Patent Description:
Converting machines can be configured to produce packaging containers such as flat-packed or folding boxes from sheets substrates which are printed, cut and scored to form blanks. These blanks can subsequently be folded and assembled into three-dimensional boxes. The boxes are designed to be folded either manually or automatically in a folder-gluer machine.

When the packaging containers and boxes are provided with a printed motif comprising a plurality of colors and various coatings, it is required that each color and coating is in the correct position on the blank and that the colors and coatings are aligned with each other.

To align the colors is referred to as setting the printing register and is often done by printing reference marks in the margins of the blanks and using a camera system to capture an image of a printed reference mark. The displacements of different elements in the reference mark can then be determined. Based on this information, printing units in the converting machine can be either manually or automatically adjusted. However, known systems have difficulties in detecting transparent coatings such as varnishes.

In view of the above-mentioned problems, it is an object of the present invention to provide an inspection device which is capable of detecting coatings with a high precision. It would also be advantageous to provide an inspection device which additionally detects different colors with high precision.

This object is solved by an inspection device according to claim <NUM>.

According to a first aspect of the present invention, there is provided an inspection device for checking the position of at least one coating on a blank transported through a converting machine, the inspection device comprising a camera configured to capture an image of a portion of the blank provided with a reference mark comprising at least one coating,.

The present invention is based on a realization that reflective coatings can be detected by creating a mirror effect from the coatings which is received into the camera. The angle of the optical axis enables the camera to capture specular reflected light rays, such that the reflective coatings can be detected.

The specular reflected light rays are captured by the camera as they are directed into an entrance pupil of a camera lens.

The entrance pupil is an optical aperture from the blank. Hence from the object side. In other words, the entrance pupil can be defined as an optical opening into the camera through which light can enter.

The surface of the blank may be a horizontal surface. Consequently, the vertical axis may be coinciding with the gravitational direction. The measuring point is located on the surface of the blank which is provided with the printed reference mark.

The term "coating" within the context of this application is transparent, i.e. colorless. It is the transparent nature of the coating which makes it invisible for a conventional camera system. The reference mark can be a composed reference mark comprising a plurality of individual reference marks. The reference mark thus comprises at least one individual reference mark printed with a coating. Each individual reference mark may be printed by a separate flexographic printing cylinder.

The first illumination module may be configured to illuminate a reference mark which generates a specular reflection. The reference mark which generates a specular reflection may comprise a varnish.

In an embodiment, the first illumination unit of the first illumination module comprises a diffusive layer. The first illumination unit may further comprise a plurality of light sources arranged side-by-side on a circuit board, and wherein the light sources are covered by the diffusive layer.

In an advantageous embodiment, the reference mark comprises at least a first individual reference mark and a second individual reference mark, and wherein the inspection device comprises a second illumination module configured to illuminate an individual reference mark configured to generate a diffuse reflection of light rays, the second illumination module comprising at least one illumination unit which is arranged at a third angle in relation to the vertical axis, the third angle being selected such that incident light rays from the at least one illumination unit are directed to the reference mark and specular reflected light rays from the reference mark are directed outside of the entrance pupil of the camera.

The second individual reference mark is printed in a color. The colors are printed with an opaque ink comprising at least one colorant, such as a dye or pigment. The colors typically generate a diffuse reflection of light.

The second illumination module may comprise at least two illumination units, wherein a first and second illumination units are arranged on opposite sides of the optical axis of the camera.

The first and second illumination units are preferably elongated and comprise a plurality of light sources arranged in a line. The longitudinal extension of the illumination units is arranged perpendicular in relation to the direction of transportation of the blank.

The first and second illumination units can be positioned at large angles towards the field of view on the blank. This enables light rays from both sides to be received outside the entrance pupil of the camera and a homogeneous illumination can be achieved.

In an embodiment, the first and second illumination units only comprise light sources located at the extremities of their elongated extension. In such a way, the illumination units are arranged in a square around the optical axis of the camera. This provides an equal distribution of the illumination units around the camera. As a result, a homogeneous illumination in the field of vision of the camera can be provided.

The light intensity from the first and second illumination modules can be varied. By varying the light intensity, the balance between glare and strong illumination of the reference mark can be optimized to achieve a precise captured image of the reference mark.

In an embodiment, the first illumination module can be disabled. This may be advantageous if the reference mark does not comprise a reflective coating.

In another embodiment, the second illumination module can be disabled. If the blank contains low-contrast color marks against a background color of the blank, but the marks exhibit different reflection characteristics as the background, it may be advantageous to illuminate only with the first illumination module.

In a preferred embodiment, the inspection device is mounted inside a housing shroud. The inspection device may further comprise a slide rail extending transversely in relation to the transportation path of the blank, and wherein the inspection device is configured to be displaced along the slide rail. The slide rail extends in perpendicular over or under the transportation path of the blank.

In an embodiment, the camera is triggered by a time signal from a control unit which is issued when an optical sensor registers a detection of a front leading edge of the blank, and wherein the time signal corresponds to an arrival time of the reference mark at a reflective illuminated area of the field of view of the camera: The reflective illuminated area can be provided by the first illumination module.

The invention will now be described by way of example and with reference to embodiments shown in the enclosed drawings, where the same reference numerals will be used for similar elements and in which:.

<FIG> illustrates an example of a blank <NUM> for a flat-packed or folding box. The blank <NUM> may be produced from cardboard, paperboard, plastics or like.

The blank <NUM> can be produced in a converting machine <NUM>, such as the one illustrated in <FIG>. The converting machine <NUM> is in the configuration of a rotary die-cutting machine <NUM>. At an entry position of the converting machine <NUM>, sheet substrates <NUM> are placed in a feeder module <NUM> and are transported in a direction of transportation T through the converting machine <NUM> in order to undergo a series of operations which print, cut and crease the sheet substrates <NUM> to form the blanks <NUM>. Hence, within the context of this application, the term "blank" applies as the sheet substrate <NUM> has been provided with a printed motif from at least one printing unit. The direction of transportation T is defined from the inlet to the outlet of the converting machine <NUM>. The blank <NUM> is transported along a transportation path P, which can be defined as the trajectory of the blank <NUM> through the converting machine <NUM>.

From the inlet of the converting machine <NUM> and in a downstream direction along the direction of transportation T, the converting machine <NUM> may comprise a prefeeder <NUM>, a feeder module <NUM>, printing module <NUM>, a die-cutting module <NUM>, a bundle stacker module <NUM> and a palletizer-breaker <NUM>. Optionally, a dryer module <NUM> (see <FIG>) can be provided after the printing module <NUM> and is configured to dry the ink before the blank <NUM> enters into the die-cutting module <NUM>. A main operator interface <NUM> may also be provided in the proximity of the converting machine <NUM>.

As illustrated in <FIG>, the printing module <NUM> comprises a plurality of flexographic printing units 16a to 16e. Each flexographic printing unit <NUM> comprises a flexographic printing assembly including a flexographic printing cylinder, and is configured to print an individual motif in a separate color or coating on the sheet substrate <NUM>. The individual motifs together form the final motif <NUM> on the blank <NUM>. Typically, at least four flexographic printing units 16a to 16d are provided in order to enable printing with different colors according to a large color palette.

As best seen in <FIG>, a reference mark <NUM> is printed by the flexographic printing units <NUM> together with the motif <NUM>. Each flexographic printing unit <NUM> is configured to print an individual reference mark <NUM>' at the same time as an individual motif is being printed onto the sheet substrate <NUM>. In such a way, a composed reference mark <NUM> is created by the different flexographic printing units <NUM>. The reference mark <NUM> is preferably located at a front leading edge <NUM> of the blank <NUM>.

Optionally, an additional second reference mark <NUM> can be provided on the rear edge <NUM> of the blank <NUM>. A reference mark <NUM> on the front leading edge <NUM> and a second reference mark <NUM> on the rear edge <NUM> facilitates the determination of rotational displacement shifts of the blank <NUM> in the flexographic printing module <NUM>.

As shown in <FIG>, the reference mark <NUM> may comprise a grid <NUM> and a plurality of dot-shaped individual reference marks <NUM>' arranged in the grid <NUM>. The grid <NUM> is typically printed by the first flexographic printing unit 16a together with a separate dot-shaped reference mark <NUM>' of a first color. The grid <NUM> is provided with a predetermined height H and length L.

As the sheet substrate <NUM> is conveyed through the flexographic printing module <NUM>, each flexographic printing unit <NUM> is printing a dot-shaped reference mark <NUM>' in the grid <NUM>. If the colors and coatings are aligned and thus perfectly registered, each dot-shaped reference mark <NUM>' is located in a predetermined position in the grid <NUM>, such as in the center of the grid <NUM>.

Alternatively, as illustrated in <FIG>, the grid <NUM> can be excluded and only a dot-shaped reference mark <NUM>' is printed by each flexographic printing unit <NUM>. This reference mark <NUM> displays the positions and alignment of the different colors and coatings in two dimensions by interrelated distances in X- and Y- coordinates.

As illustrated in <FIG>, the converting machine <NUM> comprises a printing quality control system <NUM> configured to detect the positions of the individual motifs and the alignment between the different individual motifs transferred from each flexographic printing unit <NUM> onto the sheet substrate <NUM>. The printing quality control system <NUM> comprises an inspection device <NUM>, a control unit <NUM> and a memory <NUM>.

The printing quality control system <NUM> is configured to detect and measure a longitudinal displacement and a lateral displacement between the different colors and coatings in the reference mark <NUM>. The longitudinal displacement is in the direction of transportation T, and the lateral displacement is in a direction perpendicular to the direction of transportation T. In such a way, the printing register, i.e. the positions and the alignment between the different colors and coatings can be determined. If the printing units <NUM> are not correctly registered in relation to each other, the final motif <NUM> will show a misalignment of individual motifs printed in different colors.

The printing quality control system <NUM> is configured to calculate longitudinal and lateral displacements and send corrective information to a central control system <NUM> of the converting machine <NUM>. The corrective information includes required adjustments in the angular and lateral positions of the printing cylinders of the printing module <NUM>. The converting machine <NUM> may be configured to automatically adjust the angular and lateral positions of the printing cylinders. Alternatively, the printing quality system <NUM> can display corrective information needed for a manual adjustment of the printing module <NUM> on a machine interface <NUM>.

If the printing quality control system <NUM> detects defective blanks <NUM> with misaligned colors and coatings, the central control system <NUM> can send information to an ejector module <NUM>, which discards the blank <NUM>.

As best seen in <FIG>, <FIG>, <FIG> and <FIG>, the inspection device <NUM> comprises an imaging system <NUM> and an illumination system <NUM>. The imaging system <NUM> can be a camera <NUM> with an active pixel sensor (e.g. a CMOS sensor) having an interface protocol configured to deliver images to the control unit <NUM>. The camera <NUM> is configured to receive light rays from the blank <NUM> within its field of view <NUM>.

As illustrated in <FIG>, the inspection device <NUM> can be mounted to a slide rail system <NUM>, also referred to as a "sliding rail system <NUM>". The slide rail system <NUM> comprises a longitudinal slide rail <NUM> extending in a direction perpendicular to the direction of transportation T.

Referring back to <FIG>, an optical sensor <NUM> may be placed upstream and in proximity with the camera <NUM> and can be configured to detect the arrival of the front leading edge <NUM> of the blank <NUM>. The camera <NUM> is triggered by a time signal from the control unit <NUM> which is issued when the optical sensor <NUM> registers a detection of the front leading edge <NUM> of the blank <NUM>.

The inspection device <NUM> is mounted downstream of the flexographic printing module <NUM>. As illustrated in <FIG>, the inspection device <NUM> is located below the transportation path P of the blank <NUM>. However, it is also possible to position the inspection device <NUM> above the above transportation path P of the blank <NUM>. The inspection device <NUM> is thus located such that the illumination system <NUM> and the field of view <NUM> of the camera <NUM> are directed towards a printed side of blank <NUM>. If the converting machine <NUM> is provided with a dryer module <NUM>, the inspection device <NUM> can be located after the flexographic printing module <NUM> and the dryer module <NUM>. Alternatively, the inspection device can be located between the flexographic printing module <NUM> and the dryer module <NUM>.

As best seen in <FIG>, <FIG> and <FIG>, the camera <NUM> has an optical axis A, which is a straight line passing through the geometrical center of a lens <NUM> of the camera <NUM>. The optical axis A is arranged at a first angle ϕ in relation to the direction defined by a normal vector N of the printed sheet surface of the blank <NUM>.

As illustrated in <FIG>, the illumination system <NUM> comprises a first illumination module <NUM> comprising at least one illumination unit <NUM>. As best seen in <FIG>, light emitted from the first illumination unit <NUM> towards a measurement point Pm on the blank <NUM> forms a second angle -α in relation to a vertical axis V defined by the normal vector N of the sheet surface of the blank <NUM>. The second angle -α is a negative angle. The measurement point Pm is preferably located in the reference mark <NUM>.

The absolute value of the second angle -α and the first angle ϕ may be equal. However, the first angle ϕ of the optical axis A is a positive angle.

Within the context of this application, a positive angle results from counterclockwise rotation from the vertical axis V. Consequently, a negative angle results from clockwise rotation from the vertical axis V.

As illustrated in <FIG>, a blank <NUM> with a reflective surface is illuminated by the first illumination unit <NUM>. The second angle -α of the first illumination unit <NUM> is selected such that incident light rays from the first illumination unit <NUM> are directed to the reference mark <NUM> and specular reflected light rays from the reference mark <NUM> are directed into an entrance pupil <NUM> of the camera lens <NUM>.

Coatings such as varnishes are highly reflective, which makes them difficult to detect without creating a "mirror-reflection" effect into the entrance pupil <NUM> of the camera lens <NUM>. These types of coatings generate a specular reflection when illuminated.

The first illumination unit <NUM> is configured to emit diffused light rays which are directed towards the reference mark <NUM> from multiple directions. This ensures that some reflected specular light rays are received through the entrance pupil <NUM> of the camera lens <NUM>. The first illumination unit <NUM> comprises at least one light source <NUM> and a diffusive layer <NUM>. The diffusive layer <NUM> is positioned over the at least one light source <NUM>. The diffusive layer <NUM> is configured to scatter the transmitted light rays from the light source <NUM> and provide a homogenous radiating surface of diffused light. The diffusive layer <NUM> can be made from an optically diffusive material, such as Polymethylmethacrylat.

In the illustrated embodiment, the first illumination unit <NUM> is configured such that only light rays which are reflected by a portion of the field of view <NUM> on the blank <NUM> are received through the entrance pupil <NUM> of the camera lens <NUM>. This portion is referred to as reflective illuminated area Ria. Hence, the reflective illuminated area Ria on the blank <NUM> is of a smaller surface area than area of the field of view <NUM> on the blank <NUM>. The reference mark <NUM> thus needs to be placed in the reflective illuminated area Ria of the field of view <NUM> on the blank when an image of a reflective reference mark <NUM> is captured by the camera <NUM>.

The camera <NUM> can be triggered by a time signal from the control unit <NUM> which is issued when the optical sensor <NUM> registers a detection of the front leading edge <NUM> of the blank <NUM>. The signal can be set to a time which corresponds to an arrival time of the reference mark <NUM> at the reflective illuminated area Ria of the field of view <NUM>.

In an embodiment, the first illumination unit <NUM> can be elongated with a plurality of light sources <NUM> arranged side by side. The longitudinal extension of the illumination unit <NUM> is arranged perpendicular in relation to the direction of transportation T. The longitudinal direction of the light sources is also arranged perpendicular in relation to the direction of transportation T and coincides with a longitudinal extension of the reference mark <NUM> on the blank1.

The light sources <NUM> can be arranged in a row or in a plurality of rows. The light sources <NUM> may be arranged on a printed circuit board (PCB). The distances between the light sources are selected such that a homogeneous illumination of the diffusive layer <NUM> is obtained.

When the inspection device <NUM> is mounted in the converting machine <NUM>, the camera axis A is arranged at the first angle ϕ in relation to the vertical axis V. The horizontal axis is defined by the printed surface on the blank <NUM> and the vertical axis is perpendicular thereto. The first angle ϕ enables the camera <NUM> to capture specular light rays reflected at an angle from the reference mark <NUM>. The first angle ϕ may be between <NUM>° and <NUM>°, and preferably about <NUM>°.

In a preferred embodiment, a second illumination module <NUM> is also provided. The second illumination module <NUM> is configured to illuminate printed colors which generate a diffuse reflection when illuminated with specular light rays.

These types of colors include for instance water-based or solvent-based inks. Due to the diffuse reflection of the light rays from the reference mark <NUM>, the camera <NUM> will receive reflected light rays into the entrance pupil <NUM> of the camera lens <NUM>. The second illumination module <NUM> is configured to provide a homogeneous illumination of the reference mark <NUM> on the blank <NUM>.

As best seen in <FIG> and <FIG>, which illustrate a reflective surface on a blank <NUM> which is illuminated with the second illumination module <NUM>. The second illumination module <NUM> comprises at least one illumination unit <NUM>, <NUM>, <NUM> which is arranged to emit light at a third angle β in relation to the vertical axis V defined by a normal vector N of the surface of the blank <NUM>. The third angle β is selected such that incident light rays from the at least one illumination unit <NUM>, <NUM>, <NUM> of the second illumination module <NUM> are directed to the reference mark <NUM> and specular reflected light rays from the reference mark <NUM> are directed outside the entrance pupil <NUM> of the camera lens <NUM>. This enables the camera <NUM> to capture a sharp image of the reference mark <NUM> without glare. Hence, when illuminating a reflective surface, the specular reflected light rays are not received in the entrance pupil <NUM> of the camera lens <NUM>. The full field of view <NUM> on the blank <NUM> may be illuminated by the second illumination module.

The at least one illumination unit <NUM>, <NUM>, <NUM> can be elongated and may comprise a plurality of light sources <NUM> arranged in a line. The longitudinal extension of the at least one illumination unit <NUM>, <NUM>, <NUM> is arranged perpendicular in relation to the direction of transportation T of the blank <NUM>.

The at least one illumination unit <NUM>, <NUM>, <NUM> of the second illumination module <NUM> may comprise a continuous line of light sources <NUM> arranged at a constant distance from each other. Alternatively, the at least one illumination unit <NUM>, <NUM>, <NUM> may only comprise light sources <NUM> located at the extremities of the line. In such a way, the light sources <NUM> are arranged in a square around the camera <NUM>.

In an embodiment, an additional second illumination unit <NUM> is arranged on an opposite side of the optical axis A of the camera <NUM> in relation to the first illumination unit <NUM>. In such a way, a further improved and homogenous illumination of the field of view <NUM> on the blank <NUM> can be achieved. In an embodiment, a third illumination unit <NUM> is further provided on at least one of the sides of the camera <NUM>.

Each illumination unit <NUM>, <NUM>, <NUM> may be configured to emit light towards the blank <NUM> at a different third angle β. Hence, in the illustrated example in <FIG>, there are three illumination units <NUM>, <NUM>, <NUM> and their respective angles of emitted light can be referred to as β1, β2, β3. These angles are selected such that the specular reflection of light rays is directed outside of the entrance pupil of the camera <NUM>. As long as the reflected light rays are not received in the entrance pupil <NUM> of the camera <NUM>, the angles β1, β2, β3 may all be different.

The first illumination module <NUM> and the second illumination module <NUM> can be operated at the same time, whereby the camera <NUM> captures one image of the reference mark <NUM>. Alternatively, either the first illumination module <NUM> or the second illumination module <NUM> is operated and an image can be captured by the camera <NUM>. In another embodiment, only one of a plurality of illumination units <NUM>, <NUM>, <NUM> of the second illumination module <NUM> is operated.

For inks generating a diffuse reflection when illuminated, the first illumination module <NUM> can be disabled. Depending on the color and reflective characteristics of the coatings, it can thus be sufficient to only illuminate the reference mark <NUM> with the second illumination module <NUM>. In such a way, reflections from reflective surfaces on the blank can be avoided. This is illustrated in <FIG> where the blank <NUM>, when illuminated with the first illumination module <NUM>, generates reflections in smooth areas which have been unintentionally rubbed by friction in the converting machine <NUM>. In <FIG>, the blank <NUM> is only illuminated with the second illumination module <NUM> and displays less reflections.

The light intensity from the first <NUM> and second illumination modules <NUM> can be varied. This makes it possible to adapt the illumination settings depending on the reference mark characteristics. Especially for reflective coatings (inks or varnishes), the illumination can be calibrated to obtain a detectable reflection.

As best seen in <FIG> and <FIG>, the camera <NUM> is mounted inside an external housing shroud <NUM> of the inspection device <NUM>. On a top side of the housing shroud <NUM>, a cover <NUM> is provided. The cover <NUM> is provided with a transparent surface <NUM>, such as a glass surface <NUM>. The external housing shroud <NUM> is designed to provide a hermetically sealed housing which is arranged around and encloses the camera <NUM>. The level of sealing level may for instance be IP64.

The external housing shroud <NUM> may comprise a wall <NUM> with varying thickness. The varying thickness enable a larger traversing wall for fasteners <NUM> and provides rigidity to the wall <NUM>. The wall <NUM> of the housing shroud <NUM> may further comprise a biased section <NUM>, which forms the first angle ϕ with the longitudinal extension of the external housing shroud <NUM>. This allows the optical axis A of the camera <NUM> to form the first angle ϕ with the vertical axis V. The vertical axis V coincides with the longitudinal direction of the external housing shroud <NUM>, and the camera <NUM> is thus directed through an opening <NUM> in the cover <NUM> arranged in-between the illumination modules <NUM>, <NUM>.

A thermoelectric element <NUM> is arranged between the camera <NUM> and the external housing shroud <NUM>. The thermoelectric element <NUM> can be a Peltier element <NUM>. The camera <NUM> comprises an optical module 49a and an electronic processing module 49b. The electronic processing module 49b comprises electronic parts that are heat sensitive. The camera <NUM> is preferably arranged within the inspection device <NUM> such that the electronic parts are arranged close to the thermoelectric element <NUM>. In such a way, the electronic processing module 49b is thermally connected to the thermoelectric element <NUM>.

An isolating inner housing <NUM> is arranged inside the external housing shroud <NUM> and is configured to enclose the camera <NUM>. The inner housing <NUM> may comprise a first housing part 80a arranged around the electronic processing module 49b of the camera <NUM>. A second housing part 80b can be arranged around the optical module 49a of the camera <NUM>. The second housing part 80b can be tubular.

The first housing part 80a may comprise a recess <NUM>, in which the second housing part 80b is partially received. This allows for a modular design and access to the optical module 49a of the camera <NUM> without dismantling the first housing part 80a.

As best seen in <FIG>, the first housing part 80a comprises an isolating portion <NUM> and a thermally conductive portion <NUM>. The thermally conductive portion <NUM> comprises a heat-conductive plate <NUM>, for instance a metal plate. For example, the heat-conductive plate <NUM> can be made from aluminum or silver. The thermoelectric element <NUM> is positioned between the heat-conductive plate <NUM> and the external housing shroud <NUM>. The heat-conductive plate <NUM> distributes and spreads the cold from the thermoelectric element <NUM> to the electronic processing module 49b of the camera <NUM>. The camera is fixed to the external housing shroud <NUM> by at least one fastener <NUM>. In the illustrated embodiment, a plurality of fasteners <NUM>, such as four fasteners <NUM> connect the inner housing <NUM> of the camera <NUM> to the external housing shroud <NUM>.

The thermoelectric element <NUM> produces a warm side and a cold side when a current is applied over the two sides. Consequently, the cold side of the thermoelectric element <NUM> is in contact with the thermally conductive plate <NUM> and the warm side of the thermoelectric element <NUM> is in contact with the external housing shroud <NUM>. In such a way, the processing module 49b of the camera <NUM> is cooled, while the external housing shroud <NUM> can be used to transfer the heat away from the thermoelectric element <NUM>.

As best seen in <FIG>, the inspection device <NUM> may be placed under a vacuum transfer <NUM> of the flexographic printing module <NUM>. Alternatively, the vacuum transfer can be located below the inspection device <NUM>. The vacuum suction force from the vacuum transfer <NUM> induces an airflow over the external housing shroud <NUM> which provides a heat transfer to the ambient air.

A dryer module <NUM> may be located after the flexographic printing module <NUM> to ensure that the ink is dried before the blank <NUM> travels to subsequent modules such as a die-cutting or a folding module. The dryer module <NUM> operates by blowing hot air at the printed side of the blank <NUM>.

Claim 1:
An inspection device (<NUM>) for checking the position of at least one coating on a blank (<NUM>) transported through a converting machine (<NUM>), the inspection device comprising a camera (<NUM>) configured to capture an image of a portion of the blank provided with a reference mark (<NUM>) comprising at least one coating,
wherein an optical axis (A) of the camera is arranged at a first angle (ϕ) in relation to a vertical axis (V) defined by a normal vector (N) of a surface of the blank (<NUM>), and wherein the inspection device comprises an illumination system (<NUM>) comprising a first illumination module (<NUM>) comprising at least one illumination unit (<NUM>),
and wherein the illumination unit (<NUM>) is configured to emit incident light rays towards a measuring point (Pm) on the surface of blank <NUM>, said emitted light rays forming a second angle (-α) in relation to the vertical axis (V),
and wherein the first and second angles (ϕ, -α) are selected such that incident light rays from the illumination unit (<NUM>) are directed to the reference mark and specular reflected light rays from the reference mark are captured by the camera