent. 
       FIG. 3A  is a front view of the laser-based tool of  FIG. 1  tilted to one side. 
       FIG. 3B  is a side view of the laser-based tool of  FIG. 1  tilted to one side. 
       FIG. 4  is a schematic representation of a laser-based tool projecting a reference plane onto a wall surface. 
       FIG. 5  is a schematic side view of the laser-based tool of FIG.  1 . 
       FIG. 6  is shows a support frame of a laser-based tool. 
       FIG. 7  is an exploded view of the support frame of  FIG. 6 and a  view of a projection unit, of the laser-based tool. 
       FIG. 8  is a front view of the support frame and projection unit of FIG.  7 . 
       FIG. 9  is an exploded view of the projection unit of FIG.  7 . 
       FIGS. 10A-10C  are perspective, plan, and side views, respectively, of the paths of laser beams emitted from laser diodes of the laser-based tool. 
       FIG. 11  is a partially exploded view of a mounting apparatus for a rod lens of the laser-based tool. 
       FIG. 12  is a view of the projection unit of FIG.  7 . 
       FIG. 13A  shows a front view of a plate included in the mounting apparatus of  FIG. 11 , and shown in  FIGS. 13B and 13C . 
       FIG. 14  is a side view of the mounting apparatus of FIG.  11 . 
       FIG. 15  is a view of the projection unit of  FIG. 7 and a  portion of the support frame of FIG.  6 . 
       FIGS. 16A-16C  are plan, front and side views, respectively, of a laser-based tool. 
       FIGS. 17A-17C  are plan, front and side views, respectively, of a projection unit suspended from a portion of the support frame of the laser-based tool of  FIGS. 16A-16C . 
       FIGS. 18A-18B  are plan and perspective views, respectively, of a laser beam, a beam splitter and a projected reference plane. 
   

   Like reference symbols in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a laser alignment tool  100  is shown capable of producing a level 360 degree reference plane. The tool  100  includes a housing  102  having an upper portion  104  and a lower portion  1106 . The upper portion  104  of the housing includes windows  112  from which output beams  114  are projected. The beams  114  have a substantially planar arrangement with each other (i.e., the beams  114  all project on substantially the same plane) and form horizontal reference plane indicators. In one implementation, a reference plane indicator can be the image of a substantially straight line on the surface against which an output beam is projected. For example, the reference plane alignment indicators  116  shown in  FIG. 1 , which are substantially planar to one another, can be projected onto a surface such as a wall to form a horizontal line. 
   In the embodiment shown, the upper portion  102  of the housing includes a top  108  and side walls  110 . The side walls  110  of the upper portion  104  include the windows  112 . The lower portion  106  of the housing includes lower side walls  118 , a curved lower front wall  120  and a base  122 . However, other embodiments are possible, such as a housing including rounded walls or a cylindrically shaped housing. 
     FIG. 1A  shows a plan view of the laser alignment tool  100  and the projected output beams  114 . As shown in  FIG. 2A , the reference plane alignment indicators  116  of the projected output beams overlap to form a continuous reference plane that spans 360 degrees. Alternatively, the output beams can be projected such that the reference plane alignment indicators  116  are adjacent to each other to form a continuous reference plane that spans 360 degrees, as shown in FIG.  2 B. 
   In the embodiment shown, five output beams  114  are projected from the laser alignment tool  100  to produce a reference plane that spans 360 degrees. However, more generally, any number of output beams can be projected from the laser alignment tool to produce a reference plane that spans greater than 90 degrees. For example, two or more beams  114  can be used to produce corresponding horizontal reference plane indicators  116  that can be combined to indicate a reference plane that substantially spans greater than 90 degrees from a reference point as shown in FIG.  2 A. As another example, three or more beams  114  can be used to produce corresponding horizontal reference plane indicators  116  that substantially spans greater than 180 degrees from a reference point as shown in FIG.  2 B. 
   Referring again to  FIG. 1 , the tool  100  includes a projection system or unit for projecting the output beams  114  wherein a light source and all necessary optical components for projecting the output beams are mounted in a fixed relationship to each other, preferably in a single unit. A self-leveling feature includes pendulously suspending the projection unit from a support frame, for example, by a gimbal mount, or other methods of pendulous suspension, such as by a spring or wire flexures, as known in the art. The tool  100  is operated by setting the base  122  on a substantially level surface, however the self-leveling feature can allow for accurate level reference plane indicators when the tool  100  is inclined within a certain range from horizontal, referred to herein as the “accuracy range”. 
   Referring to  FIGS. 3A and 3B , the tool  100  is shown as tilted from side-to-side at an angle q from a horizontal plane  124  (FIG.  3 A), and tilted from front-to-back at an angle q from the horizontal plane  124  (FIG.  3 B). The accuracy range, meaning the range of angle q within which the tool  100  can provide accurate horizontal reference plane indicators, depends in part on the geometry of the tool  100 . In the embodiment shown, where the tool  100  is sized to fit comfortably within the palm of a human hand, the accuracy range is between approximately 0° and plus or minus a maximum inclination in the range of approximately 4 to 7 degrees from horizontal. However, in another embodiment having a larger housing, for example, the accuracy range can be approximately plus or minus 0 to 9 degrees from horizontal. 
   The tool  100  can include an error indicator that operates to notify a user when the tool  100  is inclined from horizontal beyond the accuracy range, thereby compromising the integrity of the levelness of the horizontal reference plane indicators  116 . The error indicator can include a cut-off mechanism that cuts off power to the projection system when the cut-off mechanism senses the housing is inclined beyond the accuracy range, ceasing projection of the output beams. An example of such a cut-off mechanism is described in U.S. Pat. No. 5,144,487, issued Sep. 1, 1992, for “Portable Laser Device for Alignment Tasks”, assigned to the assignee of the subject application, the entire contents of which are hereby incorporated by reference. In this manner, the user is notified that the tool  100  is inclined beyond the accuracy range, and the tool  100  is effectively disabled, thus preventing the inadvertent use of inaccurate reference plane indicators. A further example of a cut-off mechanism is described below in reference to FIG.  15 . In another embodiment, the error indicator can be a mechanism causing the output beams to blink. 
   The tool  100  can therefore be used to indicate a level 360 degree reference plane when placed on a surface with regard to the accuracy range. Referring to  FIG. 4 , a use for the tool  100  when inclined within the accuracy range is exemplified. In this example, a user requires reference plane indicators to be projected onto walls  130 ,  132  for any convenient purpose. The tool  100  can be placed on a table  134 , such that the base  122  is substantially perpendicular with the walls  130 ,  132  and output beams  114  are directed toward the walls, providing reference plane indicators  116  on the surface of the walls. A variety of other uses in the construction and carpentry trades, for example, are also imaginable, such as reference plane indicators projected onto a ground surface for leveling purposes. 
   In one embodiment, the tool  100  can include a user-friendly interface for interacting with a user. Referring to  FIG. 5 , a side wall  110  of the tool  100  is shown. The side wall has an inclined portion  138 , which includes a button  140  and a light  142 . In this embodiment, a user can operate the tool  100  as follows. Starting with the tool  100  powered down, the light  142  is not illuminated, indicating the tool  100  is in the OFF position. A first press of the button  140  powers up the tool  100  and projects two or more horizontal linear output beams, providing reference plane alignment indicators that span substantially greater than 90 degrees from a reference point, with the error indicator activated. The light  142  shines green, indicating that the power is ON, and that the error indicator is activated. 
   A second press of the button  140  powers down the tool  100 , and the light  142  does not shine, indicating the tool  100  is in the OFF position. The next press of the button  140  would start the above described cycle over again. 
   The user interface shown in  FIG. 5  is one embodiment; however, any other convenient arrangement can be used to interact with a user, such as multiple buttons, an LCD or the like. 
   In one embodiment, the error indicator can be embodied in an implementation of the tool  100  as follows. Referring to  FIGS. 6-9 , the tool  100  can include a support frame  200 , located within the tool housing  102 , from which a projection unit  202  can be pendulously suspended by a gimbal mount  204 . 
   Referring particularly to  FIG. 6 , the support frame  200  is shown without the projection unit  202 , which would be suspended centrally within the support frame  200 . The support frame  200  includes a generally ring-shaped, rigid portion  206  supporting, for example, five columns  208  and a base  210 . An upper portion  212  is attached to the five columns  208 , for example, by using screws, and provides a receiving surface for a gimbal mount  204  used to suspend the projection unit  202 . The ring-shaped portion  206  and upper portion  212  can be made from a metal, such as aluminum, that is sufficiently rigid, yet lightweight. Other metals can be used, for example, stainless steel. A soft, padded member  214  is positioned over the upper portion to absorb impaction the tool  100  typical to its use in the construction and carpentry trades. An energy absorbent stop  216  is also positioned within the ring-shaped member to limit motion of the projection unit  202  when suspended within the support frame  200 . The padded member  214  and stop  216  can be made of rubber, such as 40 durometer EPDM rubber. A magnet  218 , used in a damping sys tem described further below, is affixed to the interior of the base of the ring-shaped portion  206 . 
   Referring particularly to  FIGS. 7 and 9 , the projection unit  202  includes a central core portion  230  which houses five laser diodes  232 . The laser diodes can be selected to emit visible light having a wavelength between about 630 to 650 nanometers (nm). The laser diode may be a model DL-4038-31 available from Sanyo Semiconductor Corporation in San Diego, Calif., or a model HL6332G available from Hitachi Semiconductor (America) Inc. of San Jose, Calif., both of which emit visible light at a wavelength of 635 nm at a power of 10 milliwatts. 
   Five collimators  234  are fixedly mounted to the central core  230  in alignment with the five laser diodes  232 , respectively. In one implementation, each collimator can be a focus lens, such as model A375 available from Eastman Kodak Company of Rochester, N.Y., or an aspheric lens, such as model AC-210-T635 available from Anteryon of Eindhoven, The Netherlands. The five collimators  234  direct five collimated beams into five optical elements  236 . 
   In the embodiment shown in  FIG. 9 , the projection unit  202  houses  5  laser diodes  232  to emit light for producing a 360 degree reference plane. However, the housing can include any number of laser diodes for emitting light to produce a reference plane that substantially spans greater than 90 degrees from a reference point. For example, the projection unit can house two laser diodes for emitting light to produce a reference plane that spans greater than 140 degrees, as shown in FIG.  2 A. 
   Referring to  FIGS. 10A ,  10 B, and  10 C, an exemplary laser diode  232 , collimator  234  and optical element  236  are shown outside the context of the projection unit  202 . A diverging light beam  238  is emitted from the laser diode  232  and directed at the collimator  234 , which collimates the beam and from which the collimated beam is immediately incident on the optical element  236  producing a linear output beam  240 . In one implementation, the five optical elements  236  within the projection unit  202  are each a rod lens, such as a micro rod lens available from Edmund Industrial Optics of Barrington, N.J., or a 5 mm rod lens available from SOTA Precision Optics, Inc. of Orange, Calif. In other implementations, other types of optical elements can be used, for example, a cylindrical lens or a Kodak™ Hybrid LG-P9 available from Eastman Kodak Company of Rochester, N.Y. Passing a collimated beam through a rod or cylindrical lens has the effect of fanning the beam into a linear output beam. The fanned linear output beams  240  can project horizontal reference plane indicators. The brightness of the projected reference plane indicators can vary, depending in part on the strength of the light beam emitted from the laser diode. An implementation using the laser diodes described above can produce highly visible, bright reference plane indicators suitable for use both indoors and outdoors. 
   In one implementation, conventional techniques are applied to modulate (or pulse) the energy level of the laser diodes  232  to increase the peak power produced by the laser diodes. In one implementation, the laser energy in the horizontal reference planes is modulated at 8 kHz. By modulating the energy level of the laser diodes at 8 kHz, the laser alignment tool can be used in conjunction with receivers for detecting projected reference plane indicators in ambient light. For example, a receiver can include circuitry such that signals produced by ambient light and by noise energy are substantially reduced relative to the signal produced by the energy in the reference plane indicators. The energy level of the laser diodes can be modulated as described in U.S. Pat. No. 4,674,870, filed Oct. 18, 1985, and is incorporated herein by reference. 
   An important feature of a portable laser alignment device is to contain the device within a relatively small housing. The tool  100  is configured such that the tool  100  can fit comfortably within the palm of a human hand, and is sufficiently lightweight to allow a user to operate the tool while holding it in one hand, and conveniently carry it on a tool belt when not in use. The mounting devices used to mount the optical elements  236  to the projection unit  202  play an important role in allowing the tool  100  to be configured into such a small footprint. The position of an optical element in relation to a collimated beam incident on the optical element, and in relation to the central core  230  of the projection unit  202  is critical, in order to produce the image of a straight line free from distortions, such as bowing slightly in one direction resulting in a curved (a frown or smile) rather than straight line. Precisely positioning the optical element to produce a straight line requires a number of adjustments. 
     FIG. 11  shows a partially exploded view of a configuration of a mounting device  241  that can have a relatively small size, and that permits the required adjustments to precisely position an optical element to produce a straight line with relative ease, is shown. Referring to  FIGS. 11 and 12 , there are at least three adjustments required to position the optical element. For the purposes of illustrating the required adjustments, the optical element shown in  FIGS. 11 and 12  corresponds to the five optical elements  236  shown in  FIG. 7 , which in the implementation shown is a rod lens. 
   Although the rod lens  236  may be centrally aligned with the collimator  234 , an inherent error in the collimated beam, due to tolerances error of laser diode packaging, typically causes the beam to fall disproportionately on one side of an axis through the center of the rod lens  236 . In this example, the rod lens  236  is oriented vertically and will produce a fanned linear horizontal output beam. If a vertical axis were drawn down the center of the rod lens  236 , the collimated beam would be seen to fall disproportionately on one side or the other of the vertical axis. As a result, the projected linear output beam is unevenly distributed in relation to where a user is pointing the tool. That is, if a user points the tool at a particular point on a wall, for example, the projected linear output beam will be shorter to one side of the point than to the other side of the point. 
   The first adjustment to position the rod lens  236  requires laterally moving the rod lens  236  slightly to one side or the other to center the collimated beam, such that the beam is incident on the center of the rod lens  236 . Referring to  FIGS. 13A-13C , the rod lens  236  is mounted on a plate  242  having a central opening  248  for the rod lens  236 . The plate  242  includes a slotted first opening  244  on one side of the central opening  248 , and a substantially round second opening  246  on the opposite side of the central opening  248 . The rod lens  236  can be mounted to the plate using, for example, an adhesive. The first and second openings  244 ,  246  are configured to receive connectors  250 ,  251  ( FIG. 13B ) for connecting the plate  242  to a base  252 . The connectors can be, for instance, screws or pins. When the rod lens  236  is mounted within the plate  242  and the connectors  250 ,  251  are in place connecting the plate  242  to the base  252 , the plate can be laterally displaced slightly from side to side (as represented by arrow “A”) due to the slotted shape of the first opening  244 , allowing some swivel movement of the plate relative to the connector  251 . The slotted first opening  244  is configured to allow sufficient movement of the plate  242  to center the rod lens  236  in relation to the collimated beam incident on the rod lens  236 . 
   The second required adjustment is necessary to ensure the collimated beam is perpendicular to the surface of the rod lens  236  upon which the beam is incident, so that the linear output beam emitted from the rod lens  236  will fall in one plane, to eliminate curvature of the linear output beam. A spring  254 , shown in  FIG. 11 , is positioned between the plate  242  and the base  252 , when the connectors  250 ,  251  are in place connecting the plate  242  and the base  252 .  FIG. 141  shows a side view of the mounting device  241  with the plate  242  secured to the base  252  and the spring  254  positioned about the connector  250  between the plate  242  and base  252 . Because the spring  254  is between the plate  242  and base  252 , the plate  242  can be moved in the direction of arrow B, for example, by tightening or loosening the connector  250  if the connector is a screw. The rod lens  236  can thereby be positioned by adjusting the connector  250  to move the plate  242  until the collimated beam is perpendicularly incident on the surface of the rod lens  236 . In other embodiments the spring  254  can be other convenient means to allow for longitudinal displacement, such as a spring flexure or a threaded assembly. 
   The first and second adjustments described above are done once the mounting device  241  is itself mounted to the projection unit  202 . As shown in  FIG. 12 , the mounting device  241  is positioned about a support  258  fixedly attached to the central core  230  of the projection unit  202 . In one implementation, as shown, the support  258  is a cylinder sized to fit within the rounded interior surface  260  of the base  252 . The mounting device  241  can be held in place by a snug fit, and once properly positioned, can be held permanently in place by any convenient means, such as an adhesive or a solder. Before the mounting device  241  is positioned on the support  258 , the collimator  234  is positioned protruding from the support  258 , such that when the mounting device is in place, the collimator  234  is positioned between the laser  232  (and the support  258 ) and the rod lens  236 . 
   The third adjustment requires moving the mounting device  241  relative to the projection unit  202  on which it is mounted, to achieve true vertical of the mounting device relative to the projection unit  202 . 
   The mounting device  241  is made of a rigid material, for example, a lightweight metal such as aluminum, although a second material, such as brass, can be used for the plate  242 . Other suitable materials can also be used, such as stainless steel. 
   The projection unit  202  is designed such that when suspended from the gimbal mount  204  the projection unit  202  will balance so the linear output beams  240  are truly horizontal. In practice, manufacturing tolerances may be such that the projection unit  202  may not balance precisely as fabricated. Thus, it may be necessary to adjust the balance of the projection unit  202  after it has been assembled. 
   Referring to  FIG. 9 , showing an exploded view of the projection unit  202 , in one implementation the projection unit  202  is balanced by inserting and adjusting one or more screws, such as two brass set screws  217 ,  219 , in suitable openings or passage ways in the lower portion of the projection unit  202 . Weight may thus be added to or removed from the projection unit  202  by adding or removing the screws  217 ,  219  and thereby adjusting the balance of the projection unit  202 . 
   The projection unit  202  can include a damping system to limit pendular motion when the tool is set down on a surface, such as the damping system described in U.S. Pat. No. 5,144,487, previously incorporated herein by reference. Referring to  FIGS. 7 and 8 , such a damping system can include the magnet  218  secured to the inside of the base  210  of the support frame  200 , and a copper damping plate  264  attached to a shaft  266  protruding downwardly from the projection unit  202 . The damping plate  264  is formed and positioned such that a precise gap  270  is maintained at a predetermined width when the damping plate  264  is in motion over the magnet  218 . The gap  270  is sufficiently small such that motion of the damping plate  264  causes eddy currents to be generated in the plate  264 . Interaction of the eddy currents in the damping plate  264  with the magnetic field of the magnet  218  causes damping of pendular motion of the projection unit  202 . The damping force may depend on the mass and thickness of the magnet  218 , the dimension of the gap  270  and the thickness of the copper plate  264 . Preferably the magnet  218  is a neodymium magnet approximately three-quarters of an inch in diameter and a quarter of an inch thick. The damping plate  264  is can be three-quarters of an inch thick and have a maximum diameter of 0.625 inches at its widest point and a minimum diameter of 0.500 at its narrowest point. Other types of magnets may be used and other arrangements of one or more magnets may be used to accomplish the damping function. In addition, other shapes and/or materials can be used for the damping plate. Alternatively, the “plate” can be an integral portion of the projection unit. 
   The laser diodes  232  can be powered by a rechargeable battery located in a battery compartment within the housing  102 . The battery can be connected to the diodes  232  by a connector extending from the battery terminal. The connector can be an ultraflexible micro-miniature connector obtainable from New England Electric Wire Company of Lisbon, N.H. In one implementation, the connector is lead to the switch  140 . The connector is then lead through the center of the gimbal mount  204  and connected to the laser diodes  232 . Leading the connector through the gimbal mount  204  is one technique to ensure the connector has a negligible effect on the balance of the projection unit  202 . 
   Provision is made to prevent excessive motion of the projection unit  202  when the tool  100  is set down on a surface which is far from level. Referring to  FIGS. 6 and 7 , the energy absorbent stop  216  is positioned such that when the tool  100  is inclined at approximately plus or minus seven degrees (7°) or greater from the horizontal, the damping plate  264  contacts the stop  216 , thus limiting further relative movement between the projection unit  202  and the support frame  200 . 
   As discussed above, the tool  100  can include a cut-off switch that will cut off power to the laser diodes  232  when the housing  102  is inclined in any direction plus or minus a predetermined angle from horizontal. In the embodiment shown, the predetermined angle can be approximately plus or minus 7°, however, the angle can vary depending on the geometry of the tool  100 , and, as already discussed above, can be in a range of approximately plus or minus 4 to 9 degrees. 
   Referring to  FIG. 15 , a partially exploded view of a portion of the support frame  200  and the projection unit  202  and is shown. The cut-off mechanism can include a wire flexure  272  attached to and protruding above the upper portion of the projection unit  202 . The cut-off mechanism further includes a small metal plate  274  attached to the top of the support frame  200 . The plate  274  includes an aperture  276 . The aperture  276  is sized and positioned such that the wire flexure  272  projects through the center of the aperture  276  when the projection unit  202  is suspended from the support frame  200 . The wire flexure  272  carries a charge from the projection unit  202 , which is charged due to the rechargeable battery used to power the laser diodes  232 . When the housing  102  is inclined beyond the accuracy range, for example 7° from horizontal, the wire flexure  272  contacts the metal interior of the aperture  276 , which in essence behaves as a contact switch, indicating to the laser power drive that power to the laser diodes  232  should be cut-off which can be accomplished, in one example, by conventional digital logic. Other embodiments are possible using different cut-off switches, for example, an omni-directional mercury switch. 
   As discussed above, using the mounting device used to mount the optical elements to the projection unit, combined with the damping system, advantageously allows the tool  100  to be embodied within a relatively compact housing with a relatively low overall weight. Referring to  FIGS. 16A-16C , in one embodiment, the tool  100  can be dimensioned to have a height X of approximately 3.83 inches, a width Y of approximately 4.9 inches and a depth Z of approximately 4.6 inches. Referring to  FIGS. 17A-17C , the upper portion of the support frame  202  can have a width R of approximately 2.0 inches. The distance S between the top of the upper portion of the support frame  202  and the bottom of the damping plate  264  is approximately 2.8 inches, and the distance T from the midpoint of the gimbal mount  204  to the bottom of the damping plate  264  is approximately 2.52 inches. In this configuration, the tool  100  can have a tool weight of approximately 15.8 ounces (without batteries). 
     FIGS. 18A-18B  show a laser diode  280  for generating a beam and a beam splitter assembly  282 . In one implementation, the laser diode  280  and the beam splitter assembly are used within a housing like the housing  102  to generate reference plane indicators. The beam splitter assembly includes beam splitters  286  for generating light beams  288 . The light beams  288  are directed towards collimators (not shown) and optical elements  290  for producing output beams  292  and corresponding reference plane indicators  294 . As described above, the collimators (not shown) can each be a focus lens or an aspheric lens. Also, the optical elements  290  can each be a rod lens, a cylindrical lens, or a hybrid lens. 
   In the embodiment shown in  FIGS. 18A-18B , five reference plane indicators  294  form a 360 reference plane. Other embodiments are possible, for example, the laser diode  280  and the beam splitter assembly  282  can be used to project only two output beams which produce corresponding horizontal reference plane indicators  116  that substantially spans greater than 90 degrees from a reference point. As another example, the laser diode  280  and the beam splitter assembly  282  can be used to project three output beams which produce corresponding horizontal reference plane indicators  292  that substantially spans greater than 180 degrees from a reference point. Also, more than one laser diode can be used in conjunction with a beam splitter assembly to form a reference plane that spans substantially greater than 90 degrees from a reference point. The output beams  292  can be projected such that the reference plane alignment indicators  294  are adjacent or overlap each other to form a continuous reference plane that spans greater than 90 degrees from a reference point. 
   A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the tool  100  can include other self-leveling features such as the use of bubble vial levels incorporated into the housing  102  for manual leveling adjustments. The self-leveling feature can also include controlled servos and/or stepper motors for automatic or manual leveling adjustments. Accordingly, other embodiments are within the scope of the following claims.