EYEGLASS LENS WITH REDUCED FRICTION PROPERTIES, MEASURING METHOD FOR DETERMINING THE SMOOTHNESS OF THE EYEGLASS LENS AND METHOD FOR MANUFACTURING THE EYEGLASS LENS

Description

[0001] The present invention relates to a measuring method for determining the smoothness of a spectacle lens.

[0002] In the prior art, spectacle lenses are known to be coated with multiple different layers. For example, a hard coating protects the lens from scratches. Reflection reduction can be achieved with an interference / anti-reflective coating. Finally, a functional coating, sometimes also called a topcoat or cleancoat, serves to repel dirt and water droplets. Starting from the surface of the spectacle lens, a typical layer structure consists of a hard coating, an interference / anti-reflective coating, and a functional coating. The latter thus represents the outermost coating layer. Each of the aforementioned coating layers can consist of a single layer or a multi-layer system.

[0003] To repel dirt and water droplets, functional coatings typically possess both oleophobic and hydrophobic properties. A measure of this is the contact angle formed by a liquid on the lens surface. Generally, the larger the contact angle, the more pronounced the oleophobic and hydrophobic properties. While lenses with functional coatings are easy to clean of dirt and water droplets that would otherwise adhere to the lens surface, they still need to be cleaned periodically, for example, by wiping with a microfiber cloth.If the lens has been previously exposed to running water, a similar wiping tool is also needed to dry it. In this context, cleaning also includes such drying.

[0004] Generally, gliding a cleaning tool across the surface of a lens requires a certain amount of force, which increases with speed. If the force applied is too weak, the cleaning tool will stick to the lens. This can cause your fingers to slip, hindering the cleaning process. As a result, the lens surface won't feel sufficiently smooth after cleaning.

[0005] Under ideal conditions, optimal cleaning results are achieved with a single wipe of a clean microfiber cloth across the lens surface, also known as "one wipe clean." However, most eyeglass wearers perform several alternating cleaning strokes across the width of the lens, i.e., alternating from left to right and then from right to left. In this case, a lens whose surface doesn't feel sufficiently smooth is perceived as particularly bothersome.

[0006] The prior art approaches to coating spectacle lenses with a functional coating typically aim to improve their oleophobic and hydrophobic properties, thereby making the lens surface even more resistant to adhesions such as dirt and water droplets. Accordingly, the focus of lens coatings to date has been on improving the dirt- and water-repellent properties of the lens, rather than on achieving particularly smooth wiping with minimal effort.

[0007] In this context, EP 2 078 765 A1 discloses a vapor deposition material, a method for producing an optical element or a plastic lens for spectacles using it, and a plastic lens for spectacles.

[0008] Furthermore, a soft contact lens with excellent wearing comfort on the eyes is known from US 2005 / 179862 A1.

[0009] The present invention is therefore based on the objective of providing a measuring method for determining the smoothness of a spectacle lens. The spectacle lens, whose smoothness is to be determined, can be coated with a functional coating, so that, in addition to repelling dirt and water droplets, particularly easy wiping with minimal effort is possible.

[0010] The foregoing problem is solved by a measuring method having the features of claim 1. Preferred embodiments are disclosed in the dependent claims.

[0011] The present invention relates to a measuring method for determining the smoothness of a spectacle lens using a rheometer by determining dynamic coefficients of friction against a microfiber cloth which describes a circular path around the center of the spectacle lens, wherein the determination of dynamic coefficients of friction is carried out at speeds in the range of 0.005 m / s to 0.4 m / s tangentially to the described circular path and tangentially to the surface of the spectacle lens, characterized in that the smoothness of the spectacle lens is expressed by a haptic smoothness coefficient HGK and the method comprises: Wiping with a microfiber cloth in a rotating motion around the center of the spectacle lens, the wiping being performed with the aid of a rotor lowered onto the convex side of the spectacle lens, the pressure of the microfiber cloth on the surface of the spectacle lens to be measured being effected by an elastic stamp with an annular contact surface, and the microfiber cloth moving tangentially to the outer radius of the annular contact surface relative to the surface of the spectacle lens at a speed in the range of 0.005 m / s to 0.4 m / s, determining the torque during the rotating movement of the microfiber cloth using a rheometer, determining the dynamic coefficient of friction based on the previously determined torque, repeating the above steps at a different speed in the range of 0.005 m / s to 0.4 m / s tangentially to the outer radius of the annular contact surface.and calculating the haptic smoothness coefficient HGK from at least two dynamic friction coefficients determined at different tangential speeds.

[0012] Lenses with a functional coating that gives them a haptic smoothness coefficient (HGK) of at most 0.28, preferably at most 0.25, and / or a haptic smoothness coefficient (R_HGK) of at most 0.85, preferably at most 0.80, as described in detail below, allow for particularly smooth wiping with minimal effort. Furthermore, the required effort increases only slightly with increasing wiping speed. Thus, the surface of such a lens feels exceptionally smooth when wiping, which is perceived as very pleasant to the touch.

[0013] The dynamic coefficient of friction can be understood as a measure of the perceived smoothness of the lens surface. It refers to the frictional force that must be overcome to move a wiping object across the lens surface, such as when cleaning the lens with a microfiber cloth or similar item.

[0014] Fundamentally, the dynamic coefficient of friction depends on the speed of the wiping motion. It is therefore sometimes also referred to as the speed-dependent dynamic coefficient of friction. Essentially, this means that the dynamic coefficient of friction would have to be continuously measured up to the maximum speed of the wiping motion in order to accurately represent the perceived smoothness. However, as the inventors have discovered, the measurement of perceived smoothness can be greatly simplified, as explained below in connection with the measurement method according to the invention.

[0015] Within the scope of the present invention, the movement used to measure the dynamic coefficient of friction describes a circular path around the center of the spectacle lens. The velocity to which the dynamic coefficient of friction refers is tangential to the described circular path and tangential to the surface of the spectacle lens. It is therefore sometimes also referred to as the tangential velocity. A range of 0.005 m / s to 0.4 m / s is considered practically relevant for this velocity, since cleaning a spectacle lens typically occurs at a speed within this range. As measurements have shown, the maximum speed of the wiping motion is typically around 0.2 m / s.

[0016] In determining the dynamic coefficient of friction, a microfiber cloth of the type "Savina MX", manufactured by "kbSeiren", Osaka, Japan, with the following properties is used: • Fiber type: ultrafine microfiber "BelimaX" • Composition: 70% polyester, 30% nylon • Thickness: 0.32 mm • Area-related mass: 165 g / m² • Length-related number of longitudinal threads: 78 / 2.54 cm • Length-related number of warp threads: 8 1 / 2.54 cm

[0017] Here, a microfiber cloth is used to wipe the lens with its functional coating in a rotating motion around its center. Before wiping, it is essential to ensure that no abrasive action has been performed on the lens surface beforehand, as this could alter its properties. The pressure of the microfiber cloth on the lens surface being measured is applied by an elastic stamp with an annular contact surface and is approximately 8380 N / m². With an inner radius of 17.5 mm and an outer radius of 20.5 mm for the annular contact surface, this corresponds to a contact force of 3 N. In cross-section, the elastic stamp has a rectangular shape and a height of 3 mm. The elastic stamp is made of silicone and has a Shore A hardness of 40.

[0018] Within the area of ​​the annular contact surface of the elastic plunger, the microfiber cloth moves relative to the surface of the spectacle lens. To account for the velocity dependence of the dynamic coefficient of friction, it is expediently determined within the aforementioned velocity range of 0.005 m / s to 0.4 m / s, where, as mentioned above, the velocity refers to the tangential velocity. Starting from an initial value, the velocity is increased incrementally.

[0019] During the rotating movement of the microfiber cloth, the torque is determined using a rheometer. This torque is related to the frictional force that the microfiber cloth must overcome for uniform movement relative to the surface of the spectacle lens via the effective radius of the annular contact surface of the elastic piston, as follows: Drehmoment = Reibungskraft × effektiver Radius

[0020] The effective radius can be approximated using the mean radius. This corresponds to the average radius of the annular contact surface of the elastic piston and can be calculated as follows: mittlerer Radius = Außenradius + Innenradius 2

[0021] The dynamic coefficient of friction, abbreviated as µ d , is related to the torque via the effective radius and the contact force as follows: μ d = Drehmoment effektiver Radius × Andruckkraft = Reibungskraft Andruckkraft

[0022] For each speed v, an individual torque(v) results, caused by an individual frictional force(v), and thus also an individual dynamic coefficient of friction(v), abbreviated as µ d (v): μ d v = Drehmoment v effektiver Radius × Andruckkraft = Reibungskraft v Andruckkraft

[0023] To accurately measure the perceived smoothness of a lens, it is crucial to understand how the velocity v is distributed along the path s across the lens surface during a cleaning stroke over time t. Investigations using a high-frame-rate camera show that the velocity distribution along the path traveled by the wiping object during a cleaning stroke can be very closely approximated by a harmonic oscillation, with the object coming to a standstill at the reversal points. Due to the symmetry of the wiping motion as a harmonic oscillation, it is sufficient to consider the velocity distribution for only a quarter of a cleaning stroke, i.e., from the center of the lens at position s = 0 at time t = 0, where the wiping motion reaches its maximum velocity vmax, to the reversal point at position s = smax at time t = T / 4, where the wiping motion comes to a standstill.Here, T denotes the duration of a complete cleaning stroke, corresponding to one full oscillation period. The following relationship applies to the distance traveled by a quarter of a cleaning stroke as a function of time: . s t = s max × sin 2 π / T × t = s max × sin ω × t mit t ∈ 0 ; T / 4

[0024] Solving for time yields the following expression: t s = 1 / ω × arcsin s / s max mit s ∈ 0 s max

[0025] If you differentiate the distance with respect to time, you obtain the speed as a function of time: v t = ds t / dt = s max × cos ω × t × ω = v max × cos ω × t

[0026] By replacing the time with the preceding expression, one finally obtains the velocity as a function of the distance: v s = v max × cos arcsin s / s max = v max × 1 − s / s max 2

[0027] The speed as a function of distance is in Fig. 1This is illustrated for a 1 / 4 cleaning stroke. The curve describing the speed can be divided into three sections. These can, in turn, be represented as bars, each with an area corresponding to the area under the respective curve segment. As can be seen from the illustration, 50% of the distance is covered at an average speed of 95% of the maximum speed. Another 35% of the distance is covered at an average speed of 72% of the maximum speed. The remaining 15% of the distance is covered at an average speed of 35% of the maximum speed. Based on the weighting of the three speeds along the path (50% of the distance at 95% v max, 35% of the distance at 72% v max, and 15% of the distance at 35% v max), a correspondingly weighted average dynamic coefficient of friction can be determined.The weighted mean dynamic coefficient of friction, abbreviated as µ d, which is a measure of perceived smoothness, is calculated as follows: . μ ¯ d = 0 , 50 × μ d 95 % v max + 0 , 35 × μ d 72 % v max + 0 , 15 × μ d 35 % v max

[0028] If v max is set to a value of 0.2 m / s, as is typical for the maximum speed of the wiping motion, the weighted mean dynamic coefficient of friction is: μ ¯ d = 0 , 50 × μ d 0 , 20 m / s + 0 , 35 × μ d 0 , 15 m / s + 0 , 15 × μ d 0 , 07 m / s

[0029] The weighted mean dynamic coefficient of friction, since it is a measure of perceived smoothness, is also referred to as the haptic smoothness coefficient (HGK). Accordingly: HGK = 0 , 50 × μ d 0 , 20 m / s + 0 , 35 × μ d 0 , 15 m / s + 0 , 15 × μ d 0 , 07 m / s

[0030] The smaller the haptic smoothness coefficient (HGK) ,The greater the perceived smoothness of the lens surface, the more tactile the friction coefficient. In metrological terms, this haptic smoothness coefficient corresponds to the weighted mean dynamic coefficient of friction, which is determined under specific parameters, such as the microfiber cloth used and the measurement geometry, as explained above. Surprisingly, determining the dynamic coefficient of friction at three tangential speeds—namely 0.20 m / s, 0.15 m / s, and 0.07 m / s—is sufficient to accurately represent the perceived smoothness.

[0031] The haptic smoothness coefficient (HGK) allows for a sufficiently accurate measurement of perceived smoothness. The smoothness perceived by touch can be measured even more accurately by including the acceleration phase near the turning point. For this purpose, a dynamic friction coefficient, determined at a comparatively low speed, is also taken into account. Excellent comparability between empirical tests on smoothness and the description of smoothness using friction curves is achieved by determining the dynamic friction coefficient at a tangential speed of 0.01 m / s, which is close to standstill, and adding it to the haptic smoothness coefficient (HGK) after multiplying by a weighting factor of 3. The resulting haptic smoothness coefficient (R_HGK) is then as follows: R _ HGK = HGK + 3 × μ d 0 , 01 m / s

[0032] The measurement method according to the invention is not limited to determining the smoothness of a spectacle lens. It can, in principle, be applied to any type of coated or uncoated surface. Using the two haptic smoothness coefficients HGK and R_HGK, for example, different surface coatings can be distinguished and their quality objectively evaluated. In this way, limit values ​​for different quality levels can be defined. Character description

[0033] Fig. 1 This graph shows the relationship between speed and distance during a wipe across a spectacle lens for a quarter cleaning stroke. Compared to the curve on the left, the curve on the right has three additional bars, each corresponding to the area under its respective curve segment. Fig. 2This figure shows the dynamic coefficient of friction as a function of the wiping speed for the coated plastic spectacle lenses from the exemplary embodiment and the reference example. The curves were obtained by determining the dynamic coefficients of friction at tangential speeds of 0.0002 m / s, 0.002 m / s, 0.01 m / s, 0.03 m / s, 0.07 m / s, 0.15 m / s, 0.20 m / s, and 0.40 m / s, and subsequently interpolating the dynamic coefficients of friction determined at these speeds. The relevant points for the calculation (0.01 m / s, 0.07 m / s, 0.15 m / s, and 0.20 m / s) are marked. Examples

[0034] The following Example of implementation This serves to further explain the present invention, without, however, being limited to this purpose.

[0035] The substrate for the coating was a tubular plastic spectacle lens with the following properties: • Diameter: 60 mm • Refractive index: 1,60 • Refractive power (spherical effect): -2.25 diopters

[0036] A hard coating was first applied to the substrate using an immersion process. An interference / anti-reflective coating was then applied to this using an ion-assisted physical vapor deposition process. After the final layer of the interference / anti-reflective coating was applied, the surface underwent plasma treatment before the functional coating was deposited. For this purpose, a "Surfclear 100M" pellet was heated in a vacuum coating system using a resistance evaporator with a boat current of 5.4 A. This resulted in the formation of a 9 nm thick functional coating on the interference / anti-reflective coating. The maximum deposition rate was 0.44 nm / s with a coating time of 50 seconds. After the functional coating was applied, the coating process was terminated and the vacuum coating system was vented.

[0037] The resulting spectacle lens was not subjected to any further treatment after coating, in particular no rubbing action beyond normal cleaning by wiping.

[0038] The spectacle lens was then measured for smoothness in accordance with the measuring method according to the invention. For this purpose, the spectacle lens was first fixed centrally on the lower support of a "Haake Mars iQ Air" rheometer using a holder, so that the functional coating applied to the convex side of the substrate pointed towards the rotor of the measuring device. The center of the spectacle lens was positioned directly below the center of the rotor. A microfiber cloth of the type "Savina MX", manufactured by "kbSeiren", Osaka, Japan, measuring 45 mm × 45 mm, was then placed on the surface of the spectacle lens. The rotor of the measuring device was designed such that a circular plate was located at its lower end, on which a silicone ring with an inner radius of 17.5 mm, an outer radius of 20.5 mm, and a height of 3 mm was mounted centrally.The silicone ring had a Shore A hardness of 40.

[0039] To measure the dynamic coefficient of friction, the rotor was lowered onto the convex side of the spectacle lens. Using the rotor's silicone ring, the microfiber cloth was pressed perpendicularly onto the lens surface with a force of 3 N. By rotating the rotor, the microfiber cloth was moved uniformly relative to the lens surface for a specific duration, corresponding to four rotations of the cloth on the lens surface, at a speed of 0.01 m / s tangential to the outer radius of the silicone ring. Simultaneously, the measuring device determined the torque required to move the microfiber cloth relative to the lens surface. From this, the dynamic coefficient of friction for the tangential speed of 0.01 m / s could then be determined.The dynamic coefficient of friction for the tangential speeds of 0.07 m / s, 0.15 m / s, and 0.20 m / s was determined analogously. The number of revolutions was adjusted to the respective speed, taking into account an acceleration phase to achieve a constant speed, and ranged from four to eight revolutions.

[0040] From the dynamic friction coefficients determined in this way, a HGK of 0.287 was obtained according to formula (XI) and an R_HGK of 0.614 according to formula (XII).

[0041] To achieve a higher quality level in the subsequent process cycle, particularly with regard to the high coefficient of performance (HGK) of 0.287, the coating parameters were adjusted. In this case, the coating parameters were adjusted such that the shutdown condition was set to a target physical layer thickness of 13 nm for the functional coating, with a maximum deposition rate of 0.43 nm / s and a coating time of 68 seconds.

[0042] Subsequently, an identical substrate was coated with a functional coating in the vacuum coating system under the adjusted coating parameters. The dynamic coefficients of friction were determined as before.

[0043] From the dynamic friction coefficients determined in this way, a HGK of 0.24 was obtained according to formula (XI) and an R_HGK of 0.50 according to formula (XII).

[0044] By adjusting the coating parameters, the haptic smoothness coefficients HGK and R_HGK could be reduced. Consequently, the spectacle lens obtained from this process cycle exhibited improved smoothness.

[0045] As Reference example A commercially available spectacle lens was used, which was identical in construction to the plastic spectacle lens from the exemplary embodiment with regard to its substrate, but was not provided with a corresponding functional coating.

[0046] Here too, the dynamic coefficients of friction were measured at tangential velocities of 0.01 m / s, 0.07 m / s, 0.15 m / s and 0.20 m / s, as described in connection with the exemplary embodiment.

[0047] From the dynamic friction coefficients determined in this way, a HGK of 0.473 was obtained according to formula (XI) and an R_HGK of 1.77 according to formula (XII).

[0048] The spectacle lens from the reference example therefore did not meet the quality requirements regarding smoothness.

[0049] How Fig. 2 As can be seen, the speed-dependent dynamic coefficient of friction is comparatively low in the case of the coated plastic lenses from the exemplary embodiment. Only a slight increase in the dynamic coefficient of friction can be observed with increasing tangential speed. Thus, particularly smooth wiping with minimal effort is possible. In contrast, the dynamic coefficient of friction is significantly higher in the lens from the reference example.

Claims

1. Measuring method for determining the smoothness of a spectacle lens using a rheometer to determine dynamic friction coefficients against a microfiber cloth, which describes a circular path around the center of the spectacle lens, wherein the determination of dynamic friction coefficients is carried out at speeds in the range from 0.005 m / s to 0.4 m / s tangential to the described circular path and tangential to the surface of the eyeglass lens, characterized in that the smoothness of the eyeglass lens is expressed by a haptic smoothness coefficient HGK and the method comprises: wiping with a microfiber cloth in a rotating motion around the center of the eyeglass lens, wherein the wiping is performed with the aid of a rotor that is lowered onto the convex side of the eyeglass lens, the pressure of the microfiber cloth onto the surface of the eyeglass lens to be measured is exerted by an elastic stamp with a ring-shaped contact surface, and the microfiber cloth moves in the area of the annular contact surface relative to the surface of the eyeglass lens at a speed in the range of 0.005 m / s to 0.4 m / s tangentially to the outer radius of the annular contact surface , Determining the torque during the rotating movement of the microfiber cloth using a rheometer, Determining the dynamic friction coefficient based on the previously determined torque, Repeating the above steps at a different speed in the range from 0.005 m / s to 0.4 m / s tangential to the outer radius of the annular contact surface , and Calculating the haptic smoothness coefficient HGK from at least two dynamic friction coefficients determined at different tangential speeds.

2. Measuring method according to claim 1, wherein dynamic friction coefficients determined at tangential velocities of 0.20 m / s, 0.15 m / s, and 0.07 m / s are included in the calculation of the haptic smoothness coefficient HGK.

3. Measuring method according to claim 2, wherein the haptic smoothness coefficient HGK is calculated using the following formula (XI): HGK = 0 , 50 × μ d 0 , 20 m / s + 0 , 35 × μ d 0 , 15 m / s + 0 , 15 × μ d 0 , 07 m / s wherein µd(0,20 m / s) is the dynamic friction coefficient determined at a tangential velocity of 0.20 m / s,µd(0,15 m / s) is the dynamic friction coefficient determined at a tangential velocity of 0.15 m / s, and µd(0,07 m / s) is the dynamic friction coefficient determined at a tangential velocity of 0.07 m / s.

4. Measuring method according to claim 3, wherein the dynamic friction coefficient at a tangential velocity of 0.01 m / s is additionally determined and this is used in the calculation of a haptic smoothness coefficient R_HGK using the following formula (XII): R _ HGK = HGK + 3 × μ d 0 , 01 m / s wherein µd(0,01 m / s) is the dynamic friction coefficient determined at a tangential velocity of 0.01 m / s.