WO2017215945A1

WO2017215945A1 - Method for the determination of intensity losses of an electromagnetic radiation propagating in a transparent sheet

Info

Publication number: WO2017215945A1
Application number: PCT/EP2017/063386
Authority: WO
Inventors: Thomas LAMBRICHT; Jean-François NOULET; Sébastien CALIARO; Carmelo Dado
Original assignee: Agc Glass Europe
Priority date: 2016-06-13
Filing date: 2017-06-01
Publication date: 2017-12-21

Abstract

The invention relates to an improved method to measure the intensity losses of an electromagnetic radiation propagating in a transparent sheet by internal reflection or total internal reflection (TIR). The method of the invention is particularly advantageous to measure the intensity losses of a transparent sheet highly transmissive to said electromagnetic radiation, with a very high accuracy, even for very thin and large dimensions sheets. The method of the invention is improved as it allows to decorrelate the "bulk loss" and the "bounce loss" of the electromagnetic radiation propagating inside said transparent sheet. The present invention also relates to a device to implement said method.

Description

Method for the determination of intensity losses of an electromagnetic radiation propagating in a transparent sheet

1. Field of the Invention

The present invention relates to a method to measure the intensity losses of an electromagnetic radiation propagating in a transparent sheet by internal reflection or total internal reflection (TIR). The method of the invention is particularly advantageous to measure the intensity losses of a transparent sheet highly transmissive to said electromagnetic radiation, with a very high accuracy, even for very thin and large dimensions sheets. The method of the invention is improved as it allows to decorrelate the "bulk loss" and the "bounce loss" of the electromagnetic radiation propagating inside said transparent sheet. The present invention also relates to a device to implement said method.

The invention finds notably its interest for characterizing sheets of material to be used in touch panels (fitted above a display surface or not) using notably the optical technology referred to as Frustrated Total Internal Reflection (FTIR), or also in light guiding plates.

2. Solutions of the Prior Art

By total internal reflection (TIR), it is meant in a known manner the propagation of an electromagnetic radiation exclusively inside a medium by reflecting bouncing at the boundary of said medium and another medium (i.e. air). This phenomenon occurs when the radiation propagates inside the medium with a higher refractive index reaching a boundary with a medium of lower refractive index, at an angle greater than or equal to the critical angle. Then, the radiation will not cross said boundary, and will be totally reflected back internally. Electromagnetic radiation could also be reflected by a coating or other layer on surface of said medium. When a radiation propagates inside a sheet of material by total internal reflection, it undergoes losses of intensity due to (i) intrinsic absorption by the material of the sheet (called the "bulk loss") and (ii) multiple reflection/bouncing at the boundaries of the sheet (called the "bounce loss"). The total loss in intensity is also usually called the "total attenuation" or α_τ.

Both bulk and bounce losses are values relevant to know/determine, together or independently, in order to evaluate the performances of the sheet when used, for example, in touch panel using FTIR. Indeed, if the losses in intensity of the radiation/signal are too high, the sensitivity of the touch panel will decrease or, alternatively, initial signal power must be increased to compensate loss, but then with an increased global energy consumption of the device integrating the touch panel. This is particularly true for sheets of large dimensions for which the optical path length is long. The "bounce loss" itself is of particular relevance, for example if one wants to study the interface/boundary effect on the performances of a touch panel for example (influence of surface quality, or an external perturbation like a touch object or water droplets) .

In a general manner, in the field of optical touch panel using the FTIR technology, there has been much effort for several years to find solutions to decrease losses in intensity of the radiation injected in the sheet to be used as touch panel. This has been particularly the case for transparent sheets made of glass for which many developments has occurred on the composition mainly to reduce the bulk loss. Indeed, basically, glass is a material of choice for touch panels as a result of its mechanical properties, its durability, its resistance to scratching and its optical clarity and also because it can be chemically or thermally strengthened. In order to quantify the "bulk loss" or "α_B" in intensity of a radiation of specific wavelength injected in a sheet (for example, in the infrared range), it is known to use the coefficient of absorption, which consequently should be as low as possible in order to obtain good performances. This coefficient of absorption is defined by the ratio of the absorbance to the distance d (or length of the optical path) travelled by an electromagnetic radiation in a given medium (expressed in m^"1). It is thus independent of the thickness of the material but it is a function of the wavelength of the radiation injected/absorbed.

In the case of glass, for example, the coefficient of absorption (a) at a chosen wavelength λ can be calculated from a measurement in transmission (T) through the thickness of the glass and from the refractive index n of the material, the values of n, p and T being a function of the chosen wavelength λ:

However, in the field of FTIR touch panels, the optical path length is long (up to tenths of centimeters, or even one meter or more), and thus classical transmission measurements made through the thickness of sheet (typically several millimeters or less) are no more relevant regarding the final application. Indeed, an accurate transmission measurement through the thickness is not possible in case of material highly transparent to a specific wavelength. For example, in a 1 mm-thick sheet of glass, a significant increase in the absorption coefficient from 1 to 3 m^"1 results in a decrease in transmission in the thickness of only 0.2%. For very highly transmissive sheets, accuracy becomes unacceptably poor, particularly if one wants to compare very transmissive sheets between each other. Such a drawback has already been pointed out and partially addressed in WO2014085535A2. In this WO2014085535A2 patent application, it is described two different methods, with two different installations and principles to implement, in order to reach 1) the "bulk loss" and 2) the total attenuation loss, which will then give access to the bounce loss by subtraction:

(i) first, it is proposed to determine the bulk loss through a multiple path lengths transmission experiment. In particular, in such a proposed method, the coefficient of absorption of a material is determined by measuring absorbance by UV-Vis-NIR spectrometer for multiple paths on a polished rectangular cuboid of the material (i.e. 10 mm x 20 mm x 50 mm polished block of glass). This results in a measurement of the fundamental absorption of the glass, or the "bulk loss". Absorption is described by the Lambert-Beer-law: I = I₀ e , where I₀ is the initial intensity, a is the coefficient of absorption and d is the distance from position d₀. Thus, taking transmission measurements for multiple path lengths allows to calculate a. This improved method has still however severe disadvantages. First of all, it requires a significant sample preparation beforehand (cuboid with different available path lengths), implying specific and not straightforward processing of the material to measure (forming and polishing). Secondly, even if the path lengths used in this method allow to improve accuracy in the determination of the coefficient of absorption, this is still not sufficient, especially if the sheet of material to evaluate is highly transmissive to the considered radiation and/or has very large dimensions.

(ii) secondly, it is proposed another method that is, when considering total internal reflection (waveguide configuration), more suitable to characterize the total intensity loss (or total attenuation) taking into account losses that result from fundamental absorption ("bulk loss") and from the geometry and surface properties of the sheet ("bounce loss"). The method proposed is as follows: a radiation (LED) is injected into a sheet of material in order to get TIR, and a photodiode is used to measure the radiation intensity at a series of points along a straight line at several different distances from the radiation source. In this method, the photodiode is temporarily bonded to the surface of the sheet and removed with a razor blade after each measurement and then moved to the next location, further from the LED, again temporarily bonded, removed, etc. This method is clearly quite time-consuming and requires a lot of steps to get the desired total attenuation value. Moreover, such a method does not allow to characterize a coating, interlayer or external perturbations (like water droplets) angle by angle and consequently, it does not allow to study the angular influence of external interfaces and inaccessible internal interfaces.

Therefore, given drawbacks of the methods of the state-of-the-art, and due to the development of the market of large optical touch screens, there is clearly a high interest to have a simple/single method that allows to reach as well the "bounce loss" and "bulk loss" of an electromagnetic radiation propagating in a transparent sheet, with a high accuracy even for highly transmissive sheet, whatever the sheet dimensions and thickness, and without the need to process a specific sampling (using merely existing sheets, eventually cut). A method allowing to get the "bounce loss" for external interfaces (for example, external interfaces like sheet material/air or even sheet material/water droplets) as well as inaccessible internal interfaces (for example, an internal interface sheet material/plastic in a laminated assembly) is of course also of high interest.

3. Objectives of the Invention

The objective of the invention is in particular to remedy the cited disadvantages and resolving the technical problem, i.e. to provide a method to measure the intensity losses of an electromagnetic radiation propagating in a transparent sheet by total internal transmission, that allows to reach a very high accuracy in the value obtained, even for sheet of material highly transparent to said electromagnetic radiation.

Another objective of the invention in at least one of its embodiments is to provide a method to measure the intensity losses of an electromagnetic radiation propagating in a transparent sheet by total internal transmission, that allows as well to decorrelate the "bulk loss" and the "bounce loss" of the sheet, preferably while using essentially the same measurement installation.

Another objective of the invention in at least one of its embodiments is to provide a method to measure the intensity losses of an electromagnetic radiation propagating in a transparent sheet by total internal transmission, that can be used without any limit in the thickness of the sheet, even for ultra-thin sheets (i.e. 1 mm thin or less) and without the need for specific sampling. In particular, another objective of the invention is to provide a method to measure the intensity losses of an electromagnetic radiation propagating in a sheet of material by total internal transmission, which is not destructive for the sample.

Another objective of the invention in at least one of its embodiments is to provide a method to measure the intensity losses of an electromagnetic radiation propagating in a transparent sheet by total internal transmission, that can be used for sheets of large dimensions. Another objective of the invention in at least one of its embodiments is to provide a method to measure the intensity losses of an electromagnetic radiation propagating in a transparent sheet by total internal transmission, that allows characterizing as well external interfaces and inaccessible internal interfaces (in a laminated or coated sheet, for example).

Another objective of the invention in at least one of its embodiments is to provide a device to implement the method allowing to measure the intensity losses of an electromagnetic radiation propagating in a transparent sheet by total internal transmission. Finally, another objective of the invention is to provide a solution to the disadvantages of the prior art that is simple, quick and, above all, economical.

4. Outline of the Invention

The invention relates to a method for determination of bulk and bounce intensity losses of an electromagnetic radiation propagating in a transparent sheet, which comprises the following steps :

1) a "bulk step" during which, in order: (i) said electromagnetic radiation of intensity I₀ is injected with an optical coupling system CI in the sheet through one of its edge without contacting the two main faces of the sheet, the optical path length being di ; and (ii) an output signal (II) is recorded 2) a "bounce step" during which, in order: (i) said electromagnetic radiation of intensity I'₀ is injected with an optical coupling system C2 in the sheet through one of its faces with a given bounce angle Θ enabling propagation of the radiation inside the sheet between its two main faces by reflection and/or total internal reflection, the optical path length being d₂ ; and (ii) an output signal (I₂) is recorded

3) a "computation step" during which equations system (I) is solved, thereby allowing to determine the bulk loss, α_B and the bounce loss, α_s ; I₀ and I'o being known beforehand:

wherein step 1 and 2 are carried out independently in whatever order and step 3 is carried out after steps 1 and 2.

1) a "bulk step" during which, in order: (i) an electromagnetic radiation of intensity I₀ is injected with an optical coupling system CI in the sheet through one of its edge without contacting the two main faces of the sheet; the optical path length being di ; and (ii) an output signal (I₁ is recorded

2) a "bounce step" during which, in order: (i) an electromagnetic radiation of intensity I'₀ is injected with an optical coupling system C2 in the sheet through one of its faces with a given bounce angle Θ enabling propagation of the radiation essentially inside the sheet between its two main faces by reflection and/or total internal reflection, the optical path length being d₂ ; and (ii) an output signal (I₂) is recorded

3) a step during which step 1 or step 2 is repeated with an optical path length d₃ or d₄, with d₃≠d₁ or d₄≠d₂, respectively (output signal is I₃ or I₄, respectively) ;

4) a "computation step" during which the equations system (II) or (III) is solved, thereby allowing to determine the bulk loss, α_B and the bounce loss, α_s ; I₀ and I'₀ being equal or one of I₀ and I'₀ being known beforehand :

wherein steps 1-3 are carried out independently in whatever order and step 4 is carried out after steps 1-3. Finally, the invention also relates to a method for determination of bulk and bounce intensity losses of an electromagnetic radiation propagating in a transparent sheet, which comprises the following steps :

1) "bulk step" during which, in order: (i) an electromagnetic radiation of intensity I₀ is injected with an optical coupling system CI in the sheet through one of its edge without contacting the two main faces of the sheet; the optical path length being di ; and (ii) an output signal (I₁ is recorded

3) a step during which step 1 is repeated with an optical path length d₃, with d₃≠di (output signal is I₃);

4) a step during which step 2 is repeated with an optical path length d₄, with d₄≠d₂ (output signal is I₄);

5) a "computation step" during which the equations system (IV) is solved, thereby allowing to determine the bulk loss, α_B and the bounce loss, α_s ; I₀ and I'o being unknown:

wherein steps 1-4 are carried out independently in whatever order and step 5 is carried out after steps 1-4.

Hence, the invention rests on a novel and inventive approach, since it enables a solution to be found for the disadvantages of prior art. In particular, the inventors have found that it is possible, with one of the methods of the invention, to reach a very high accuracy in the values of bulk and bounce losses obtained, even for a sheet highly transparent to the used electromagnetic radiation. But, above all, the invention allows to decorrelate the "bulk loss" and "bounce loss" of a sheet without any limit in the thickness/dimensions of the sheet and without the need to process a specific sampling, without destroying the sample and while using essentially the same measurement device.

Throughout the present text, when a range is indicated, the extremities are included. In addition, all the integral and subdomain values in the numerical range are expressly included as if explicitly written.

Other features and advantages of the invention will be made clearer from reading the following description of preferred embodiments and figures, given by way of simple illustrative and non-restrictive examples. The invention relates to a method for determination of bulk and bounce intensity losses of an electromagnetic radiation propagating in a transparent sheet.

According to the invention, by a "sheet", it is meant a tridimensional object with two dimensions being significantly larger than the third one. According to the invention, the sheet has the following faces : two main faces and one or several edge(s). The sheet of the invention may be flat or shaped. In the case of a shaped sheet, the condition of the "bulk step" of the invention according to which the propagation of the radiation is done inside the sheet between its two main faces should be maintained in any manner (i.e. small curvature, large thickness, cylindrical curvature along the "generating line").

According to the invention, by a "transparent sheet", it is meant a sheet which is transparent to the electromagnetic radiation. Advantageously, the transparency of the sheet allows a detectable part of the input signal to reach the acquisition system. In particular, the transparent sheet of the invention has a transmission of the electromagnetic radiation of at least 0.000001%. By transmission, it is herein meant a transmission as obtained by the ratio I₁/I₀. Preferably, the transparent sheet has a transmission of the electromagnetic radiation of at least 0.000005%, or even at least 0.00001%. More preferably, the transparent sheet has a transmission of the electromagnetic radiation of at least 0.0001%, or even at least 0.001%, or even at least 0.01%. In a very preferred embodiment, the transparent sheet has a transmission of the electromagnetic radiation of at least 0.1%, or even at least 1%, or even at least 5%.

The sheet of the invention may be made of any type of material. For example, it may be a sheet made of glass, ceramic or polymer. The method of the invention gave particularly interesting results for a sheet of glass.

The sheet of the invention may also be composed of an assembly of two or more transparent sheets, identical or not, or an assembly of at least one transparent sheet and one or more transparent layer (s), identical or not. For example, the sheet may be composed of two sheets of glass laminated with an interlayer of PVB, all transparent to the electromagnetic radiation chosen.

The sheet of the invention may also textured or patterned.

The sheet of the invention may additionally be combined with a non- transparent element like a layer, or another sheet. The sheet of the invention may have any thickness, for example from

0.1 to 25 mm.

According to the invention, the electromagnetic radiation may be any type of electromagnetic radiation. It may be monochromatic or polychromatic.

Preferably, the electromagnetic radiation is in the UV-visible-IR wavelengths domain. In particular, according to this embodiment, the electromagnetic radiation is in the wavelengths range 100-2500 nm.

More preferably, the electromagnetic radiation is in the visible wavelengths domain (380-780 nm). In such an embodiment, the method of the invention is advantageous to evaluate color, illumination, light guiding properties of the transparent sheet.

More preferably also and alternatively, the electromagnetic radiation is in the IR wavelengths domain (780-2500 nm). According to this embodiment, preferably, the electromagnetic radiation is in the wavelengths range 780-1800 nm. More preferably, the electromagnetic radiation is in the wavelengths range 850-950 nm. In such an embodiment, the method of the invention is advantageous to evaluate transparent sheets for touch applications using IR technologies. The methods of the invention comprises at least one "bulk step" during which, in order: (i) an electromagnetic radiation of intensity I₀ is injected with an optical coupling system CI in the sheet through one of its edge without contacting the two main faces of the sheet; the optical path length being d_1; d₃ ; and (ii) an output signal I₃) is recorded. The optical path of the radiation during the "bulk step" of the invention is schematized at Figure 1(a).

The optical coupling system CI in the invention is done in order to inject the radiation into the sheet (F) through one of its edge without contacting the two main faces of the sheet (F) (in other words, the radiation propagates in the sheet thickness). The optical coupling system CI uses a coupling element which may be vacuum or air or at least one specifically designed material (i.e. a prism), transparent and/or reflecting to the electromagnetic radiation, or combination thereof. Depending on the edge quality (mainly geometry) of the sheet, the optical coupling system CI may also imply the use of an optical index matching material (solid or liquid) between said coupling element and the sheet.

The propagation direction is then essentially parallel to the main faces in order to guaranty that at least a part of the radiation does not contact the main faces of the sheet (F). Indeed, any part of the input signal which touches the main faces will impact negatively the accuracy of the measurement because of contamination of bulk loss by bounce loss. Thus, an electromagnetic radiation which is only faintly divergent or convergent is advantageous, even more if sample length is important and/or sample thickness is low. In particular, advantageously, a radiation divergence γ lower than 10° is better. Preferably, the radiation divergence γ is lower than 5°, or even better lower than 0.5° or even more better, lower than 0.1°. In a most preferable embodiment, the radiation is collimated (radiation divergence γ is ~0). For example, the use of laser beam as a source for the electromagnetic radiation is particularly interesting.

The methods of the invention comprise at least once "bounce step" during which, in order: (i) said electromagnetic radiation of intensity I'₀ is injected with an optical coupling system C2 in the sheet (F) through one of its faces with a given bounce angle Θ enabling propagation of the radiation inside the sheet (F) between its two main faces by reflection and/or total internal reflection, the optical path length being d₂, d₄ and (ii) an output signal (I₂, I₄) is recorded. In the bounce step, the radiation of intensity I'₀ is injected in the sheet either through one main face or through one edge. Possible optical path of the radiation during the "bounce step" of the invention is schematized at Figure 1 (b) and (c) in the case of a propagation by total internal reflection ((b): injection through one edge, and (c) : injection through one main face) .

The optical coupling system C2 in the invention is done in order to inject the radiation in the sheet (F) through one of its faces with a given bounce angle Θ. The optical coupling C2 uses a coupling element which may be vacuum, air or at least one specifically designed material (i.e. a prism), transparent and/or reflecting to the electromagnetic radiation, or combination thereof.

Depending on the edge or main faces quality (mainly geometry) of the sheet, the optical coupling system C2 may also imply the use of an optical index matching material (solid or liquid) between said coupling element and the sheet (F).

By "bounce angle", it is meant the angle at bouncing between the main propagation direction of the radiation and the hit main face. This angle is defined in the plane perpendicular to the hit main face and containing the main propagation direction of the radiation. It is illustrated in Figure 1(b) and (c).

According to an embodiment of the invention, the bounce angle Θ is as follows : 0° < Θ < 90°. According to this embodiment, preferably, the bounce angle Θ is as follows : 0° < Θ ≤ 60° or better : 0° < Θ ≤ 45°. More preferably, the bounce angle Θ is as follows : 10°≤ Θ ≤ 40° or better : 20°≤ Θ ≤ 30°. Such values for bounce angle allows better propagation of the radiation inside the sheet between its two main faces by total internal reflection.

Alternatively, the bounce angle Θ is as follows : 45°≤ Θ < 90°. Such values for bounce angle allows propagation of the radiation inside the sheet between its two main faces by reflection.

In the methods of the invention, an output signal is recorded I₂, I₃, and/or I₄). The output signal may be recorded according to the invention using any acquisition system able to collect, convert the output signal and quantify it in intensity.

According to the invention, an output signal means a part of the initial signal (injected electromagnetic radiation of intensity I₀ or I'₀) collected along the optical path on one main face or one edge of the sheet. In the "bulk step" according to the invention, the ouptut signal ^ or I₃ is collected from the edge opposite to that from which the signal is injected. In the "bounce step" according to the invention, the ouptut signal I₂ or I₄ is collected either from a main surface or from one edge.

Depending on the injected electromagnetic radiation, the acquisition system may be sensitive to one wavelength (e.g. a photodiode when using a monochromatic radiation) or may be sensitive to several wavelengths (e.g. a spectrophotometer when using polychromatic radiation). With the acquisition system, the output signal is collected, then converted in an electric signal and finally, quantified in intensity, as a numerical value I₂, I₃, and/or I₄).

According to an embodiment, the step of recording the output signal I₂, I₃, and/or I₄) according to the invention is carried out by using a sensor with a sufficient area to directly collect and convert the output signal.

Alternatively, according to another embodiment, the step of recording the output signal I₂, I₃, and/or I₄) according to the invention is carried out by using an optical device that concentrates/focalizes the output signal towards a sensor which collect and convert the signal. Alternatively, according to another embodiment, the step of recording the output signal I₂, I₃, and/or I₄) according to the invention is carried out by spatially uniformizing the output signal which is then converted by a sensor having a smaller area than the area covered by the output signal. For example, one may use a integration sphere combined with a sensor.

According to those embodiments for the step of recording the output signal, the sensor may be a photoelectric or a pyroelectric sensor.

According to an embodiment, the "bulk step" and/or the "bounce step" may comprise an optical out-coupling CI' and/or C2', respectively. This is advantageous in some cases in order to efficiently extract the corresponding output signal from the sheet before recording step. Described coupling elements above may as well also be used as decoupling elements. None specific condition is required between selected coupling and outcoupling elements (same or different).

The invention covers three methods for determination of bulk and bounce intensity losses of an electromagnetic radiation propagating in a transparent sheet, comprising at least: a "bulk step" during which, in order: (i) said electromagnetic radiation of intensity I₀ is injected with an optical coupling system CI in the sheet through one of its edge without contacting the two main faces of the sheet, the optical path length being di ; and (ii) an output signal (II) is recorded a "bounce step" during which, in order: (i) said electromagnetic radiation of intensity I'₀ is injected with an optical coupling system C2 in the sheet through one of its faces with a given bounce angle Θ enabling propagation of the radiation inside the sheet between its two main faces by reflection and/or total internal reflection, the optical path length being d₂ ; and (ii) an output signal (I₂) is recorded a "computation step" during which an equations system is solved, thereby allowing to determine the bulk loss, α_B and the bounce loss, α_s. Depending on whether I₀ and I'₀ (initial intensity of the electromagnetic radiation) are known beforehand or not, are equal or not, some additional steps may be required and the equations system to be solved at the computation step will be different. This is illustrated at figure 2.

If I₀ and I'₀ are known beforehand, the determination of the bulk and bounce losses requires a computation step with a system of two equations to solve. In such an event, the method of the invention comprises the following steps :

1) a "bulk step" during which, in order: (i) said electromagnetic radiation of intensity I₀ is injected with an optical coupling system CI in the sheet through one of its edge without contacting the two main faces of the sheet, the optical path length being di ; and (ii) an output signal (II) is recorded

2) a "bounce step" during which, in order: (i) said electromagnetic radiation of intensity I'₀ is injected with an optical coupling system C2 in the sheet through one of its faces with a given bounce angle Θ enabling propagation of the radiation inside the sheet between its two main faces by reflection and/or total internal reflection, the optical path length being d₂ ; and (ii) an output signal (I₂) is recorded

3) a "computation step" during which equations system (I) is solved, thereby allowing to determine the bulk loss, α_B and the bounce loss, α_s:

If I₀ and I'₀ are unknown but are equal, the determination of the bulk and bounce losses requires a computation step with a system of three equations to solve. This implies that at least the "bulk step" or the "bounce step" has to be repeated while keeping all respective measurement parameters constant except the optical path length which is varied.

In such an event, the method of the invention comprises the following steps : 1) a "bulk step" during which, in order: (i) an electromagnetic radiation of intensity I₀ is injected with an optical coupling system CI in the sheet through one of its edge without contacting the two main faces of the sheet; the optical path length being di ; and (ii) an output signal (I₁ is recorded

3) a step during which step 1 is repeated with an optical path length d₃, with d₃≠di (output signal is I₃) ;

4) a "computation step" during which the equations system (II) is solved, thereby allowing to determine the bulk loss, α_B and the bounce loss, α_s :

Alternatively, in such an event, the method of the invention comprises the following steps :

1) a "bulk step" during which, in order: (i) an electromagnetic radiation of intensity I₀ is injected with an optical coupling system CI in the sheet through one of its edge without contacting the two main faces of the sheet; the optical path length being ; and (ii) an output signal (I₁ is recorded

2) a "bounce step" during which, in order: (i) an electromagnetic radiation of intensity I'₀ is injected with an optical coupling system C2 in the sheet through one of its faces with a given bounce angle Θ enabling propagation of the radiation essentially inside the sheet between its two main faces by reflection and/or total internal reflection, the optical path length being d₂ ; and (ii) an output signal (I₂) is recorded 3) a step during which step 2 is repeated with an optical path length d₄, with d₄≠d₂ (output signal is I₄);

4) a "computation step" during which the equations system (III) is solved, thereby allowing to determine the bulk loss, α_B and the bounce loss, α_s :

If I₀ is known beforehand but I'₀ is unknown, the determination of the bulk and bounce losses requires also a computation step with a system of three equations to solve. This implies that at least the "bounce step" has to be repeated while keeping all respective measurement parameters constant except the optical path length which is varied. In such an event, the method of the invention comprises the following steps :

3) a step during which step 2 is repeated with an optical path length d₄, with d₄≠d₂ (output signal is I₄);

If I₀ is unknown but I'₀ is known beforehand, the determination of the bulk and bounce losses requires also a computation step with a system of three equations to solve. This implies that at least the "bulk step" has to be repeated while keeping all respective measurement parameters constant except the optical path length which is varied. In such an event, the method of the invention comprises the following steps :

According to the invention, if I₀ and/or I'₀ is/are known beforehand, it is, for example, from pre-calibration or from values coming from estimated bulk/bounce values available in the literature or from values coming from previous measurements (done with the method of the invention or not). The accuracy of the methods of the invention will at least partially be determined by the accuracy of these previously known values for I₀ and I'₀. If I₀ and I'₀ are unknown, the determination of the bulk and bounce losses requires a computation step with a system of four equations to solve. This implies that the "bulk step" and the "bounce step" have to be repeated while keeping all respective measurement parameters constant except the optical path length which is varied. In such an event, the method of the invention comprises the following steps:

5) a "computation step" during which the equations system (IV) is solved, thereby allowing to determine the bulk loss, α_B and the bounce loss, α_s :

The methods of the invention comprise a "computation step" during which an equations system (I), (II), (III) or (IV) is solved, thereby allowing to determine the "bulk loss", α_B and the "bounce loss", α_s. During the "bounce step", the intensity loss of the radiation due to repeated bouncing on the main surfaces decreases globally in an exponential manner (I₂, I₄). However, the intensity loss due to the interface occurs at each bounce so that the exponential decreasing occurs actually by a number (corresponding to the bounce number) of little steps. This phenomenon is illustrated schematically at figure 3, showing the decrease of I₀' (actual and approximated) with the optical path/sheet length and considering α_B equal to zero.

According to the invention, using a classical exponential function for evaluating the decrease of I₀' is an acceptable approximation, in particular if the sheet has a significant length (corresponding to the optical path of the radiation) and/or if the number of bounces is relatively high (i.e. if the bounce angle is high). In such a case, the equations system (I), (II), (III) or (IV) to be solved during the computation step of the invention is as follows:

However, if one wants to reach very actual and accurate values for intensity losses (α_B, α_s) and/or if the number of bounces N is relatively low (i.e. if the sheet length/optical path is relatively short and/or if the bounce angle is very low), the used function for evaluating the decrease of I₀' should preferably not be approximated but should integrate the decreasing by steps. In such a case, the equations system (I), (II), (III) or (IV) to be solved during the computation step of the invention is as follows:

« LPB » being the average loss at the interface of each individual bounce.

In the method of the invention, the optical path lengths d₂ and d₄ can be determined using sample length and the bounce angle Θ.

The methods of the invention also allows to evaluate the effect of an additional material in contact with the sheet of the invention or of a specific surface treatment. For example, it is possible with the methods of the invention to evaluate the effect on the intensity losses of : a layer deposited on the sheet ;

a contamination on the sheet (like dust or adsorbed molecules or fingerprints) ;

a surface treatment like an etching/texturing treatment;

an object coming into contact with the sheet, like a stylus, a finger or water droplets.

In this case, a given sheet with an additional material/treatment will be compared to a reference sheet (same as the given sheet but without the additional material/treatment). Measurements done on both sheets could be compared and additional "bounce loss" due to the specific surface could thus be extracted. For example, in FTIR touchscreen devices, reduction of these signal losses are critical. An accurate characterization of these losses will thus help to select the most appropriate components (anti-reflective coating, anti-glare, hydrophobic, ...). Surface characterization principle could also be used to detect environment changes on or in the vicinity of the sheet surface.

In the methods of the invention, in general, if the sheet is asymmetric (for example if it bears on one of its main face a layer, the opposite main face being naked or covered with a different layer), the "bounce loss" α_s determined at the computation step is an average value coming from contribution of both main faces of the sheet. Hence, for example, if the sheet bears one different layer on each face, if one wants to isolate the effect of one of the two layers, the method of the invention may be implemented with the sheet coated with the two layers, and then repeated with the same sheet bearing only one layer.

The invention also relates to a device to carry out the methods of the invention. According to the invention, the device comprises :

a "bulk part" to carry out the "bulk step", comprising a source SI of electromagnetic radiation of intensity I₀ and an optical coupling system CI;

a "bounce part" to carry out the "bounce step", comprising a source S2 of electromagnetic radiation of intensity I'₀ and an optical coupling system C2; a support to position the transparent sheet;

an acquisition system.

According to the invention, the "bulk part" of the device comprises a source SI of electromagnetic radiation of intensity I₀ and an optical coupling system CI.

According to the invention, the "bounce part" of the device comprises a source S2 of electromagnetic radiation of intensity I'₀ and an optical coupling system C2.

Embodiments of the electromagnetic radiation described above in relation with the methods of the invention apply also to the device of the invention.

Embodiments of the optical coupling system CI or C2 described above in relation with the methods of the invention apply also to the device of the invention.

In one embodiment, the source SI of electromagnetic radiation and the source S2 of electromagnetic radiation are independent (i.e. two laser beams). In an alternative embodiment, the source SI of electromagnetic radiation is also the source S2 of electromagnetic radiation. In such an event, the device also comprises advantageously a splitting system allowing to split the electromagnetic radiation from the unique source into two beams that can then be directed towards the "bulk part" and "the bounce part" of the device, respectively.

According to the invention, the device comprises an acquisition system. The acquisition system may be of any type suitable to collect, convert the output signal and quantify it in intensity. Embodiments of the acquisition system (or step of recording the output signal) described above in relation with the methods of the invention apply also to the device of the invention.

In addition to allow implementing methods according to the invention, the device of the invention also notably allows (i) to determine the refractive index of a coating/layer deposited on the medium; and (ii) to measure the efficiency of a solar concentrator as described in European patent application EP 15 196 313.9.

Advantageously, the device of the invention allows to determine the refractive index of a coating/layer deposited on the transparent sheet. In such a case, the refractive index of the transparent sheet has to be known and the evaluated stack (sheet/coating) should involve a third medium on the coating side opposite to the sheet which has a transmission different from said sheet (i.e., sheet/coating/air). An electromagnetic radiation is injected with an optical coupling in the sheet through one of its faces with different bounce angles Θ enabling propagation of the radiation inside the sheet between its two main faces by reflection and/or total internal reflection; and an output is recorded. Refractive index of the coating/layer can then be deducted from the bounce angle Θ at which one observes a significant decrease of the output signal.

Specific embodiments linked to the device according to the invention will be illustrated in the followings, with some figures amongst which :

Figure 4 (a)-(c) illustrates schematically an embodiment of device according to the invention, from different views ((a) : general view; (b) side view ; (c) top view) .

Figure 5 illustrates an embodiment of (a) a coupling system CI from the "bulk part" and of (b) a coupling system C2 from the "bounce part" of a device according to the invention (side views).

Figure 6 illustrates an embodiment of (a) a source SI from the "bulk part" and of (b) a source S2 from the "bounce part" of a device according to the invention (side views). Tables 1-5 give a description of each reference indicated in Figures 4-

6, as well as its technical effect in the illustrated embodiment(s) of the device of the invention.

Table 1

The aim in this example is to evaluate the bulk and bounce losses at a wavelength at 852 nm of a sample being a 4mm-thick sheet of a chromium- containing low-iron glass (as described in patent application WO2014/128016A1) which has high infrared transmission.

The device used (references are from Figures 4-6) is composed of two sources : SI for bulk measurements and S2 for bounce measurements, comprising each a laser (1.1 and 2.1, respectively). The laser beams from the sources (1.2 and 2.2, respectively) are coupled with the sheet (3.9) using a coupling system (CI and C2 for bulk and bounce measurements, respectively). The extremity of the sheet is inserted in an integrating sphere (3.13) by the slit of a custom port entrance (3.5). The integrating sphere collect the beam (output signal) out-coupled from the sheet without any specific coupling element ("air coupling"). The beam/radiation intensity is converted by a photodiode (3.11) in a current. SI and S2 are also monitored by photodiodes (1.13 and 2.13, respectively), which allow to compensate a potential variation in the power of the lasers. All photodiodes are connected to an acquisition system, and each photodiode current is integrated over time. For bulk (bounce) measurements, the currents of the photodiodes 1.13 (2.13) and 3.11 are integrated at the same moment and during the same time to reduce the noise as much as possible.

Using this configuration, the initial intensities of the beam (electromagnetic radiation) that is coupled in the sheet, I₀ and I'₀, were not known. In this case, at least two sheets of different lengths are needed. To increase the accuracy and the reliability of the evaluation, in this example, five sheets with lengths of 20, 40, 60, 80 and 100 cm were measured. These sheets were cut from a same large glass sheet to guarantee they have the same characteristics (this can also be achieved by using a large sheet for first step, which is then shortened progressively at each next step of the method).

The implemented method comprised the following consecutive steps 1) The system was turned on (lasers (1.1) and (1.2), the acquisition card, computer, ... ) .

2) Wait at least 15 minutes for laser warming up.

3) The sphere (3.13) was moved along the guiding rails (3.12) and placed at a position where the port entrance (3.5) is at position A.

20cm-sheet measurements

4) The optical breadboard (3.4) was moved along the guiding rails (3.3) and placed at a position where the coupling system CI is at position C.

5) An index liquid with a refraction index of 1.51 was applied on the top of the prism (3.7) and on the edge of 20cm-sheetsheet where it will be in contact with the coupling glass (5.1). The shortest sheet is preferably measured first and used to set the integration times because it has the lowest absorption from samples set.

6) The sheet was placed on the support (3.10) and in contact with the coupling glass (5.1) and the top of the prism. At this position, the extremity of the sheet is support-free.

7) The sphere (3.13) was moved along the guiding rails (3.12) and placed at a position where the sheet enter of few millimetres in the sphere. The port entrance (3.5) was at position B.

"Bulk step":

8) The bulk shutter (1.9) was then opened.

9) Using the rotation mount (1.10), the beam height was adjusted to be centred in the sheet thickness.

10) Top (3.14) and bottom (3.15) brushes were placed in contact with the sheet to block any residual beam that could travel under and above the sheet.

11) The "bulk integration time" of the acquisition system was adjusted to be in the upper range of the acquisition range. This integration time should remain at the same value for all sheet lengths. 12) The acquisition was started for both output photodiode (3.11) and bulk reference photodiode (1.13).

13) The bulk output value was normalized using the bulk

reference value

14) Top and bottom brushes (3.14, 3.15) were removed.

15) The bulk shutter (1.9) was closed.

"Bounce step":

16) The bounce shutter was opened (2.9).

17) The bounce angle Θ of the beam in the sheet was set at 15° using the linear (4.1) and rotative (4.2) motors; the mirror (4.3) was positioned to reflect the beam (2.14) towards the prism (3.7), the beam (2.14) is then reflected on the mirror (4.4) and sent in the sheet (3.9) at a position where it cannot re-enter the prism after its first reflection inside the sheet.

18) The "bounce integration time" of the acquisition system was adjusted to be in the upper range of the acquisition range. This integration time should remain at the same value for all sheet lengths.

19) The acquisition was started for both output photodiode (3.11) and bounce reference photodiode (2.13).

20) The bounce output

values were normalized using the bounce reference value

21) The bounce shutter (2.9) was closed.

22) The sphere (3.13) was moved along the guiding rails (3.12) and placed at a position where the port entrance (3.5) was at position A.

23) The 20cm-sheet was removed and the coupling systems CI and C2 were cleaned.

40cm-sheet measurements

24) The optical breadboard (3.4) was moved along the guiding rails (3.3) and placed at a position where the coupling system CI is at position D. 25) An index liquid with a refraction index of 1.51 was applied on the top of the prism (3.7) and on the edge of the 40cm-sheet where it will be in contact with the coupling glass (5.1).

26) The sheet was placed on the support (3.10) and in contact with the coupling glass and the top of the prism.

27) The sphere (3.13) was moved along the guiding rails (3.12) and placed at a position where the sheet enter of few millimetres in the sphere. The port entrance (3.5) was at position B.

"Bulk step": 28) The bulk shutter (1.9) was then opened.

29) Top (3.14) and bottom (3.15) brushes were placed in contact with the sheet to block any residual beam that could travel under and above the glass.

30) The acquisition was started for both output photodiode (3.11) and bulk reference photodiode (1.13).

31) The bulk output value

was normalized using the bulk reference value

32) Top and bottom brushes were removed.

33) The bulk shutter (1.9) was closed.

"Bounce step":

34) The bounce shutter was opened (2.9).

35) The bounce angle Θ in the sheet was set using the linear (4.1) and rotative (4.2) motors.

36) The acquisition was started for both output photodiode (3.11) and bounce reference photodiode (2.13).

37) The bounce output values

values were normalized using the bounce reference value

38) The bounce shutter (2.9) was closed.

39) The sphere (3.13) was moved along the guiding rails (3.12) and placed at a position where the port entrance (3.5) was at position A.

40) The 40cm-sheet was removed and the coupling systems CI and C2 were cleaned.

Other sheets measurements

41) Steps 24 to 40 were then repeated for remaining 3 sheets (steps 24) : position E for 60cm-sheet, position F for 80 cm-sheet,...).

"Computa tion step "

42) An exponential fitting of collected bulk output values was then performed to determine the absorption coefficient ("bulk loss" or α_B). Values obtained are summarized in Table 6 and exponential fitting is illustrated at Figure 7.

43) The beam path length in the bounce step was calculated for each sheet. An exponential fitting of collected bounce output values as a function of the calculated beam path length (or optical path length) was then performed to find the total attenuation (α_τ = α_Β + α_s). Values obtained are summarized in Table 7 and exponential fitting is illustrated at Figure 8. From α_τ and α_B, one can recompute the value of α_s.

44) The "loss per bounce" was finally also calculated as follows : a. The optical path length between two bounces was calculated as

b. "Loss per bounce" or LPB at 15° was calculated by applying the factor on the optical path length : Loss per bounce

Table 6

Example 2

Bounce measurements from Example 1 were also repeated with different bounce angles Θ varying from 14° to 35°. Therefore, graphs showing the evolution of total attenuation and loss per bounce according to bounce angle Θ were obtained. Such graphs are showed at Figure 9 (a)-(b).

Example 3

In the following example, it is illustrated how the device according to the invention is able to determine the refractive index of a coating/layer deposited on a transparent sheet. The refractive index of a coating/layer deposited on a transparent sheet can be indirectly measured using the device according to the invention and using the "bounce step" according to the invention.

The aim in this example is to evaluate at a wavelength at 852 nm the refractive index of a low-index interlayer in the following laminated sample : 4mm- thick transparent sheet of a chromium-containing low-iron glass (as described in WO2014/128016A1, highly transparent to IR) laminated with a colored glass (that has a very high absorption of IR) through a "low n" interlayer (layer to evaluate). The use of a highly absorptive (colored) glass sheet as second sheet to realize the laminate allows to highly simplify the measurement because the beam is directly and completely absorbed by the colored glass sheet when the beam is not anymore in total internal reflection in the transparent sheet.

This measurement only requires one sample length (i.e. 20 cm-length) and requires the "bounce step" to be repeated with a bounce angle Θ increasing.

Refractive index of the interlayer in the laminated sample was then deducted from the bounce angle at which one observes a significant decrease of the bounce output signal (called the "critical angle" and being 20.3°). This is illustrated at Figure 10.

In this example, the refractive index of the interlayer was calculated as follows:

Claims

1. Method for determination of bulk and bounce intensity losses of an electromagnetic radiation propagating in a transparent sheet, which comprises the following steps :

1) a "bulk step" during which, in order: (i) said electromagnetic radiation of intensity I₀ is injected with an optical coupling CI in the sheet through one of its edge without contacting the two main faces of the sheet, the optical path length being di ; and (ii) an output signal (I₁ is recorded

2) a "bounce step" during which, in order: (i) said electromagnetic radiation of intensity I'₀ is injected with an optical coupling C2 in the sheet through one of its faces with a given bounce angle Θ enabling propagation of the radiation inside the sheet between its two main faces by reflection and/or total internal reflection, the optical path length being d₂ ; and (ii) an output signal (I₂) is recorded

2. Method for determination of bulk and bounce intensity losses of an electromagnetic radiation propagating in a transparent sheet, which comprises the following steps :

1) a "bulk step" during which, in order: (i) an electromagnetic radiation of intensity I₀ is injected with an optical coupling CI in the sheet through one of its edge without contacting the two main faces of the sheet; the optical path length being di ; and (ii) an output signal (I₁ is recorded

2) a "bounce step" during which, in order: (i) an electromagnetic radiation of intensity I'₀ is injected with an optical coupling C2 in the sheet through one of its faces with a given bounce angle Θ enabling propagation of the radiation essentially inside the sheet between its two main faces by reflection and/or total internal reflection, the optical path length being d₂ ; and (ii) an output signal (I₂) is recorded

3) a step during which step 1 or step 2 is repeated with an optical path length d₃ or d₄, respectively (output signal is I₃ or I₄, respectively);

wherein steps 1-3 are carried out independently in whatever order and step 4 is carried out after steps 1-3.

3. Method for determination of bulk and bounce intensity losses of an electromagnetic radiation propagating in a transparent sheet, which comprises the following steps :

3) a step during which step 1 is repeated with an optical path length d₃ (output signal is I₃);

4) a step during which step 2 is repeated with an optical path length d₄ (output signal is I₄); 5) a "computation step" during which the equations system (IV) is solved, thereby allowing to determine the bulk loss, α_B and the bounce loss, α_s ; I₀ and I'o being unknown:

4. Method according to any one of claims 1-3, characterized in that the electromagnetic radiation is monochromatic.

5. Method according to any one of claims 1-4, characterized in that the radiation divergence g is lower than 5°.

6. Method according to preceding claim, characterized in that the electromagnetic radiation is in the wavelengths range of 100-2500 nm.

7. Method according to any one of claims 1-6, characterized in that the transparent sheet is a sheet of glass.

8. Device to carry out method according to claims 1-7, comprising :

- a "bulk part" to carry out the "bulk step", comprising a source

SI of electromagnetic radiation of intensity I₀ and an optical coupling system CI;

a "bounce part" to carry out the "bounce step", comprising a source S2 of electromagnetic radiation of intensity I'₀ and an optical coupling system C2;

- a support to position the transparent sheet;

an acquisition system.