Light beam splitter assembly and colorimeter comprising such an assembly

Abstract

A spot measurement colorimeter includes a light beam splitter assembly comprising at least a first beam splitter, being a polka dot beam splitter or, alternatively, a light beam splitting optical plate or cube, and a second beam splitter, being a polka dot beam splitter or, alternatively, a light beam splitting optical plate or cube, arranged in series with the first beam splitter. A light beam incident on the first beam splitter is partially reflected by the first beam splitter as a first light beam and partially transmitted through the first beam splitter as a second light beam. The second light beam incident on the second beam splitter is partially reflected by the second beam splitter as a third light beam and partially transmitted through the second beam splitter as a fourth light beam.

Description

[0001] P36878PCOO / CHO

[0002] Title: Light beam splitter assembly and colorimeter comprising such an assembly

[0003] In the field of visual display manufacturing product testing is an important aspect. During the production process, different display technologies, like LCD and LED panels, are subject to variations in color, luminance and flicker levels. To guarantee a constant quality of the displays at the end of the production line it is necessary to check for and correct eventual changes and variations in the parameters concerned such as color intensity and luminance levels.

[0004] One type of display measurement is a spot measurement to determine color or light intensity. For this purpose it is known to apply a colorimeter. Known colorimeters use a lens assembly to catch the light from a spot on the display. The incoming light is split into multiple sections via different optical fibre light guiding lines which guide the light to different light sensors corresponding to different color channels or by segmented sensors directly. A problem of these known devices is that the optical fibres “lose” light due to the behaviour of the fibre, the acceptance angle and “empty” sections between fibres. This decreases the sensitivity of the colorimeter, which becomes in particular a problem when displays have to be measured at lower light emitting levels. A known solution for this problem is using larger lenses to catch the light and measure larger spots (e.g. 27 mm) on the display. Another option is to increase the lens acceptance angle. In general these known solutions are aiming at increasing the amount of light received and thereby increase the overall sensitivity of the spot measurement device (i.e. the colorimeter).

[0005] A drawback of the above-mentioned solutions is that larger and / or more complex lenses are required, which increase the costs of the optical components. For some display technologies larger acceptance angles can also cause measurement deviations.

[0006] Furthermore, in some applications a smaller spot size is required. If a spot with a smaller spot size - for example of 5 mm or smaller - at a low luminance level has to be measured with the known solutions, the measurement period has to be increased, thus decreasing the measurement speed. The latter has a slowing effect on the production and testing line which is disadvantageous. Another effect is that at low luminance levels less accurate or reliable measurement data is obtained which may negatively affect the testing quality. The present invention has for an object to provide an improvement in spot measurement of displays at lower luminance levels.

[0007] This object is achieved by a light beam splitter assembly comprising at least a first polka dot beam splitter and a second polka dot beam splitter arranged in series with the first polka dot beam splitter such that a light beam incident on the first polka dot beam splitter is partially reflected by the first polka dot beam splitter as a first light beam and partially transmitted through the first polka dot beam splitter as a second light beam, and such that said second light beam incident on the second polka dot beam splitter is partially reflected by the second polka dot beam splitter as a third light beam and partially transmitted through the second polka dot beam splitter as a fourth light beam.

[0008] The beam splitter assembly according to the invention advantageously loses less light than the known separations with optical fibres. Thereby the light measurement device in which the beam splitting assembly is incorporated can have a sensitivity which is 3-5 times higher than the optical fibre solution.

[0009] The first light beam, the third light beam and the fourth light beam generated in the light beam splitter assembly according to the invention are directed on respective light sensors. The respective light sensors may be adapted to detect light within a wavelength range, e.g. by positioning an optical bandpass filter in front of each of the light sensors. The bandpass filters are then configured to transmit light within the wavelength range associated with the corresponding light sensor. There may thus be a bandpass filter for an X-, a bandpass filter for an Y- and a bandpass filter for a Z-wavelength range of a tristimulus (XYZ) color space.

[0010] In an embodiment of the beam splitter assembly according to the invention, the first polka dot beam splitter is configured to reflect 1 / 3 of the incident light and to transmit 2 / 3 of the incident light, and the second polka dot beam splitter is configured to reflect 1 / 2 of the incident light and to transmit 1 / 2 of the incident light. In this embodiment the first light beam, the third light beam and the fourth light beam, possibly corresponding to three color channels (e.g. X, Y and Z) receive the same amount of light, which is 1 / 3 of the original light beam.

[0011] In an embodiment of the beam splitter assembly according to the invention the first polka dot beam splitter and the second polka dot beam splitter each are polka dot mirrors provided with holes in a polka dot pattern. The polka dot mirrors are mirrors with a predefined surface portion (%) of “holes” in them, where the light is not reflected but transmitted through the mirror. Thereby the polka dot mirrors act in a similar way to a beamsplitter. The holes are uniformly spread over the surface, whereby the light transmission / reflection is evenly spread over the surface, so this does not influence sample uniformity.

[0012] A polka dot beam splitter may for example also have a transmissive substrate with reflective dots in a polka dot pattern thereon.

[0013] As an alternative for a polka dot beam splitter it is also possible to use a beam splitter which is not provided with a pattern of reflective areas and a pattern of transmissive areas, but which reflects light over the entire surface and at the same time transmits light over the entire surface. To this end a transparent optical substrate is provided with specific coatings which control the specific reflection and transmission of light. The coatings allow light to be reflected and transmitted over a broad range of wavelengths. Such a beam splitter achieves the same as a polkadot beam splitter, i.e. a part of a light beam (e.g. 50%) reflects from the beam splitter and the other part of the light beam is transmitted through the beam splitter.

[0014] One example of such a beam splitter is a non-polarizing beam splitter.

[0015] This may be a non-polarizing beam splitter plate, which includes an optical substrate provided with a partially reflective dielectric coating applied to one surface. The coating is chosen such that the incident light is reflected by it and is transmitted through it over the whole relevant spectrum. An anti-reflection coating may be applied on the opposite surface of the substrate to prevent unwanted secondary reflections.

[0016] Another example is a non-polarizing cube beam splitter. This may include two right-angle optical (transparent) prisms that are positioned against each other along their hypotenuse faces (thereby forming a cube). A dielectric coating is applied to the interface between the prisms to control how light is split.

[0017] The invention also relates to a colorimeter comprising a light beam splitter as described in the above.

[0018] In particular the invention relates thus to a colorimeter comprising:

[0019] - a housing, - a lens assembly for receiving light from a light emitting object to be measured, such as a display, and creating a light beam to be guided into the housing,

[0020] - at least three light sensors arranged in the housing for measuring light intensity incident on the respective light sensor,

[0021] - a light beam splitter assembly arranged in the housing for splitting and guiding the light beam from the lens assembly to the at least three respective light sensors, wherein the light beam splitter assembly comprises at least a first polka dot beam splitter and a second polka dot beam splitter arranged in series with the first polka dot beam splitter such that a light beam incident on the first polka dot beam splitter is partially reflected by the first polka dot beam splitter as a first light beam and partially transmitted through the first polka dot beam splitter as a second light beam, and such that said second light beam incident on the second polka dot beam splitter is partially reflected by the second polka dot beam splitter as a third light beam and partially transmitted through the first polka dot beam splitter as a fourth light beam.

[0022] In particular the colorimeter according to the invention may be a spot measurement colorimeter.

[0023] Preferably, the colorimeter is configured and arranged to perform spot measurements on visual displays.

[0024] In this colorimeter the light beam splitter assembly may, instead of polka dot beam splitters, be provided with the beam splitters as described in the above, which beam splitters include an optical substrate, such as an optical plate or cube having a coated surface. The coating reflects light over the entire coated surface and at the same time allows light to be transmitted over the entire coated surface.

[0025] In an embodiment of the colorimeter each of the at least three separate light sensors is adapted to detect light within a wavelength range.

[0026] In a further embodiment each or part of the at least three separate light sensors is associated with a separate color channel, i.e. the wavelength ranges associated with the at least three separate light sensors are different.

[0027] In a practical embodiment three of the color channels correspond to the X, Y and Z wavelength ranges of a tristimulus (XYZ) color space. The XYZ color space is the XYZ color space defined by the CIE, the International Commission on Illumination. In a further embodiment the colorimeter has at least four light sensors, wherein next to the sensors associated with the three color channels corresponding to said X, Y and Z wavelength ranges, a sensor associated with an additional color channel is included.

[0028] The additional color channel may correspond to an additional X, Y or Z wavelength range. For example there may be two color channels Xi and X2, together covering the X wavelength range, next to the usual Y and Z color channels. The X wavelength range contains two “bumps” in its graph. When using an Xi channel and an X2 channel, each of those channels can be adapted to fit to one of the bumps.

[0029] In another embodiment the colorimeter has at least four light sensors, wherein next to the sensors associated with the three color channels corresponding to said X, Y and Z wavelength ranges, a sensor configured as a radiance meter is included. With a radiance meter the integral amount of light over the whole (relevant) spectral range is determined without diversifying over colors.

[0030] In embodiments of the colorimeter an optical bandpass filter is positioned in front of at least a part of the light sensors, said bandpass filter being configured to transmit light within the wavelength range associated with the corresponding light sensor. In case there are only three sensors, there is positioned a filter in front of each of the sensors. In case there is an additional color channel used, a bandpass filter may be positioned in front of the additional sensor. In case a sensor is to be used as a radiance meter, a bandpass filter may be omitted in front of that particular sensor.

[0031] Preferably, the lens assembly is configured and arranged to observe a spot on the display, such that a so called spot measurement can be performed with the colorimeter. The lens assembly can be focussed on a spot on a display to be tested and catches the light of the spot on the display.

[0032] The colorimeter may be configured and arranged to perform spot measurements with a spot size smaller than 27 mm. However, the colorimeter according to the invention can also be configured and arranged to perform relatively large spot measurements of 27 mm and larger for some applications.

[0033] In a practical embodiment the colorimeter according to the invention is configured and arranged to perform spot measurements on visual displays with a spot size within a range of 1-11 mm, for example a spot size of 10 mm, 5 mm, 2 mm or 1 mm. The invention also relates to a testing line for visual displays, wherein a colorimeter as described in the above is provided to perform spot measurements on the displays.

[0034] The invention also relates to a method for spot measurement of a display using a colorimeter as defined in the above.

[0035] The invention will be further elucidated in the following description with reference to the drawing, in which:

[0036] Fig. 1 shows a schematic cross section of an embodiment of a colorimeter including a beam splitter arrangement according to the invention,

[0037] Fig. 2 shows schematically an embodiment of a colorimeter according to the invention having four light sensors and three beam splitters.

[0038] In Fig. 1 is shown a colorimeter 1. The colorimeter 1 has a housing 2. A lens assembly 3 is arranged on one side 21 of the housing 2. The lens assembly 3 is configured and arranged to receive light from a light emitting object to be measured. In a practical application, such a light emitting object could for example be a display of for example a laptop computer, a tablet computer or a smartphone. The lens assembly 3 creates a light beam that is guided into the housing 2.

[0039] The lens assembly 3 may be configured and arranged to observe a spot on the display, such that a so called spot measurement can be performed with the colorimeter 1. The configuration of the lens assembly 3 determines the acceptance angle and the spot size. In some applications a relatively small spot size, e.g. of 10 mm or smaller is used. In other applications a relatively large spot size is used, e.g. of 27 mm.

[0040] Within the housing 2 three light sensors 41, 42, 43 are arranged. In a practical embodiment these sensors 41, 42, 43 each include a photodiode. Optical elements are positioned in front of the sensors to condition the light beam that is incident on the sensor. These optical elements at least include respective optical bandpass filters 51, 52, 53 in front of the respective light sensors 41 , 42, 43. Moreover the optical elements includes respective focussing lenses 61 , 62, 63 and 71, 72, 73, which focus the light beam transmitted through the respective filters 51, 52, 53 on the respective sensors 41, 42, 43. The colorimeter 1 also comprises a light beam splitter assembly 8 comprising a first polka dot beam splitter 81 and a second polka dot beam splitter 82 arranged in series with the first polka dot beam splitter 81. A light beam incident on the first polka dot beam splitter 81 is partially reflected by the first polka dot beam splitter 81 as a first light beam 91 and partially transmitted through the first polka dot beam splitter 81 as a second light beam 92. The second light beam 92 incident on the second polka dot beam splitter 82 is partially reflected by the second polka dot beam splitter 82 as a third light beam 93 and partially transmitted through the second polka dot beam splitter 82 as a fourth light beam 94.

[0041] The first polka dot beam splitter 81 and the second polka dot beam splitter 82 each are polka dot mirrors provided with holes in a polka dot pattern.

[0042] The first polka dot beam splitter 81 is configured to reflect 1 / 3 of the incident light and to transmit 2 / 3 of the incident light, and the second polka dot beam splitter 82 is configured to reflect 1 / 2 of the incident light (i.e. second beam 92), and to transmit 1 / 2 of the incident light (i.e. second beam 92). The first light beam 91, the third light beam 93 and the fourth light beam 94 thus each make up 1 / 3 of the initial light beam incident on the first polka dot mirror 81. Thus each of the filters 51, 52, 53 positioned in front of the respective sensors 41, 42, 43 receives 1 / 3 of the initial light.

[0043] The first light beam 91 is directed by a mirror 10 to the first sensor 41.

[0044] It is noted here that the polka dot beam splitters 81 and 82 in the colorimeter 1 of Fig. 1 can be replaced respectively by another light beam splitting optical plate, such as a non-polarizing beam splitter plate. The configuration of the colorimeter 1 does not change of such a replacement, i.e. the drawing of Fig. 1 would remain substantially the same, as a skilled person will understand. Non-polarizing beam splitter plates are as such known in the art and include an optical substrate provided with a partially reflective dielectric coating applied to one surface. The coating is chosen such that the incident light is reflected by it and is transmitted through it over the whole relevant spectrum. An anti-reflection coating may be applied on the opposite surface of the substrate to prevent unwanted secondary reflections.

[0045] Alternatively, the polka dot beam splitters 81 and 82 in the colorimeter 1 of Fig. 1 could be replaced respectively by a light beam splitting cube, and in particular by a non-polarizing cube beam splitter. Such a cube beam splitter generally takes up more space than a light beam splitting plate. Thus, the configuration and arrangement of the components may stay essentially the same as is shown in Fig. 1, but the housing and spaces between components might need increased dimensions. Non-polarizing cube beam splitters are as such known in the art and include two right-angle optical (transparent) prisms that are positioned against each other, and usually cemented along their hypotenuse faces, thereby forming a cube. A dielectric coating is applied to the interface between the prisms to control how light is split. On the outer surfaces of the cube an anti-reflection coating may be applied to prevent unwanted secondary reflections.

[0046] The non-polarizing beam splitter plates or the non-polarizing beam splitting cubes may be made such that they have the same beam splitting ratio as was described for the polka dot beam splitters 81 and 82. Thus a first non-polarizing beam splitter plate or non-polarizing beam splitting cube is configured to reflect 1 / 3 of the incident light and to transmit 2 / 3 of the incident light, and the second non-polarizing beam splitter plate or non-polarizing beam splitting cube is configured to reflect 1 / 2 of the incident light, and to transmit 1 / 2 of the incident light.

[0047] It is also noted that within the scope of the invention it is conceivable to combine different types of beam splitters in one colorimeter. As non-exhaustive examples can be mentioned a combination of a polka dot beam splitter and a non-polarizing cube beam splitter, or a combination of a polka dot beam splitter and a non-polarizing beam splitter plate, or a combination of a non-polarizing beam splitter plate and a non-polarizing cube beam splitter.

[0048] In a practical embodiment the optical bandpass filters 51, 52, 53 correspond to an X, Y and Z filter, respectively, of a tristimulus color space (CIE XYZ color space). In such an embodiment the sensor 41 for example detects the luminance in the X wavelength range, the sensor 42 detects the luminance in the Y wavelength range and the sensor 43 detects the luminance in the Z wavelength range. The sensors 41, 42 and 43 thus provide three so called color channels by which the color and light intensity of a spot on the display can be measured.

[0049] Instead of XYZ filters it is also possible to use other filters, for example RGB filters (red, green and blue filters) of an RGB color space.

[0050] In the example of the colorimeter shown in Fig. 1 there are only three sensors and two polka dot splitters present. It is however also feasible to provide a colorimeter in which for example three polka dot splitters are arranged and four sensors in a similar way as is outlined for two polka dot splitters and three sensors. In such an embodiment the first polka dot splitter reflects % of the incident light and transmits 3 / 4. The second polka dot splitter reflects 1 / 3 of the incident light and transmits 2 / 3. The third polka dot splitter reflects 14 of the incident light and transmits 14. In this way each sensor receives 14 of the original amount of light.

[0051] For example, next to the three sensors associated with the three color channels corresponding to said X, Y and Z wavelength ranges, the fourth sensor may be associated with an additional color channel. For example, the additional color channel may correspond to an additional X, Y or Z wavelength range. For example, as is illustrated in Fig. 2, there may be two color channels Xi and X2, together covering the X wavelength range, next to the usual Y and Z color channels. The X wavelength range contains two “bumps” in its graph. When using an Xi channel and an X2 channel, each of those channels can be adapted to fit to one of the bumps. The colorimeter 101 in Fig. 2 has a housing 102 and a lens assembly 103. Within the housing 102 it has for example three polka dot beam splitters 181 , 182 and 183, which divide the incoming light beam into four beams which are guided to four sensors 141 , 142, 143 and 144 associated with the color channels Xi, X2, Y and Z, respectively. Optical bandpass filters 151 , 152, 153 and 154 are arranged in front of the sensors 141 , 142, 143 and 144 , respectively.

[0052] In another example, next to the three sensors associated with the three color channels corresponding to said X, Y and Z wavelength ranges, a fourth sensor configured as a radiance meter may be included. With a radiance meter the integral amount of light over the whole (relevant) spectral range is determined without diversifying over colors. The colorimeter could essentially have the same layout as in Fig. 2, wherein the filter 151 is omitted and the sensor 141 could be the radiance meter. The sensor 142 could be the sensor for the X wavelength range. The sensors 143 and 144 remain the sensor for Y and Z channel.

[0053] It is for example also feasible to provide a colorimeter in which for example four polka dot splitters are arranged and five sensors. In such an embodiment the first polka dot splitter may for example reflect 1 / 5 of the incident light and transmit 4 / 5. The second polka dot splitter reflects 1 / 4 of the incident light and transmits 3 / 4. The third polka dot splitter reflects 1 / 3 of the incident light and transmits 2 / 3. The fourth polka dot splitter reflects 1 / 2 of the incident light and transmits 1 / 2. In this way each sensor receives 1 / 5 of the original amount of light. In this embodiment there may be, next to the Y and Z color channels, two color channels Xi and X2 as described in one of the examples above in combination with a sensor configured as a radiance meter, as described in another one of the examples above. This is the same if the other abovementioned beam splitters are used instead of the polka dot beamsplitters. It is to be noted that although practical in many applications, the end result of the light splitting not necessarily needs to be that each sensor in the light measuring device receives the same amount of light, as is the case in the examples set out in the above. Also other light distributions over the sensors are possible and can be achieved by the configuration and arrangement of different polka dot splitters. In this regard, it is for example possible to take the behaviour of the sensor for a certain color into account (i.e. the sensitivity of a photodiode for a certain color of light). With three color channels it is for example possible to guide 50% of the light to one color channel and 25% to each of the other two color channels, such that the color channels end up with the same sensitivity.

[0054] The colorimeter 1 or 101 can for example be used in a manufacturing site of visual displays, e.g. for smartphones, tablet computers, laptop computers or televisions. The colorimeter will typically be used in testing line which is arranged downstream of a production line for the displays. The colorimeter as described in the above is preferably provided to perform spot measurements on the displays. The spot measurements can be used to calibrate the individual displays such that the displays leaving the manufacturing site all have the same desired colored light emitting properties.

[0055] The invention can be summarised by the following clauses:

[0056] 1. Light beam splitter assembly comprising at least a first polka dot beam splitter and a second polka dot beam splitter arranged in series with the first polka dot beam splitter such that a light beam incident on the first polka dot beam splitter is partially reflected by the first polka dot beam splitter as a first light beam and partially transmitted through the first polka dot beam splitter as a second light beam, and such that said second light beam incident on the second polka dot beam splitter is partially reflected by the second polka dot beam splitter as a third light beam and partially transmitted through the second polka dot beam splitter as a fourth light beam.

[0057] 2. Light beam splitter assembly according to clause 1, wherein the first polka dot beam splitter is configured to reflect 1 / 3 of the incident light and to transmit 2 / 3 of the incident light, and the second polka dot beam splitter is configured to reflect 1 / 2 of the incident light and to transmit 1 / 2 of the incident light.

[0058] 3. Light beam splitter assembly according to any one of the preceding clauses, wherein the first polka dot beam splitter and the second polka dot beam splitter each are polka dot mirrors provided with holes in a polka dot pattern. 4. Light beam splitter assembly comprising at least a first light beam splitting optical plate or cube and a second light beam splitting optical plate or cube arranged in series with the first light beam splitting optical plate such that a light beam incident on the first light beam splitting optical plate is partially reflected by the first light beam splitting optical plate as a first light beam and partially transmitted through the first light beam splitting optical plate as a second light beam, and such that said second light beam incident on the second light beam splitting optical plate is partially reflected by the second light beam splitting optical plate as a third light beam and partially transmitted through the second light beam splitting optical plate as a fourth light beam.

[0059] 5. Light beam splitter assembly according to clause 4, wherein the first light beam splitting optical plate is a non-polarizing beam splitter plate.

[0060] 6. Light beam splitter assembly according to clause 4, wherein the first light beam splitting optical plate is a polka dot beam splitter.

[0061] 7. Light beam splitter assembly according to clause 4, wherein the first light beam splitting cube is a non-polarizing cube beam splitter.

[0062] 8. Light beam splitter assembly according to any one of the clauses 4-7, wherein the second light beam splitting optical plate is a non-polarizing beam splitter plate.

[0063] 9. Light beam splitter assembly according to any one of the clauses 4-7, wherein the second light beam splitting optical plate is a polka dot beam splitter.

[0064] 10. Light beam splitter assembly according to claim 4-7, wherein the second light beam splitting cube is a non-polarizing cube beam splitter.

[0065] 11. Light beam splitter assembly according to any one of the clauses 4-10, wherein the first beam splitting optical plate is configured to reflect 1 / 3 of the incident light and to transmit 2 / 3 of the incident light, and the second beam splitting optical plate is configured to reflect 1 / 2 of the incident light and to transmit 1 / 2 of the incident light.

[0066] 12. Colorimeter comprising:

[0067] - a housing,

[0068] - a lens assembly for receiving light from a light emitting object to be measured, such as a display, and creating a light beam to be guided into the housing, - at least three light sensors arranged in the housing for measuring light intensity incident on the respective light sensor,

[0069] - a light beam splitter assembly according to any one of the preceding clauses, arranged in the housing for splitting and guiding the light beam from the lens assembly to the at least three respective light sensors.

[0070] 13. Colorimeter according to clause 12, wherein each of the at least three separate light sensors is adapted to detect light within a wavelength range.

[0071] 14. Colorimeter according to clause 13, wherein each or part of the at least three separate light sensors is associated with a separate color channel, i.e. the wavelength ranges associated with the at least three separate light sensors are different.

[0072] 15. Colorimeter according to clause 14, wherein three of the color channels correspond to the X, Y and Z wavelength ranges of a tristimulus (XYZ) color space.

[0073] 16. Colorimeter according to clause 15, wherein the colorimeter has at least four light sensors, wherein next to the sensors associated with the three color channels corresponding to said X, Y and Z wavelength ranges, a sensor associated with an additional color channel is included.

[0074] 17. Colorimeter according to clause 16, wherein the additional color channel corresponds to an additional X, Y or Z wavelength range.

[0075] 18. Colorimeter according to any one of the clauses 15-17, wherein the colorimeter has at least four light sensors, wherein next to the sensors associated with the three color channels corresponding to said X, Y and Z wavelength ranges, a sensor configured as a radiance meter is included.

[0076] 19. Colorimeter according to any one of the clauses 13-17, wherein an optical bandpass filter is positioned in front of at least a part of the light sensors, said bandpass filter being configured to transmit light within the wavelength range associated with the corresponding light sensor.

[0077] 20. Testing line for visual displays, wherein a colorimeter according to any one of the clauses 12-19 is provided to perform spot measurements on the displays. 21. Method for spot measurement of a display using a colorimeter according to any one of the clauses 12-19.

Claims

CLAIMS1. Spot measurement colorimeter comprising:- a housing,- a lens assembly for receiving light from a light emitting object to be measured, such as a display, and creating a light beam to be guided into the housing,- at least three light sensors arranged in the housing for measuring light intensity incident on the respective light sensor, and- a light beam splitter assembly arranged in the housing for splitting and guiding the light beam from the lens assembly to the at least three respective light sensors, the light beam splitter assembly comprising at least a first polka dot beam splitter and a second polka dot beam splitter arranged in series with the first polka dot beam splitter such that a light beam incident on the first polka dot beam splitter is partially reflected by the first polka dot beam splitter as a first light beam and partially transmitted through the first polka dot beam splitter as a second light beam, and such that said second light beam incident on the second polka dot beam splitter is partially reflected by the second polka dot beam splitter as a third light beam and partially transmitted through the second polka dot beam splitter as a fourth light beam.

2. Colorimeter according to claim 1, wherein the first polka dot beam splitter is configured to reflect 1 / 3 of the incident light and to transmit 2 / 3 of the incident light, and the second polka dot beam splitter is configured to reflect 1 / 2 of the incident light and to transmit 1 / 2 of the incident light.

3. Colorimeter according to any one of the preceding claims, wherein the first polka dot beam splitter and the second polka dot beam splitter each are polka dot mirrors provided with holes in a polka dot pattern.

4. Spot measurement colorimeter comprising:- a housing,- a lens assembly for receiving light from a light emitting object to be measured, such as a display, and creating a light beam to be guided into the housing,- at least three light sensors arranged in the housing for measuring light intensity incident on the respective light sensor, and- a light beam splitter assembly arranged in the housing for splitting and guiding the light beam from the lens assembly to the at least three respective light sensors,the light beam splitter assembly comprising at least a first light beam splitting optical plate or cube and a second light beam splitting optical plate or cube arranged in series with the first light beam splitting optical plate such that a light beam incident on the first light beam splitting optical plate is partially reflected by the first light beam splitting optical plate as a first light beam and partially transmitted through the first light beam splitting optical plate as a second light beam, and such that said second light beam incident on the second light beam splitting optical plate is partially reflected by the second light beam splitting optical plate as a third light beam and partially transmitted through the second light beam splitting optical plate as a fourth light beam.

5. Colorimeter according to claim 4, wherein the first light beam splitting optical plate is a non-polarizing beam splitter plate.

6. Colorimeter according to claim 4, wherein the first light beam splitting optical plate is a polka dot beam splitter.

7. Colorimeter according to claim 4, wherein the first light beam splitting cube is a nonpolarizing cube beam splitter.

8. Colorimeter according to any one of the claims 4-7, wherein the second light beam splitting optical plate is a non-polarizing beam splitter plate.

9. Colorimeter according to any one of the claims 4-7, wherein the second light beam splitting optical plate is a polka dot beam splitter.

10. Colorimeter according to claim 4-7, wherein the second light beam splitting cube is a non-polarizing cube beam splitter.

11. Colorimeter according to any one of the claims 4-10, wherein the first beam splitting optical plate is configured to reflect 1 / 3 of the incident light and to transmit 2 / 3 of the incident light, and the second beam splitting optical plate is configured to reflect 1 / 2 of the incident light and to transmit 1 / 2 of the incident light.

12. Colorimeter according to any one of the preceding claims, wherein each of the at least three separate light sensors is adapted to detect light within a wavelength range.- 16 -13. Colorimeter according to claim 12, wherein each or part of the at least three separate light sensors is associated with a separate color channel, i.e. the wavelength ranges associated with the at least three separate light sensors are different.

14. Colorimeter according to claim 13, wherein three of the color channels correspond to the X, Y and Z wavelength ranges of a tristimulus (XYZ) color space.

15. Colorimeter according to claim 14, wherein the colorimeter has at least four light sensors, wherein next to the sensors associated with the three color channels corresponding to said X, Y and Z wavelength ranges, a sensor associated with an additional color channel is included.

16. Colorimeter according to claim 15, wherein the additional color channel corresponds to an additional X, Y or Z wavelength range.

17. Colorimeter according to any one of the claims 14-16, wherein the colorimeter has at least four light sensors, wherein next to the sensors associated with the three color channels corresponding to said X, Y and Z wavelength ranges, a sensor configured as a radiance meter is included.

18. Colorimeter according to any one of the claims 15-17, wherein the light beam splitter assembly includes at least a third light beam splitter, for example a polka dot light beam splitter, a light beam splitting optical plate or a light beam splitting optical cube.

19. Colorimeter according to any one of the claims 12-18, wherein an optical bandpass filter is positioned in front of at least a part of the light sensors, said bandpass filter being configured to transmit light within the wavelength range associated with the corresponding light sensor.

20. Colorimeter according to any one of the preceding claims and configured and arranged to perform spot measurements on visual displays.

21. Colorimeter according to any one of the preceding claims, wherein the lens assembly is configured and arranged to observe a spot on a display.

22. Colorimeter according to claim 20 or 21 , configured and arranged to perform spot measurements with a spot size smaller than 27 mm.

23. Colorimeter according to claim 20 or 21, configured and arranged to perform spot measurements with a spot size within a range of 1-11 mm, for example a spot size of 10 mm, 5 mm, 2 mm or 1 mm.

24. Colorimeter according to any one of the preceding claims, wherein the light sensors each include a photodiode.

25. Testing line for visual displays, wherein a colorimeter according to any one of the preceding claims is provided to perform spot measurements on the displays.

26. Method for spot measurement of a display using a colorimeter according to any one of the claims 1-24.

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