Background light suppression for a laser projector

A laser projector steers an outgoing beam of light onto an object, passing light returned from the object through a focusing lens onto an aperture rigidly coupled to an optical detector.

BACKGROUND

The subject matter disclosed herein relates to a light projection system, often referred to as a “laser projection system,” and in particular to a light projection system that projects a glowing light pattern onto an object without requiring retroreflective or cooperative targets.

Light projection devices are used in a variety of applications to project images onto objects. In some applications, an illuminated three-dimensional (3D) pattern, also referred to as a “template,” is projected onto an object. The template may be formed, for example, by projecting a rapidly moving, vector-scan, light beam onto the object. In some systems, the projected light beam is a laser beam. The light beam strikes the surface of the object following a predetermined trajectory in a repetitive manner. When repetitively moved at a sufficiently high beam speed and refresh rate, the trace of the projected beam on the object appears to the human eye as a continuous glowing line. The projected pattern of light appears as the glowing template that can be used to assist in the positioning of parts, components and work pieces. In some cases, the projected template is based partly on computer aided design (CAD) data of the object.

A challenge faced by light projection devices is minimizing noise from unwanted scattered light or from background light in captured images while at the same time ensuring that the light projector operates properly from distances relatively near to the scanner to distances relatively far from the scanner. An additional related challenge is obtaining quality scanned images from black objects that return very little light to white objects that return high levels of light.

Accordingly, while existing systems and methods of patterned light projection are suitable for their intended purposes, the need for improvement remains, particularly in suppressing unwanted scattered light while retaining the ability to measure from near to far with high dynamic range.

BRIEF DESCRIPTION

According to one aspect of the disclosure, an apparatus comprises: a light source operable to emit a beam of outgoing light; a beam-steering system operable to steer the beam of outgoing light onto an object; a first focusing lens operable to receive light returned from the object; an aperture operable to receive light from the first focusing lens; and an optical detector rigidly coupled to the aperture.

According to yet another aspect of the disclosure, a method comprises: providing a light source, a beam-steering system, a first focusing lens, an aperture, and an optical detector rigidly coupled to the aperture; emitting a beam of outgoing light with the light source; steering the beam of outgoing light onto an object with the beam-steering system; receiving through the first focusing lens light returned from the object; and sending the returned light through the aperture and onto the optical detector.

DETAILED DESCRIPTION

Embodiments of the present invention provide improved operating range and operating consistency for a light projector device.

FIGS. 1A, 1B, 1Care isometric, front, and bottom views of a light projector10according to an embodiment of the present invention. In an embodiment, the light projector10includes a front cover20, a window25, a base housing30, a fan assembly40, and venting slots50. In an embodiment, a beam of light is sent out of and returned back through the window25.

In an embodiment, the light source assembly210includes a light source212and a mounting block214. In an embodiment, the light source212is a diode-pumped solid state laser (DPSS) that emits a round beam of green laser light having a wavelength of about 532 nm. In other embodiments, the light source212is a different type of laser such as a diode laser or is a non-laser source. In an embodiment, the fold mirror assemblies220A,220B include fold mirrors224A,224B, respectively, and adjustable mirror mounts222A,222B, respectively. In an embodiment, light from the light source reflects off the fold mirrors224A,224B and then travels through a beam expander230, which includes a beam expander lens234and a beam expander mount232. The expanded beam of light from the beam expander230travels through a collimating/focusing lens assembly240, which acts to focus the beam leaving the light projector10onto an object of interest. Because the light leaving the light projector10is relatively far from the light projector10, the beam of light is nearly collimated and converges relatively slowly to a focused spot. In an embodiment, the collimating/focusing lens assembly240includes a lens241, a lens mount242, and a motorized focusing stage243. The motorized focusing stage243adjusts the position of the lens241and lens mount242to focus the beam of light onto the object of interest. In an embodiment, the motorized focusing stage243includes a servomotor assembly244that drives a rotary actuator245attached to shaft246affixed to an attachment247. As the rotary actuator245rotates, it causes the lens mount242to be translated on a ball slide248.

In an embodiment, the beamsplitter assembly250includes entrance aperture251A, exit aperture251B, and beamsplitter252. In an embodiment, the beamsplitter252is a 50/50 beamsplitter, which is to say that the beamsplitter252transmits half and reflects half the incident optical power. Half of the light arriving at the beamsplitter assembly250from the collimating/focusing lens assembly240is reflected onto a beam absorber assembly255, which absorbs almost all the light, thereby keeping unwanted reflected light from passing back into the electro-optical plate assembly200. In an embodiment, the beam absorber assembly255includes a neutral density filter256, a felt absorber257, and a felt absorber258.

The two-axis beam-steering assembly260includes beam steering assemblies260A,260B. Each beam steering assembly260A,260B includes respectively a light weight mirror261A,261B, a mirror mount262A,262B, a motor263A,263B, a position detector264A,264B, and a mounting block265A,265B. The first mirror261A steers the beam of light to the second mirror261B, which steers the beam out of the window25to the object of interest. The beam-steering assembly260steers the beam in each of two orthogonal axes, sometimes referred to as x-y axes. In an embodiment, the beam-steering assembly260is provided steering directions to move the beam of light in a predetermined pattern by a processor312(FIG. 4). Light reflected or scattered off the object of interest retraces the outgoing path, striking first the mirror261B and then the mirror261A before passing through the exit aperture251B, and reflecting off the beamsplitter252. Beam steering assemblies such as260A,260B are also each referred to as galvanometers or galvos, which is an electromechanical device that works as an actuator that produces a rotary deflection, in this case of the mirrors261A,261B.

The mirror assembly270includes mount271and return mirror272. The focusing mirror assembly275includes focusing lens276and lens mount277. In an embodiment, light arriving at the return mirror272from the beamsplitter252passes through the focusing lens276. In an embodiment, the focusing lens276is a doublet. In an embodiment, an opaque cone280smoothly slides over lens mount277and attaches rigidly to adjustment stage285. The purpose of the opaque cone280is to block background light from within the light projector10from contaminating the light emitted by the light source210and reflected off the object of interest and passing through the lens276. Aperture assembly includes aperture291and aperture mount292. In an embodiment, the aperture assembly290is rigidly affixed to the optical detector assembly295by an interface element292. In an embodiment, the aperture assembly290is further rigidly coupled to the adjustment stage285. The adjustment stage285is adjusted in the x direction by an x adjuster286, in they direction by a y adjuster287, and in the z direction by a z adjuster288. The purpose of the adjustment stage285is to adjust the position of the aperture291and the optical detector assembly295in x, y, and z relative to the beam of light to enable the focused beam of light281to pass through the aperture for the object of interest located within the rated range of distances of the object being scanned with the light from the light projector10. The purpose of the aperture is to block unwanted background light, especially light scattered from within the enclosure of the laser projector10, for example, off the mirrors216A,216B, the beamsplitter252, the components of the beam block255, the return mirror272, and the focusing lens276. In addition, the aperture291helps to block unwanted background light from the environment outside the enclosure of the light projector10. Examples of such unwanted background light blocked by the aperture include artificial light and sunlight, both direct and reflected.

In an embodiment, the aperture291is a circular aperture. In an embodiment, the circular aperture has a diameter of 150 micrometers and a centering accuracy of +/−20 micrometers. A circular aperture is often referred to as a pinhole, and the element291may alternatively be referred to as an aperture or a pinhole. In other embodiments, the aperture is not circular but has another shape.

The optical detector assembly295receives light on an optical detector within the assembly295and produces an electrical signal in response. In an embodiment, the optical detector is a photomultiplier tube (PMT). In an embodiment, the PMT is includes a high-voltage supply circuit and a low-noise amplifier. In an embodiment, the amplifier is connected close to the PMT anode output pin to reduce the effect of external noise on the produced electrical signal. In an embodiment, the PMT is a Hamamatsu H11903 photosensor manufactured by Hamamatsu Photonics K.K., with headquarters in Shimokanzo, Japan. An advantage of a PMT for the present application includes high sensitivity to small optical powers and ability to measure both very weak optical signals and very strong optical signals. In an embodiment, the gain of the PMT can be adjusted by a factor of 100,000 or more according to the selected gain level, which is determined by the voltage applied to the PMT. This wide range of achievable gains enables the light projector to measure object regions ranging from dark black to bright white or shiny (i.e. highly reflective).

As explained herein above, the motorized focusing stage243adjusts the position of the lens241and lens mount242to focus the beam of light from the light projector10onto the object of interest. A method for determining the correct focusing position for the motorized stage243is now described with reference toFIG. 5A,FIG. 5B, andFIG. 5CandFIG. 6A,FIG. 6B, andFIG. 6C. In an embodiment, the motorized focusing stage243adjusts the position of the collimating/focusing lens assembly240to each of several positions, thereby producing scanning lines of different widths as illustrated inFIG. 5A,FIG. 5B, andFIG. 5C.FIG. 5Ashows the case in which the scanning line is adjusted to its minimum width, whileFIG. 5BandFIG. 5Cillustrate wider out-of-focus scanner lines produced by non-optimum focusing of the lens241by the motorized focusing stage243. In an embodiment, the desired focusing of the collimating/focusing lens assembly240is found by stepping the lens241to each of several positions. At each of those positions, the galvo mirrors261A,261B are used to steer the projected light along a line. An example is shown inFIG. 6A,FIG. 6B, andFIG. 6C, where the observed relative optical powers observed for each correspond to the levels of focus illustrated inFIG. 5A,FIG. 5B, andFIG. 5C, respectively. As can be seen inFIG. 5A-FIG. 5CandFIG. 6A-FIG. 6C, improved focus corresponds to relatively larger variations in the returned optical power as received by the optical detector assembly295as the beam is steered from point to point on the object of interest. Note that the average level of optical power in each ofFIG. 6A,FIG. 6B, andFIG. 6Cis the same, namely about 2.0 arbitrary units (au), in each ofFIG. 6A,FIG. 6B, andFIG. 6C. In contrast, the peak relative optical power observed inFIG. 6Ain around 5.0 au, around 3.0 au higher than the average value, while the peak relative optical power observed inFIG. 6Cis only around 2.5 au, which is only around 0.5 au higher than the average relative optical power. Without being bound to a particular theory, it is believed the reason for this change in relative optical power level is speckle, which is an effect in which laser light scattered off different portions of an object interfere constructively or destructively to produce the fluctuations in returned optical power. When a laser beam is focused, the relative change in the returned optical power is increased as the beam is swept along the object. In an embodiment, the motorized focusing stage243is adjusted until the maximum change in relative optical power is achieved in scanning a line. This ensures that the lens241has been adjusted to the position of optimal focus.

In an embodiment, a pre-scan is performed to determine the desired level of gain for a given scan region. For example, if a region is scanned with some elements in the region having a relatively high reflectance, for example because the elements are white, the gain of the PMT is set to a relatively low value since the optical power returned to the PMT is relatively high. On the other hand, if scanning is performed on a region containing only elements having relatively low reflectance, for example because the elements are black or dark, the gain of the PMT is set to a relatively high value. In an embodiment, a pre-scan is performed on a region to be measured as a way to obtain relatively high measurement sensitivity without saturating the PMT. In other words, the use of a pre-scan enables relatively dark objects to be measured even at relatively large distances from the light projector10. When a region includes both white or light objects as well as black or dark objects, in an embodiment, the region may be broken into sub-regions, with separate scans performed for at least some of the sub-regions.

The light from the light source212that leaves the light projector10travels to the object of interest and scatters off the object in a solid angle, afterwards retracing its path as it returns to the light projector10. After reflecting off the mirrors261B,261A, the solid angle of returning scattered light is limited in size by the exit aperture251B. The light then reflects off beam splitter252before passing through the lens276to form the focused light beam281. The direction of focused light beam281is determined by the path from a first point at which light from the light projector10strikes the object to a second point through the center of the entrance pupil of the lens276. In an embodiment, the aperture291is further aligned to the path that extends from the first point to the second point and into the optical detector assembly295. Furthermore, in an embodiment, the position of the aperture291as adjusted in the z direction to cause the beam waist of the returning beam of light to pass through the aperture291when the object is in the range of 5 to 7 meters from the light projector10. In an embodiment, the aperture291is large enough to pass nearly all of the return light through the exit aperture251B onto the active area of the optical detector at the range of 5 to 7 meters. In an embodiment, the light begins to clip slightly at larger distances such as 10 to 15 meters from the light projector10. At distances closer to the light projector10than 5 meters, the light will clip more significantly, but this is not usually a problem because the optical power scattered off an object point closer than 5 meters has larger scattered intensity than light scattered off an object point farther from the light projector10.

In an embodiment, the aperture291is rigidly affixed to the aperture assembly290, which in turn is rigidly affixed to the optical detector assembly295. In an embodiment, the optical detector assembly295and aperture assembly290are further aligned to ensure that returning light passing through the center of the entrance pupil of the lens276not only passes through the center of aperture291but also the center of the active area of the optical detector in the optical detector assembly295. As a result, the range of operation of the light projector10is made as large as possible. This is to say that the rigid attachment of the aperture291to the photodetector assembly295in combination with alignment of the aperture291, the photodetector assembly295, the lens276, and the exit aperture251B helps to ensure that the best sensitivity is obtained for objects both near to and far from the light projector10. With this alignment, the pre-scan is also expected to give consistent results in determining the PMT gain settings required for each combination of object distance and object reflectance.

FIG. 3is an isometric view of the electrical assembly300within the light projector10, andFIG. 4is an electrical block diagram for the light projector10. The electrical assembly300includes an electronics plate302and a number of circuit boards including a carrier board310, first galvo driver320A, second galvo driver320B, analog circuit340, multi-voltage power supply350, +24 volt power supply360A, and −24 volt power supply360B. The circuit block diagram representation for the electrical assembly300is shown inFIG. 4. The carrier board310includes a processor312that controls many functions within the light projector10. Control cables322A,322B run from the carrier board310to digital-to-analog converters (DACs)324A,324B on the first and second galvo driver boards320A,320B, respectively. Control signals sent from the carrier board310to the DACs324A,324B control the angles of the mirrors261A,261B, thereby controlling the direction to which the beam is steered. Power supplies360A,360B supply +24 volts, −24 volts, respectively, to the galvo drivers320A,320B, which in turn supply voltages to the galvo motor/position-sensing components328through cables326A,326B. In an embodiment, a jumper cable232is used to connect the first and second galvo driver boards320A,320B when synchronized steering is needed in two dimensions (such as X and Y directions).

The analog circuit board340includes an analog-to-digital converter (ADC)341. The ADC341receives an analog electrical signals from the optical detector295, which in an embodiment is a PMT. The ADC341converts the analog signals into digital electrical signals, which it sends over an Ethernet cable342to the carrier board310. The carrier board provides the digital data to the processor312and, in an embodiment, to an external computer attached to input/output (I/O) panel370through a USB cables313,314, an Ethernet cable315,316, and/or a wireless channel. In an embodiment, the processor312or external computer420constructs a gray-scale image of the optical powers received by optical detector295. Such an image may be displayed to a user, may be used to identify features in the scanned object, and may be used for other functions such as setting the position of the focusing lens241with the motorized focusing stage243. In an embodiment, the analog circuit board340receives voltages over the cable343from the multi-voltage power supply350. In an embodiment, the carrier board310further provides control signals to the motorized focusing stage243over the cable317and control signals to the light source212over the cable318. A connector316is attached to the circuit board to override the laser bypass circuit. In an embodiment, the carrier board310is further provided with a cable319operable to send a signal to reset the software on the carrier board. The carrier board310receives voltages over the cable311from the multi-voltage power supply350. In an embodiment, additional voltages are provided from the multi-voltage power supply350to the I/O panel370and to the fan assembly380.