Transparent material processing with an ultrashort pulse laser

Abstract

A method for scribing transparent materials uses ultrashort laser pulses to create multiple scribe features with a single pass of the laser beam across the material, with at least one of the scribe features being formed below the surface of the material. This enables clean breaking of transparent materials at a higher speed than conventional techniques.

Claims

1. A method of scribing a transparent material, comprising: using a single pass of a focused beam of ultrashort laser pulses to create a surface groove in said material and at least one modified region within the bulk of said material during motion of said transparent material relative to said focused laser beam, wherein an intensity of said focused beam produces non-linear absorption within a material region, and said ultrashort pulses are generated at a repetition rate sufficiently high such that spatial overlap between said pulses, and interaction between said laser pulses and a modified region, creates at least one additional region of material modification within the material, wherein said surface groove and said at least one modified region are each formed by interaction of said focused beam with said material, said surface groove in said material and said at least one modified region within the bulk being formed along a beam propagation direction, and wherein said surface groove and said at least one modified region are discontinuous and separated in depth.

2. The method according to claim 1, wherein a location of said surface groove and said at least one modified region are separated by a determined distance.

3. The method according to claim 1, wherein said transparent material comprises sapphire.

4. A method for scribing a transparent material, comprising: using a single pass of a focused beam of ultrashort laser pulses to create a plurality of modified regions within the bulk of said material during motion of said transparent material relative to the focused laser beam, wherein said method comprises focusing said beam within the bulk of said material such that an intensity of said focused beam produces non-linear absorption within said material and causes first material modification at a first location within said material in the beam propagation direction and second material modification at a second location at a greater distance along said beam propagation direction within said material, wherein each of said first and second material modifications are formed along said beam propagation direction as discontinuous regions separated in depth, wherein said beam of ultrashort pulses is generated at a repetition rate sufficiently high such that said first and second material modifications are formed with spatially overlapping pulses during said single pass.

5. The method according to claim 4, wherein said first location is at or near the surface of the material.

6. A transparent material scribed at two or more locations in a depth direction thereof with a single pass of a focused beam of ultrashort laser pulses, wherein said scribed locations comprise a surface groove scribed in said material and at least one scribe feature formed in the bulk of said material, wherein said surface groove and said at least one scribe feature are formed along said beam propagation direction, said surface groove and said at least one scribe feature being discontinuous and separated in depth, and wherein a location of said surface groove and said at least one scribe feature are separated by a determined distance.

7. A method of scribing a transparent material, comprising: irradiating said material with a beam of focused and spatially overlapping ultrashort laser pulses, in a single pass and during motion of said transparent material relative to said focused laser beam, to modify said material at two or more locations and produce scribe features in a depthwise direction thereof, wherein an intensity of said focused beam produces non-linear absorption within a material region, wherein at least a portion of said irradiating is carried out with a diffractive optical element that generates multiple regions of high optical intensity at different depthwise locations along a propagation axis of said focused laser beam, said diffractive optical element receiving an input beam and generating multiple laser modified discontinuous regions separated in depth.

8. The method of claim 7, wherein a surface groove is formed in said material, and at least one modified region is formed within the bulk of said material.

9. A method of scribing a transparent material, comprising: generating a zero-th order Bessel beam of laser light, and irradiating said material in a single pass with said beam during motion of said transparent material relative to the beam to modify said material at two or more locations in a depthwise direction thereof using multiple regions of high optical intensity generated by said beam at different depthwise locations within said material, wherein an intensity of said beam produces non-linear absorption within said material.

10. The method according to claim 9, wherein a surface groove is formed in said material, and at least one modified region is formed within the bulk of said material.

11. A system for scribing a transparent material, comprising: an ultrashort laser source to generate a beam of ultrashort laser pulses, wherein said beam of ultrashort pulses comprises a pulse duration in the range from 50 fs to 500 ps; an optical system to focus and deliver said beam of ultrashort laser pulses to said material with optical intensity sufficiently high so as to produce non-linear absorption within said material, and to modify said material at two or more locations in a depthwise direction thereof so as to produce scribe features; and a motion system to produce motion of said transparent material relative to said focused laser beam, said motion system operatively connected to said ultrashort laser source and said optical system, wherein said ultrashort laser source and said motion system are configured in such a way that said ultrashort pulses are generated at a repetition rate sufficiently high such that said-material modifications and scribe features are simultaneously formed with spatially overlapping pulses during said motion and in a single pass of said laser beam with respect to said material.

12. The system for scribing a transparent material according to claim 11, wherein said optical system produces a focused beam waist in the bulk of said material such that ablation or material modification occurs simultaneously at said beam waist position and at one or more material locations between said beam waist and said optical system.

13. The system for scribing a transparent material according to claim 11, wherein said one or more material locations are in the bulk of said material.

14. The system for scribing a transparent material according to claim 11, wherein said one or more material locations comprise a surface of said material, and said ablation or material modification forms a groove in said surface.

15. The system for scribing a transparent material according to claim 11, wherein an interaction between said laser pulses and a modified region creates at least one additional region of material modification within the material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be more clearly understood from the following description in conjunction with the accompanying drawings, wherein:

(2) FIG. 1 is a diagram of a system used in a method for scribing transparent materials according to one embodiment of the current invention, where (a) shows the system configuration, and (b) shows a detail view of the scribing and subsequent cleaving;

(3) FIG. 2 is an illustration of the surface and bulk scribe features that are generated by a focused Gaussian beam according to one embodiment of the current invention;

(4) FIG. 3 is a diagram of a system that uses an axicon lens to generate multiple sub-surface scribe lines according to one embodiment of the current invention;

(5) FIG. 4 is an intensity contour plot of a focused Gaussian astigmatic beam used in one embodiment of the current invention;

(6) FIG. 5 is an illustration of a diffractive optical element (DOE) used in one embodiment of the current invention;

(7) FIG. 6 is a diagram of a system used in a method for welding transparent materials according to one embodiment of the current invention, where (a) shows the system schematic, and (b) is an enlarged view showing the detail of beam focusing within the adjoining materials;

(8) FIG. 7 is an illustration of the welding process where a raised ridge is used to fill the gap between two pieces. (a) shows the gap, (b) shows the ridge formed by focusing the laser beam slightly below the surface of the lower piece, and (c) show the weld formed when the laser focus is moved up to the interface between the raised ridge and the upper piece to be bonded.

(9) FIGS. 8-10 show illustrations of the sub-surface marking, wherein an arrow mark has been used as an example of the markings possible according to the invention.

(10) FIG. 11 is an optical micrograph showing experimental results of one embodiment of the current invention;

(11) FIG. 12 is an image sequence showing a fused silica weld according to one embodiment of the current invention. (a) shows the fused silica before breaking apart the weld, (b) shows the bottom surface of the fused silica after breaking apart the weld, and (c) shows the top surface of the fused silica after breaking apart the weld.

(12) FIGS. 13-15 are photos of a glass marked sample made according to the present invention; and.

(13) FIGS. 16a and 16b are photos of a prior art decorative article made by laser marking using a long-pulse laser, and an individual mark thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(14) 1. Ultrashort Pulse Laser Scribing

(15) FIG. 1 illustrates one embodiment of the current invention, which is a method for scribing transparent materials for subsequent cleaving. This embodiment employs a laser system (1) producing a beam of ultrashort laser pulses (2), an optical system (6) that generates a desired laser beam intensity distribution, and a target material (7) to be scribed that is transparent to the wavelength of the laser pulses. In addition, a Z-axis stage (8) is used for beam focus position control (depth), and an automated X-Y axis stage assembly (9) is generally required for moving the work pieces (7) laterally relative to the focused laser beam. Alternatively, the laser beam (2) could be moved relative to a stationary target material with the use of scanning mirrors (3), (4), and (5).

(16) The laser beam (2) is directed through the optical system (6), which transforms the laser beam (2) to create a desired 3-dimensional intensity distribution. Particular regions of the transformed laser beam have sufficient intensity to cause ablation and/or modification of the target material via nonlinear absorption processes. Material ablation generally refers to the vaporization of material from intense laser radiation. Material modification more broadly refers to a change in the physical and/or chemical structure of the irradiated material, which can affect the propagation of a crack through the material. Laser modification generally requires lower optical intensities than laser ablation for a particular material.

(17) The transformed beam is directed toward the target transparent material (7) to cause ablation/modification of the material (7) at multiple determined locations, within and/or on the surface, of the material (7). The ablated and/or modified regions are generally located in the material (7) along the optical propagation axis and are separated within the material (7) by a determined distance. The transformed beam and the target material (7) are moved relative to each other, resulting in the simultaneous generation of multiple laser-scribe features in the material (7). The multiple scribe features allow for cleaving of the material (7) with the application of suitable force(s) (See FIG. 1(b)).

(18) FIG. 2 illustrates another embodiment of the current invention, in which a laser beam (10) having a Gaussian spatial intensity distribution is focused to create sufficient intensity for nonlinear absorption and subsequent ablation or modification of the target material (11). The region of tightest focus is positioned below the material's surface to a chosen location within the bulk of the material (11). In addition, by employing suitable focusing optics and laser pulse energy, a region of intensity sufficient to cause material ablation is at the same time generated on or near the surface of the material (11).

(19) The important aspect is that the pulse energy and focusing geometry are chosen such that there is sufficient intensity to simultaneously cause ablation or modification not only within the bulk of the material (where the focused beam waist is positioned), but also at another point on the optical propagation axis prior to the beam waist (12) (either in the bulk or on the surface of the material). When the laser pulses encounter the target material (11), their high-intensity region (near the center of the radial Gaussian intensity distribution) is absorbed nonlinearly by the material and ablation or modification occurs. The outer spatial regions of the laser beam (outer edges of the Gaussian intensity distribution), however, are too low in intensity to be absorbed by the material, and continue to propagate to the beam waist, located further within the bulk of the material. At the beam waist location, the beam diameter is small enough to once again generate sufficient intensity for nonlinear absorption and subsequent laser modification to occur in the bulk of the material.

(20) A region directly below the surface ablation may also be modified by diffraction and constructive interference of succeeding pulses after the initial surface feature is created (spot of Arago). A relatively high repetition rate laser source better enables this process at reasonable translation speeds.

(21) Under these focusing and pulse energy conditions, translation of the material (11) relative to the laser beam (10) results in the simultaneous generation of multiple laser-modified regions (i.e. a surface groove (13) and one or more bulk modified regions (14), or two or more bulk modified regions), which together allow for precise cleaving of the material.

(22) FIG. 3 illustrates another embodiment of the current invention, in which an axicon (cone-shaped) lens (20) is used to generate the multiple internal scribe lines (21). When illuminated with a laser beam (22), the axicon lens (20) creates what is known as a 0th-order Bessel beam. The name arises from the fact that the mathematical description of the optical intensity distribution in the plane normal to the axis of propagation is defined by the 0th-order Bessel function, with the radial position from the beam center being the independent variable. This beam has the unique property of containing a high-intensity central beam spot (23) that can propagate with the same small size for much larger distances than for the case of a similarly-sized beam spot generated by conventional focusing methods (i.e. much longer than the Rayleigh range of a conventionally-focused beam). The central intensity field is surrounded by a plurality of concentric rings of light (not shown), the intensity of which decreases with increasing radius. Due to an inward radial component of their propagation vector, these rings of light continually reconstruct the small, central beam spot (25) as the Bessel beam propagates. Therefore, a small, high intensity central beam spot (23) can be generated that maintains its small diameter through the entire thickness of a target material (24). Because of the extended range of tight beam focusing, the Bessel beam is also commonly referred to as a non-diffracting beam.

(23) Because the outer rings reconstruct the intense center spot (23), if the center spot (23) is intense enough to cause ablation of the material at the surface (26), the rings (which have a larger diameter than the ablated region) will converge to the center of the beam a short distance later, causing reconstruction of the intense center beam spot, at which point ablation or material modification can occur again. With proper optical system design and sufficient pulse energy, this process of ablation and subsequent beam reconstruction can repeat through the entire bulk of the transparent material (24). Other optical components, such as graded-index lenses and diffractive optical elements, can also be used to generate Bessel beams.

(24) In additional embodiments of this invention, alternative beam intensity transforming techniques, well known to those skilled in the art, are employed in the optical system of the invention to tailor the beam intensity to generate multiple scribe lines in the target material. One such method utilizes astigmatic beam focusing to create two distinct regions of high optical intensity, separated by a determined distance. FIG. 4 displays an intensity distribution plot of a focused astigmatic Gaussian beam, in which the focal planes of the X and Y axes are separated by a distance of 20 m. Note the presence of two distinct high intensity regions (distinguished by the constant-intensity contour lines). When directed at the target material, these two regions can be used to create multiple laser scribe features.

(25) Another method for generating multiple scribe features in a transparent material employs a diffractive optical element (DOE) that is designed to generate multiple regions of high optical intensity at different locations along the beam propagation axis. FIG. 5 illustrates how such a DOE could function. These multiple intense regions, when directed at the target material, generate multiple scribe features for subsequent cleaving of the material.

(26) For a variety of beam-focusing and/or intensity-mapping methods used to generate multi-scribe ablation features, additional optical components could be introduced to generate an elliptical component to the overall beam shape. By orienting the elliptical beam such that the long axis is parallel to the direction of beam scanning, higher scanning speeds can be achieved. Higher scanning speeds can be achieved because the elliptical beam shape allows for sufficient pulse-to-pulse overlap for the machining of smooth and continuous scribe features (as opposed to dotted scribe features resulting from spatially-separated pulses ablating the material). While increased pulse overlap, and higher scanning speeds, could also be achieved with a larger circular beam spot, this would at the same time result in a wider scribe feature width, which is often undesirable.

(27) 2. Ultrashort Pulse Laser Welding

(28) Another embodiment of the current invention relates to a process for laser-welding of transparent materials. As shown in FIG. 6, this embodiment requires the use of a laser system (50) producing a beam of ultrashort laser pulses (51) at a high repetition rate; a focusing element (55) (e.g. lens, microscope objective) of sufficient focusing power; and at least two materials (56) and (57) to be joined together, at least one of which is transparent to the wavelength of the laser. In addition, a beam focus positioning stage (58) is used to adjust the focus position of the laser beam (51), and an automated motion stage assembly (59) is generally required for moving the work pieces (56) and (57) relative to the focused laser beam.

(29) In this embodiment, the two materials (top piece (56) and bottom piece (57)) to be laser-welded are placed in contact with each other to create an interface with little or no gap between their surfaces; a clamping force may or may not be applied to the two pieces. A lens (55) is then positioned in the path of the laser beam to create a focal region of high intensity laser radiation. The two transparent materials (56) and (57) are positioned relative to the focused laser beam such that the beam focal region spans the interface of the top piece (56) and the bottom piece (57). With sufficient laser intensity, welding of the material interface will occur. By moving the transparent materials (56) and (57) relative to the beam focal region, while at the same time keeping the interface of the materials (56) and (57) in close proximity to the beam focal region, a determined length of laser welding can be achieved. In a particularly unique application of this embodiment, the materials (56) and (57) could be positioned such that the focused laser beam travels through the top (transparent) piece (56) and forms the focal region near to the interface of the top piece (56) and the bottom piece (57), resulting in welding of the two materials.

(30) Unlike other laser welding processes, the process of the invention welds by utilizing primarily nonlinear absorption rather than linear absorption. Because of this, there are unique properties in this welding process. The nonlinear absorption is very intensity dependent so the process can be limited to the focus of the laser beam. Thus the absorption can be made to occur only deep within a transparent material around the focus. Typically nonlinear absorption by ultrashort pulses leads to plasma formation and very little (if any) heat deposition, thus ablation with ultrafast lasers leads to a very small heat affected zone (HAZ). However, by keeping the intensity low enough so ablation does not occur but high enough for nonlinear absorption to occur, some heat is deposited. If the repetition rate of the laser is increased sufficiently then the heat can be accumulated sufficiently in the material to lead to melting.

(31) The laser system (50) emits an approximately collimated laser beam (51) of pulses having a pulse duration in the range of about 200-500 fs and a wavelength of about 1045 nm at a pulse repetition rate between 100 kHz and 5 MHz. The first beam steering mirror (52) directs the laser beam to the power adjust assembly (53), which is used to adjust the pulse energy that is used for the welding process; specific methods for achieving such attenuation are well known to those skilled in the art. The second beam-steering mirror (54) directs the beam onto the beam focusing objective (55). The beam focusing objective (55) focuses the laser pulses to achieve the appropriate fluence (energy/unit area) for the process, which has a maximal value at approximately a distance (F) from the beam focusing objective (55). The beam focus positioning stage (58) translates the beam-focusing objective (55) such that this maximal fluence region is located at the interface of the target materials (56) and (57). The XY stage assembly (59) moves the target materials (56) and (57) relative to the focused beam so as to provide for the ability to generate a linear weld feature, or an array of circular weld features, at the interface of the target materials (56) and (57).

(32) FIG. 7 shows another embodiment of this invention where welding is desired between two pieces separated by a small gap (60). First, the laser beam (51) is focused below the surface of the bottom piece (57). With the proper control of the pulse energy and focusing conditions, a raised ridge (61) is formed as the sample is translated relative to the beam focus (or as the beam is moved relative to the target). This raised ridge (61) bridges the gap between the top and bottom targets. A second pass of the laser with the beam focus raised to the height near the interface between the top of the raised ridge (61) and the top piece (56) then forms the weld (62).

(33) 3. Visible/Invisible Laser Marks

(34) The same system shown in FIG. 1a can be used to make sub-surface marks in transparent materials where the applied laser beam is focused below the surface of the transparent material substrate

(35) FIG. 8 shows an illustration of the top-view of an arrow pattern (63) written in a transparent material (64) such as glass. A light source (65) injects light into an optical waveguide (66) that delivers the light to the arrow mark (63) to illuminate the pattern. The output numerical aperture of the optical waveguide should be properly designed to efficiently illuminate the desired source. Multiple optical waveguides can be used to illuminate different regions of a pattern. Controlling the timing of the different illuminating light sources can produce different decorative and signaling effects. Alternatively, the pattern can be illuminated directly from a properly focused light source, rather than using an optical waveguide.

(36) FIG. 9(a) shows an illustration of a close-up of the top-view of the arrow mark (63) that is made up of parallel lines, all perpendicular to the direction of the illumination light. These parallel lines are generated by tightly focusing the laser light within the target substrate to create regions of material modification. FIG. 9(b) shows an illustration of the side-view of the arrow mark (63). The arrow mark is composed of a group of marks at different depths. These marks scatter the light delivered by the optical waveguide (66) towards the viewer (67). The brightness can be controlled by the intensity of the illuminating light, the size of the individual marks and the density of the marks.

(37) FIG. 10 shows an illustration of the concept where the pattern is composed of a pixels (68) and where each pixel is made up of a group of parallel lines at different depths (69) formed by tightly focusing the laser light to modify the substrate material.

Experimentally Demonstrated Results

(38) 1. Ultrashort Pulse Laser Scribing

(39) As shown in FIG. 11, with a single pass of the laser beam, a pair of scribe lines (a surface groove (70) and a sub-surface scribe feature (71)) were simultaneously machined in a 100-m thick sapphire wafer using a 20 aspheric focusing objective (8-mm focal length). The cleave facet exhibits good quality. The scanning speed was 40 mm/s (not optimized).

(40) For the case of a surface scribe line only, using the same laser pulse energy and repetition rate, and under identical processing conditions (ambient atmosphere environment, etc.), the fastest scribing speed which resulted in good cleaving of the material was 20 mm/s.

(41) 2. Ultrashort Pulse Laser Welding

(42) After a number of laser pulses are absorbed within a particular region of the materials to be welded, heating, melting and mixing of the materials occurs and, upon cooling, the separate materials are fused together. The number of pulses required to weld the materials together depends on other process variables (laser energy, pulse repetition rate, focusing geometry, etc.), as well as the physical properties of the materials. For example, materials with a combination of high thermal conductivity and high melting temperature require higher pulse repetition rates and lower translation speeds to achieve sufficient thermal accumulation within the irradiated volume for welding to occur.

(43) A. Polycarbonate Welding

(44) Experiments with a high-repetition rate, femtosecond pulse laser source operating at a pulse repetition rate of 200 kHz and having a wavelength of 1045 nm have resulted in the laser-joining of two optically-transparent materials. In particular, 2 J laser pulses were focused with a 100 mm focal length lens through the top surface of a -thick piece of transparent polycarbonate, and onto its bottom surface interface with the top surface of a similarly-sized piece of transparent polycarbonate. The polycarbonate pieces were translated linearly and in a plane perpendicular to the direction of laser propagation, maintaining positioning of the beam focal region near-to the interface of the materials. The two pieces were fused together at the laser-irradiated interface and significant force was required to break them free from one another.

(45) B. Fused Silica Welding

(46) A 200-m thick fused silica plate was welded to a 1-mm thick fused silica plate using a 40 aspheric lens and a laser repetition rate of 5 MHz. The 1/e.sup.2 beam diameter of the laser was 3.6 mm and the aspheric lens focal length was 4.5 mm, resulting in an operating NA (numerical aperture) of 0.37. FIG. 12 shows a weld feature in fused silica, with images taken both before and after breaking the two silica plates apart. The first image (a) shows the intact weld feature exhibiting regions of smooth melted glass, and the subsequent images (b) and (c) show the two glass surfaces after the weld was fractured, exhibiting facets of fractured glass.

(47) Welding speeds ranged from 0.1 to 1.0 mm/s, though speeds greater than 5 mm/s are possible, and the maximum speed could be increased with an increased pulse repetition rate. The nominal fluence range for the process is 5-15 J/cm.sup.2 and the nominal pulse duration range is 10-1000 fs. Within these fluence and pulse duration ranges, the nominal pulse repetition rate range is 1-50 MHz. With rigorous process optimization, these ranges may be extended to 1-100 J/cm.sup.2, 1 fs-500 ps, and 100 kHz-100 MHz for the fluence, pulse duration, and repetition rate, respectively. The high repetition rate is required for sufficient thermal accumulation for the onset of melting in the fused silica.

(48) With the availability of higher energy pulses at similar repetition rates, looser focusing is possible to generate a larger focal volume with the required fluence. The size and shape of this welding focal volume can be tailored based on the region to be welded.

(49) 3. Visible/Invisible Laser Marks

(50) FIG. 13 shows a glass sample with the arrow mark illuminated by a green light source from the side. Here, the arrow pattern is clearly visible. The illustrations in FIGS. 8 and 9 show the details of the arrow pattern, where lines at different depths, perpendicular to the illuminating light source (green light in this case) were generated by tightly focusing the laser light.

(51) FIG. 14 shows the same glass sample with the illuminating light source off. Clearly, the arrow pattern cannot be seen.

(52) FIG. 15 shows a microscope photo of an individual pixel that is used to define the arrow mark in FIG. 13. FIG. 16(a) shows a photo of a decorative pattern inside glass and FIG. 16(b) shows a microscope image of an individual mark.

(53) The mark in FIG. 16(b) is approximately 200 m in size and very rough, composed of several distinct cracks radiating from the center. The pixel in FIG. 15 is made up of a series of parallel lines, each line is roughly 10 m wide and 250 m long. The line spacing is 50 m. The difference in size and smoothness difference between the features in FIGS. 15 and 16(b) explains why the glass sculpture in FIG. 16(a) is clearly visible in most lighting conditions while the arrow in FIGS. 13 and 14 requires side illumination to be visible. The size and smoothness of the generated feature is controlled by the pulse energy, pulse duration, wavelength of the laser and the translation speed of the beam through the target. The optimal parameters depend on the particular target material. The visibility of the pixel in FIG. 15 can be controlled by controlling the width and length of each line in the pixel and the line density within the pixel as well as the smoothness.

(54) Thus, one method for generating visible patterns of laser-modified features below the surface of the transparent material proceeds by first forming a plurality of lines at different depths within the material using a tightly focused ultrafast pulse laser, while controlling the roughness of the lines by controlling parameters of said laser as described. The lines are then illuminated using light propagating or directed generally perpendicular to the lines. The patterns formed in this way are clearly visible to the unaided eye when illuminated from the perpendicular direction, although they are substantially invisible to the unaided eye when not illuminated; i.e., under normal ambient light conditions as in FIG. 14. The illumination is conducted by directing a focused light source upon the lines or by directing the light to the lines via an optical waveguide with an output numerical aperture selected to efficiently illuminate the pattern.

(55) Different ones of said lines, for example the lines of different pixels, can be at defined angles relative to one another, and can be illuminated separately or simultaneously by arranging multiple light sources so that they each direct light generally perpendicular to a subset of said lines.

(56) Thus, the invention provides a transparent material having patterns of sub-surface markings formed by a laser, e.g. an ultrafast pulse laser, where the markings are formed of lines at different depths within the material, with the lines being substantially visible to the unaided eye only when illuminated with a light source directed generally perpendicular to the lines.