Method for determining stray light artifacts of an optical device and measuring apparatus

Abstract

The invention relates to a method for determining stray light artifacts of an optical device (102), wherein the method can be carried out using a measuring apparatus (100), wherein the method comprises a step of outputting light (106) from a light source (104) through a diffractive optical element (108) to the optical device (102), wherein the diffractive optical element (108) is arranged between the light source (104) and the optical device (108) in order to diffract the light (106) into a plurality of partial beams (110), each of which is assigned an order of diffraction, and a step of reading in image data from the optical device (102) via an interface, wherein the image data represent partial beams of the plurality of partial beams (110) that have been captured by means of the optical device (102), wherein the captured partial beams form a projected diffraction pattern (114) by projection onto an image plane, and a step of evaluating the projected diffraction pattern (114) using a reference diffraction pattern in order to determine the stray light artifacts.

Description

[0001] Description

[0002] title

[0003] Method for determining stray light artifacts of a tear and

[0004] State of the art

[0005] The invention is based on a device or method according to the class of the independent claims. The present invention also relates to a computer program.

[0006] The object detection performance of cameras used in advanced driver assistance systems (ADAS) is highly influenced by the contrast ratios in the captured images. Contrast is typically determined by the luminance of various objects and their background in a scene, but can also be reduced by unwanted scattering or reflections in an optical system. Light that does not follow the system's intended optical path is referred to as stray light. Typical scenarios in which the contrast of an image captured by an ADAS camera can be affected by stray light include at night with oncoming traffic with headlights, or at sunrise and sunset when the sun is close to the horizon.

[0007] Test methods for stray light testing are described, for example, in ISO standards 9358 and 18844, as well as in the IEEE P2020 standard. The stray light testing methods described in these standards can be divided into two categories based on the type of light excitation. In the integral method, the camera is illuminated by a broad, uniform light source with dark areas in between, while in the analytical method, the camera is illuminated by a light source with a narrow angular range in a dark environment.

[0008] Disclosure of the invention

[0009] Against this background, the approach presented here presents a method, a control unit that uses this method, and finally a corresponding computer program according to the main claims. The measures listed in the dependent claims enable advantageous further developments and improvements of the device specified in the independent claim.

[0010] The presented approach describes a possibility for reducing cycle times in a stray light test. Furthermore, the presented approach can advantageously generate a large number of collimated partial beams, which can simultaneously inspect a large number of positions of an optical device to be inspected and thus detect stray light artifacts. Furthermore, the presented approach can reduce costs and simplify the design of a measuring device. Since the presented approach can also be used for geometric camera calibration, no additional hardware is required, which also enables cost savings.

[0011] A method for determining stray light artifacts of an optical device is presented, wherein the method can be carried out using a measuring device. The method comprises a step of outputting light from a light source of the measuring device through a diffractive optical element of the measuring device to the optical device. The diffractive optical element is arranged between the light source and the optical device in order to diffract the light into a plurality of partial beams, each of which is assigned a diffraction order. The method further comprises a step of reading in image data via an interface of the optical device, wherein the image data represents partial beams of the plurality of partial beams detected by means of the optical device. The detected partial beams form a projected diffraction pattern by projection onto an image plane.In an evaluation step, the projected diffraction pattern is evaluated using a reference diffraction pattern to determine the stray light artifacts.

[0012] By means of the method, the optical device can advantageously be checked for irregularities which can be attributed, for example, to a damaged or contaminated surface. The optical device can also be referred to as a test object and implemented, for example, as a camera or as a lens. The optical device can advantageously be coupled to the measuring device so that the optical device can be checked by the measuring device. The optical device can be arranged in an optical path of the measuring device for checking. In addition, the optical device can be connected to a control unit of the measuring device. The optical device can have a detection device for detecting the partial beams and, additionally or alternatively, the projected diffraction pattern.The diffractive optical element (DOE) can, for example, be arranged directly in front of the optical device. The light source can, for example, be in the form of a laser diode, which can be designed to emit the light in the form of a collimated laser beam. The majority of the partial beams of the light can advantageously have the same properties as the light before it hits the DOE. The DOE can advantageously change only one radiation direction of the light, so that the partial beams advantageously form a light cone. This in turn means that each diffraction order of the partial beams can advantageously be identical in phase, wavelength, and polarization, and only the beam direction and amplitude can be changed. The projected diffraction pattern can advantageously be a Fraunhofer diffraction pattern.The projected diffraction pattern can be compared with the reference diffraction pattern during the evaluation step. This means that the reference diffraction pattern can, for example, represent an optimum for the optical device, according to which the stray light behavior of the optical device can be determined. Advantageously, this allows many positions of the optical device, such as a lens or aperture, to be checked for stray light using a single image.

[0013] According to one embodiment, in the outputting step, the light can be output as a collimated light beam to the diffractive optical element in order to diffract the light into a plurality of collimated partial beams, each of which is assigned a diffraction order. Advantageously, this allows scattered light to be determined for a plurality of different positions of the optical device.

[0014] Furthermore, in the outputting step, the light can be output and diffracted such that a diameter of the light incident on the optical device can be larger than a diameter of an input opening of the optical device. This advantageously allows the entire input opening of the optical device to be checked for unevenness or irregularities.

[0015] Irregularities are checked. The entrance opening can, for example, be a camera aperture through which the light enters the optical device.

[0016] According to one embodiment, the light can be output as a laser beam in the outputting step. This advantageously enables highly precise determination of the scattered light artifacts. Thus, a collimated light beam and a plurality of collimated partial beams can be advantageously generated.

[0017] Furthermore, in the outputting step, the light can be output through the diffractive optical element to the optical device and diffracted at a maximum diffraction angle that can be smaller than a field of view of the optical device. This means that, for example, the light can strike the optical device in a light cone that can be smaller than the entrance opening in order to be able to check an area of ​​the optical device for the presence of stray light. The method can comprise a step of causing a relative movement between the diffractive optical element and the optical device, wherein in a repeated input step, further image data can be input, which can represent further partial beams of the plurality of partial beams detected by means of the optical device.The captured additional partial beams can be projected onto the image plane to form a further diffraction pattern projected by the relative movement. In a repeated evaluation step, the projected additional diffraction pattern can be evaluated using the reference diffraction pattern to determine the stray light artifacts. The movement can advantageously be caused by or be a rotation of the DOE and, additionally or alternatively, the optical device relative to one another. This advantageously allows previously untested areas of the optical device to be checked for existing stray light artifacts. This can be particularly advantageous if the light can be diffracted by the DOE with a maximum diffraction angle that is smaller than the field of view of the optical device.

[0018] This method can be implemented, for example, in software or hardware or in a mixed form of software and hardware, for example in a control unit.

[0019] The approach presented here further provides a control unit configured to perform, control, or implement the steps of a variant of a method presented here in corresponding devices. This embodiment of the invention in the form of a control unit also allows the problem underlying the invention to be solved quickly and efficiently.

[0020] For this purpose, the control unit can have at least one computing unit for processing signals or data, at least one memory unit for storing signals or data, at least one interface to a sensor or an actuator for reading sensor signals from the sensor or for outputting control signals to the actuator, and / or at least one communication interface for reading or outputting data embedded in a communication protocol. The computing unit can be, for example, a signal processor, a microcontroller, or the like, wherein the memory unit can be a flash memory or a magnetic storage unit.The communication interface can be designed to read in or output data wirelessly and / or wired, wherein a communication interface that can read in or output wired data can read this data, for example, electrically or optically from a corresponding data transmission line or output it to a corresponding data transmission line.

[0021] In this context, a control unit can be understood as an electrical device that processes sensor signals and outputs control and / or data signals depending on them. The control unit can have an interface that can be implemented in hardware and / or software. In a hardware implementation, the interfaces can, for example, be part of a so-called system ASIC, which contains a wide variety of functions of the control unit. However, it is also possible for the interfaces to be separate integrated circuits or to consist at least partially of discrete components. In a software implementation, the interfaces can be software modules that are present, for example, on a microcontroller alongside other software modules.

[0022] Furthermore, a measuring device for determining scattered light artifacts of an optical device is presented, wherein the measuring device comprises a light source for emitting light, a diffractive optical element for diffracting the light into a plurality of partial beams, each of which is assigned a diffraction order, wherein the diffractive optical element is arranged between the light source and the optical device, and a control unit in a previously mentioned variant. The control unit is coupled to the light source and can be coupled to the optical device via the interface.

[0023] If the control unit is coupled to the light source, this means that the control unit and the light source are connected to each other in a way that enables signal transmission. If the control unit can be coupled to the optical device via the interface, this means that the control unit and a detection device of the optical device can be connected to each other in a way that enables signal transmission. The measuring device can, for example, be designed as a maintenance tool that can be used to monitor the functionality of the optical device or to check whether the optical device meets specified criteria.

[0024] According to one embodiment, the diffractive optical element can comprise a polymer. A suitable choice of material for the DOE can advantageously reduce manufacturing costs. Furthermore, the choice of material can depend, for example, on the application of the optical device.

[0025] Furthermore, the diffractive optical element can have a maximum diffraction angle that can be smaller than the field of view of the optical device. This can advantageously enable the use of a more cost-effective DOE.

[0026] Also advantageous is a computer program product or computer program with program code that can be stored on a machine-readable carrier or storage medium such as a semiconductor memory, a hard disk memory or an optical memory and is used to carry out, implement and / or control the steps of the method according to one of the embodiments described above, in particular when the program product or program is executed on a computer or a device.

[0027] Examples of the approach presented here are shown in the drawings and explained in more detail in the following description. It shows:

[0028] Fig. 1 is a schematic representation of an embodiment of a measuring device;

[0029] Fig. 2 is a schematic representation of a reference diffraction pattern;

[0030] Fig. 3 is a schematic representation of a diffraction pattern; Fig. 4 is an enlarged schematic representation of a diffraction pattern;

[0031] Fig. 5 is a schematic representation of a diffraction pattern with stray light artifacts;

[0032] Fig. 6 is an enlarged schematic representation of a diffraction pattern with stray light artifacts;

[0033] Fig. 7 is a flowchart of an embodiment of a method for determining stray light artifacts of an optical device; and

[0034] Fig. 8 is a block diagram of a control unit according to an embodiment.

[0035] In the following description of advantageous embodiments of the present invention, the same or similar reference numerals are used for the elements shown in the various figures and having a similar effect, whereby a repeated description of these elements is omitted.

[0036] First, however, the background will be briefly outlined to better understand the topic. Sources of stray light are reflections and scattering of the incident light on the various optical surfaces in a camera lens system. While the former are primarily due to the optical design of the system and its coating, the latter are primarily caused by damage to the optical surfaces of the lenses in the system. This damage includes scratches and dents, for example, but also contamination with small scattering particles or liquids such as grease. To ensure that all parts with visible stray light artifacts are rejected during camera production, a stray light test is performed at the end of a production line.Because such damage or contamination is highly random and unpredictable, a large number of inspection positions in the image, for example, using a high sampling rate, is required to evaluate the performance of a camera and its production processes. The following description describes such an approach.

[0037] Fig. 1 shows a schematic representation of an embodiment of a measuring device 100. The measuring device 100 is designed to determine scattered light artifacts of an optical device 102. For this purpose, the measuring device 100 has a light source 104 for emitting light 106, a diffractive optical element 108, also referred to herein as a DOE, and a control unit 112. The optical device 102 is, for example, a camera.

[0038] The diffractive optical element 108 is configured to diffract the light 106 into a plurality of partial beams 110, each of which is assigned a diffraction order. The diffractive optical element 108 is arranged between the light source 104 and the optical device 102. The DOE 108 optionally comprises a polymer and additionally or alternatively has a maximum diffraction angle that is smaller than a field of view of the optical device 102. Advantageously, the diffraction angle is larger than the field of view of the optical device 102. If the diffraction angle is smaller, this can be compensated by multiple image acquisition.

[0039] The control unit 112 is coupled to the light source 104 and can be coupled to the optical device 102 via an interface. The measuring device 100 is used, for example, for a method for determining stray light artifacts of the optical device 102, as mentioned and / or described in more detail in at least one of the following figures. The method is carried out and / or controlled in corresponding units of the control unit 112. This means that, according to this exemplary embodiment, a test setup is shown in which the optical device 102, in particular a lens or an objective thereof, is examined for stray light artifacts. This is done, for example, by the optical device 102 detecting the plurality of partial beams 110 and projecting them onto an image plane.This makes a diffraction pattern 114 visible, for example, which is evaluated by the control unit 112 according to the method described in Figure 7. In other words, the approach described here enables a measurement of scattered light using the DOE 108. A space- and cost-saving method for generating many collimated beams with different propagation directions is possible through the use of a DOE 108. When a monochromatic plane wave, such as a collimated laser beam as the light 106, strikes the DOE 108, the incoming beam is diffracted into several diffraction orders by the precisely manufactured microstructure of the DOE 108. Each diffraction order or each partial beam 110 is a copy of the incoming beam 106 and has, among other things, the same intensity profile, the same wavelength and / or the same wavefront.Only the propagation direction changes depending on the periodicity of the microstructure. On a sensor or screen located, for example, in the far field, the Fraunhofer diffraction pattern 114 is visible. Using powerful optical field simulation tools, it is possible to calculate the required microstructure to generate any desired diffraction pattern 114. This results in the diffraction pattern 114 being projected onto the image plane. Without stray light artifacts, the plane waves of each diffraction angle are projected onto the image plane as individual points, as shown in Fig. 2.

[0040] The diameter of the incident laser beam is larger than the camera's entrance aperture or aperture. If this is the case, a damaged lens in the beam path will result in stray light artifacts, as shown in Figures 5 to 6. If each diffraction order is a collimated light beam entering the lens system, the resulting image is a superposition of several different test positions. In this way, using the DOE 108, it is possible to easily test several hundred diffraction orders with a single image acquisition. This results in very short cycle times combined with excellent coverage of the entire field of view. In addition, the DOE 108, used for a DOE-based approach to intrinsic calibration, can also be used for stray light testing.For example, the DOE 108 covers an entire field of view with high-density diffraction orders, meaning a small angular separation between the diffraction orders. In this case, it may be an option to use the DOE 108 with a maximum diffraction angle that is smaller than the field of view of the camera or optical device 102. By rotating the camera 102 in front of the DOE 108 or vice versa and capturing multiple images, the entire field of view can be covered with a high density of test positions.

[0041] Fig. 2 shows a schematic representation of a reference diffraction pattern 200. The reference diffraction pattern 200 is, for example, stored on a control unit or accessible to a control unit, as described in Figure 1, and can be used, for example, for a method for determining stray light artifacts, as described in Figure 7. This means that the reference diffraction pattern 200 is similar to the diffraction pattern 114 mentioned in Fig. 1, wherein the reference diffraction pattern 200 represents a diffraction pattern achieved under optimal conditions without stray light artifacts.

[0042] In Fig. 2, a multitude of points 202 arranged in a matrix are visible, each representing a diffraction order of a partial beam. Scattered light artifacts are not detectable.

[0043] Figures 3 to 6 show diffraction patterns as false-colored camera scattered light images, which are created, for example, by irradiating two cameras with several laser beams generated by a DOE.

[0044] Fig. 3 shows a schematic representation of a diffraction pattern 300 similar to the diffraction pattern mentioned in Fig. 1. The diffraction pattern 300 shown here is an example result obtained using a measuring device as also described in Fig. 1. Based on the diffraction pattern 300 shown here, it is possible to conclude that the optical device to be tested is intact, since no stray light artifacts are detectable. All diffraction orders represented as points are sharp, bright spots. The light intensity between the diffraction orders is very low. The brightest spot 302 in the center, the zero diffraction order, is very large but has a sharp edge and thus no stray light artifacts.

[0045] Fig. 3 shows an example result of a “good” sample or an artifact-free diffraction pattern 300, which can also be used as a reference diffraction pattern, for example.

[0046] Fig. 4 shows an enlarged schematic representation of a diffraction pattern 300, which corresponds to the diffraction pattern in Fig. 3. Here, only the diffraction orders represented as points are enlarged to clarify that no stray light artifacts are recognizable here either.

[0047] In other words, this is a close-up of the diffraction pattern 300 described in Fig. 3.

[0048] Fig. 5 shows a schematic representation of a diffraction pattern 500, similar to the diffraction pattern described in Fig. 3, for example, with stray light artifacts 502. This shows an image captured by an optical device exhibiting a clearly visible stray light artifact 502. In other words, a strong beam is shown emerging from the zero diffraction order located centrally in the image. The higher diffraction orders, previously referred to as spots and attributable to the partial beams, also show signs of stray light artifacts, which are further illustrated in Fig. 6.

[0049] Fig. 5 shows a generated image or diffraction pattern 500 of a camera with a surface scratch.

[0050] Fig. 6 shows an enlarged schematic representation of a diffraction pattern 500 with stray light artifacts, as described, for example, in Fig. 5. Since the edges of the points are only blurred, it is also possible to infer the presence of stray light artifacts here. In other words, Fig. 6 shows a close-up of the image or diffraction pattern 500 generated by the scratched camera. Fig. 7 shows a flowchart of an embodiment of a method 700 for determining stray light artifacts of an optical device. The method 700 is controlled and / or carried out, for example, by a control unit of a measuring device, as described, for example, in Fig. 1 and / or mentioned in at least one of Figs. 1 to 6. The method 700 comprises an output step 702, a reading step 704, and an evaluation step 706.Only optionally additionally, the method 700 comprises a step 708 of effecting.

[0051] In step 702 of outputting, light, such as a laser beam, is output from a light source of the measuring device through a diffractive optical element of the measuring device to the optical device in order to diffract the light into a plurality of partial beams, each of which is assigned a diffraction order. The diffractive optical element is arranged between the light source and the optical device. In step 704 of reading, image data is read in from the optical device via an interface, wherein the image data represents partial beams of the plurality of partial beams detected by the optical device. The detected partial beams form a projected diffraction pattern, which is, for example, a Fraunhofer diffraction pattern, by projection onto an image plane.In step 706 of evaluation, the projected diffraction pattern is evaluated using a reference diffraction pattern to determine the stray light artifacts.

[0052] Only optionally, in step 708 of causing, a relative movement between the diffractive optical element and the optical device is additionally caused. Then, in a repeated step 704 of reading, further image data are read in, representing further partial beams of the plurality of partial beams detected by the optical device, wherein the detected further partial beams form a further diffraction pattern projected by the relative movement by projection onto the image plane. In a repeated step 706 of evaluating, the projected further diffraction pattern is evaluated using the reference diffraction pattern to determine the scattered light artifacts. In this case, step 702 of outputting is continuously executed.

[0053] According to one embodiment, in step 702 of outputting, the light is output as a collimated light beam to the diffractive optical element in order to diffract the light into a plurality of collimated sub-beams, each of which is assigned a diffraction order. For example, the light is output and diffracted such that a diameter of the light incident on the optical device is larger than a diameter of an input aperture or aperture of the optical device.

[0054] Alternatively, the light is output through the diffractive optical element to the optical device and diffracted at a maximum diffraction angle that is smaller than a field of view of the optical device. In this case, the optional effecting step 708 is triggered or performed, in which a relative movement, such as a rotational movement, is effected between the diffractive optical element and the optical device. In the repeated reading step 704, for example, further image data is then read in, which represent further partial beams of the plurality of partial beams detected by means of the optical device, wherein the detected further partial beams form a further diffraction pattern projected by the relative movement by projection onto the image plane.In the repeated step 706 of evaluation, the projected further diffraction pattern is further evaluated using the reference diffraction pattern in order to determine the scattered light artifacts.

[0055] For example, the approach explained here receives a very large number of collimated laser beams generated by DOEs and checks a very large number of test positions within the camera's field of view for stray light artifacts. Furthermore, the acquisition of a single image covers the entire camera field of view with a high spot density, allowing for very short cycle times for stray light testing. The requirements for the DOE itself are not very high, which allows for the use of inexpensive polymer DOEs. This allows for a simple, small, and cost-effective setup. Compared to visual inspection, this method enables a reliable, algorithm-based approach for detecting stray light artifacts.

[0056] Fig. 8 shows a block diagram of a control unit 112 according to an exemplary embodiment, which corresponds, for example, to the control unit 112 described in Fig. 1 and is designed to control and / or carry out a method as described in Fig. 7. The control unit 112 has an output unit 800 which outputs light from a light source 104 of the measuring device through a diffractive optical element of the measuring device to the optical device 102, wherein the diffractive optical element is arranged between the light source 104 and the optical device 102 in order to diffract the light into a plurality of partial beams, each of which is assigned a diffraction order.Furthermore, the control unit 112 has a reading unit 802 for reading image data 804 via an interface 805 from the optical device 102, wherein the image data 804 represents partial beams of the plurality of partial beams captured by the optical device 102. The captured partial beams then form a projected diffraction pattern by projection onto an image plane. Furthermore, the control unit 112 has an evaluation unit 806 for evaluating the projected diffraction pattern using a reference diffraction pattern 200 to determine the scattered light artifacts.

[0057] Optionally, the control unit 112 additionally has an actuating unit 808 for effecting a relative movement between the diffractive optical element and the optical device 102, for example, using a control signal 810. In this case, the read-in unit 802 is configured to read in further image data 812 representing further partial beams of the plurality of partial beams detected by the optical device 102, wherein the detected further partial beams form a further diffraction pattern projected by the relative movement by projection onto the image plane, wherein the evaluation unit 806 is consequently configured to evaluate the projected further diffraction pattern using the reference diffraction pattern 200 in order to determine the scattered light artifacts.If an embodiment comprises an “and / or” link between a first feature and a second feature, this is to be read as meaning that the embodiment according to one embodiment has both the first feature and the second feature and according to another embodiment has either only the first feature or only the second feature.

Claims

Claims 1. A method (700) for determining scattered light artifacts of an optical device (102), wherein the method (700) is executable using a measuring device (100), wherein the method (700) comprises the following steps: Outputting (702) light (106) from a light source (104) of the measuring device (100) through a diffractive optical element (108) of the measuring device (100) to the optical device (102), wherein the diffractive optical element (108) is arranged between the light source (104) and the optical device (108) in order to diffract the light (106) into a plurality of partial beams (110), each of which is assigned a diffraction order; Reading (704) image data (804) via an interface from the optical device (102), wherein the image data (804) represent partial beams of the plurality of partial beams (110) detected by the optical device (102), wherein the detected partial beams form a projected diffraction pattern (114; 300; 500) by projection onto an image plane; and Evaluating (706) the projected diffraction pattern (114; 300; 500) using a reference diffraction pattern (200) to determine the stray light artifacts.

2. The method (700) according to claim 1, wherein in the outputting step (702), the light (106) is output as a collimated light beam to the diffractive optical element (108) to diffract the light (106) into the plurality of collimated sub-beams (110), each of which is assigned a diffraction order.

3. The method (700) according to any one of the preceding claims, wherein in the outputting step (702), the light (106) is output and diffracted such that a diameter of the light (106) incident on the optical device (102) is larger than a diameter of an input opening of the optical device (102).

4. The method (700) according to any one of the preceding claims, wherein in the outputting step (702) the light (106) is output as a laser beam.

5. The method (700) according to any one of the preceding claims, wherein in the outputting step (702), the light (106) is output through the diffractive optical element (108) to the optical device (102) and is diffracted at a maximum diffraction angle that is smaller than a field of view of the optical device (102).

6. The method (700) according to claim 5, comprising a step (708) of causing a relative movement between the diffractive optical element (108) and the optical device (102), wherein in a repeated step (704) of reading in, further image data (812) are read in, which represent further partial beams of the plurality of partial beams (110) detected by means of the optical device (102), wherein the detected further partial beams form a further diffraction pattern projected by the relative movement by projection onto the image plane, wherein in a repeated step (706) of evaluating, the projected further diffraction pattern is evaluated using the reference diffraction pattern (200) in order to determine the scattered light artifacts.

7. Control device (112) which is configured to execute and / or control the steps (702, 704, 706, 708) of the method (700) according to one of the preceding claims in corresponding units (800, 802, 806, 808).

8. A measuring device (100) for determining scattered light artifacts of an optical device (102), the measuring device (100) having the following features: a light source (104) for emitting light (106); a diffractive optical element (108) for diffracting the light (106) into a plurality of partial beams (110), each of which is assigned a diffraction order, the diffractive optical element (108) being arranged between the light source (104) and the optical device (102); and a control unit (112) according to claim 7, the control unit (112) being coupled to the light source (104) and being capable of being coupled to the optical device (102) via the interface.

9. Measuring device (100) according to claim 8, wherein the diffractive optical element (108) comprises a polymer 10. Measuring device (100) according to one of claims 8 to 9, wherein the diffractive optical element (102) has a maximum diffraction angle that is smaller than a field of view of the optical device (102).

11. A computer program configured to execute and / or control the steps (702, 704, 706, 708) of the method (700) according to any one of claims 1 to 6.

12. A machine-readable storage medium on which the computer program according to claim 11 is stored.

Images