Omnidirectional lenses for low cost laser scanners

Transcription

1 Omnidirectional lenses for low cost laser scanners M. Aikio, VTT Technical Research Center of Finland Abstract There is a need for small sensor that could provide 360-degree field of views in the intelligent vehicle applications. The usual technique has been to use a catadioptric system where a conical shaped mirror is placed in front of a camera, providing 360-degree horizontal field of view and of order tens of degrees of vertical view. The downside of these kinds of systems has been their size, usually ranging around 20 centimetres. A so-called omnidirectional lens can fold the optical path inside the lens decreasing the volume requirements considerably, while still providing comparative optical performance. In this work, two different omnidirectional lens systems are presented, more common type of this lens images a whole surrounding scenery to a image sensor, providing instant 360-degree field of view. The other lens can select a known position from the 360-degree scenery, and provide an undistorted image of it. The other application for this type of lens is laser scanner that necessitates direction selectivity. 1 Short survey of omnidirectional vision sensors Omnidirectional vision systems are not a new invention; the usual approach to expand the field of view of a known camera lens has been to place a conical or hyperbolical mirror in front of the camera [1-3]. Outside of scientific publications, Olympus has several press releases dating from 2008 that show a combined camera and a lens comprising of several refractive and reflective surfaces to provide omnidirectional vision, but it is unknown for the author if this system is currently on the markets. The author would like to point out that there is some engineering or scientific interest behind the 360-degree vision systems, evidenced by frequent surveys to omnidirectional vision systems [4-5] and omnidirectional workshops (Omnivis) in conferences like ICCV 2005 and ECCV For the surprise of the author of this paper, the exact dimensions of the sensors using a conical mirror are usually not mentioned in the publications, but their field of views are. The other interesting note is that the when such a catadioptric component is placed in front of a camera lens, the F-number of the camera used in the tests and measurements is often not mentioned in the texts. The importance of the F-number, or the light gathering capability, relates to the achievable frame rate in varying illumination conditions if the sensor is operating in a moving system. The reason behind this is likely related to the optical design procedure of the conical mirrors and the conditions placed on the camera lens during the design process: more often than not, the camera lens aperture is described as a pinhole. There is no mention of the prices of hyperbolic or, in general, conical mirrors that are used in the systems. The other standard technique is to use a fish-eye lens in front of a camera, but these systems usually suffer from relatively low amount of information content at the horizontal plane - this region is more useful in vehicle applications - while a large fraction of the image is used by what is directly above or below the fish-eye lens. In addition, it should be noted that when using a fish-eye lens in daylight conditions and when the lens is pointed upwards, the sun is very likely in the field of view of the sensor, possibly complicating the exposure control. There are several patents relating to the omnidirectional lens systems presented in this paper [6-10], especially Ehud Gal et al. describe several omnidirectional vision systems that are in principle, the same as the first case presented in this paper. The other example of similar lens is shown in [7], where an omnidirectional lens is used to track incoming projectiles. The second case presented in this paper, we believe, is unique and has not been published formerly. 2 Developed omnidirectional lens systems In this paper, we present two different omnidirectional lens systems, the first one similar to former work as discussed earlier and provides an image of the surrounding scenery for the image sensor. The second type of an omnidirectional lens uses a beam steering mirror in order to select the scanning angle, and it could be used to get an undistorted image from a known direction. The other application of this lens is laser scanners, where direction selectivity is very important.

2 2.1 Omnidirectional lens with an image sensor Originally initiated and developed in 2006 for mobile conferencing applications, the omnidirectional lens was designed to be used with a mobile phone camera, providing overall small vision system module. Figure 1 represents the working principle of the omnidirectional lens, while a picture of the lens module is shown in Figure 2. A picture taken with this lens is represented in Figure 3, and shows typical distortion patterns for this type of lens. The lens was designed in VTT, and manufactured by diamond turning by a Danish company Kaleido Technologies. The resulting omnidirectional lens diameter is roughly 33 mm, and the height is roughly 25 mm, including the mobile phone camera. The vertical field of view is 30 degrees, and 360 degrees horizontal, without any shadowing support mechanics. The omnidirectional lens tilts and preserves the collimation state of the incoming beam, and the mobile phone camera itself is used for focusing. The illumination levels are limited by the camera lens aperture stop, the design aperture was F/2.8. The physical aperture diameter of the camera in the design is 1.75 mm, and the focal length was 4.95 mm and full field of view is 40 degrees. The diameter of the collimated pencil of rays that strikes the cylindrical part of the omnidirectional lens is averagely 0.5 millimeters per field point, when the target is assumed to be far. The vertical field of view is designed for video conferencing, so that the lens system does not see below horizontal plane, as the phone was assumed to be located on a table. The 30 degree vertical FOV allows for photographing persons around the table, if they are sitting about 1.5 meters from the mobile phone. The lens material is ZEONEX E48R, which is a plastic that has a good environmental resistance. The lens has been designed injection molding process in mind, which enables low cost manufacturing of optical components. In orders of pieces, the price of a single lens is usually counted in cents. Figure 1. The working principle of the omnidirectional lens. Under the main lens there is a wavefront correcting element, and only the aperture stop of the camera lens is shown below. Figure 2. Omnidirectional lens placed on top of a mobile phone camera. The system size is easily understood from the image.

3 There is an additional wavefront correcting element in the omnidirectional lens system as shown in Figure 1, it is placed under the omnidirectional lens and just above the mobile phone camera aperture. The role of this lens is to improve the incoming wavefront so that a better quality image can be obtained and it is not necessary for all applications, depending on the needed image quality and camera optics. In this case, the mobile phone camera had a sensor of 1280 x 960 pixels, and it was determined that in order to keep the drawing capability of the mobile phone camera unaffected by the omnidirectional lens, and additional lens was required. Our experiences with this kind of combination of a mobile phone camera (or similar) and the omnidirectional lens are that this configuration will allow for a small surround vision sensor size. There are some issues when directly attaching the omnidirectional lens on top of a mobile phone camera with intelligent exposure control; even inside a building, it is common to find a window or other brightly lit area that will dominate the image exposure, leaving other regions slightly under-exposed. When considering the applications of this type of sensor for example in a vehicle, the important consideration is that the sun is surprisingly often in the direct field of view (Fig. 3) of the camera in the northern latitudes and it is our recommendation that this effect should be considered in the design phase. Software correction of this could be possible, but during the project, this was judged to be outside of the scope of the omnidirectional lens research. Figure 3. A picture captured by the omnidirectional lens, seen directly from the camera before the polar transformation or other image correction algorithms. The sky is often slightly overexposed in pictures taken with this camera. Third important consideration is the sensor type itself. It the sensor were constructed with a mobile phone camera or similar, and the application would be a vehicle moving with higher speeds, it is important to select a sensor that does not have a rolling shutter. Otherwise as with normal cameras, there will be additional image distortion caused by the shutter. 2.2 Omnidirectional lens with direction selectivity The development history of this lens is related to laser scanner applications, and the general objective of the current Minifaros-project is to replace a large rotating mirror from laser scanners with a MEMS mirror. Instead of imaging a whole scenery around the lens, a rotating mirror is used to select a portion of the scenery to be imaged on the sensor or to be measured with a laser scanner. Without this property, the laser scanner would not be possible. This kind of lens is new to author s knowledge, and no prior art work has been published of it. The working principle of the lens is shown in Figure 4, and one manufactured lens is shown in Figure 5.

4 Figure 4. A sketch of an omnidirectional lens that has a beam direction capability. The tilted plane below the lens is a beam steering mirror, that is used to deflect and rotate the scanning position. This provides a 360 degree scanning capability. The lens diameter is roughly 50 millimetres and the height is roughly 25 millimetres. The outgoing beam is slightly elliptical, resulting in divergences of 30 mrad x 22 mrad with a circular receiver of 200 µm in diameter. The beam area on the cylindrical surface is 23.8 mm 2, which gives the physical limit for power that can be collected on the receiver. When the diameter of the source is less than the diameter of the receiver, the resulting divergence is improved, which allows slightly better resolution. The lens can be used both in biaxial and coaxial configurations, depending on the needs of the application. Figure 5. A manufactured omnidirectional lens which is used in conjunction with a beam steering mirror. A biaxial laser scanner consisting of two lenses as shown in Figure 5 was constructed, and the performance was evaluated. The divergence of the sensor was 30 milliradians with a detector of diameter 200 µm. The signal to noise ratio allowed the usage of the sensor up to 10 metres, with a black diffuse target. Expanding the measurement distance from this is one of the objectives in Minifaros project.

5 3 Importance of omnidirectional sensor Omnidirectional vision and sensor systems are important in autonomous vehicle operation if the amount of sensors needs to be reduced. By using a large field of view sensor, there is no need to have multiple sensors in a vehicle. However, one constraint on using them has been the size, manufacturing tolerances and the price of the resulting system. In this work, we have presented two separate omnidirectional vision systems that are small and easier to install for vehicle and robot applications; the first one uses a mobile phone camera to reduce the size of the omnidirectional vision system. The second lens is used in laser scanner application with a rotating beam steering mirror. This type of omnidirectional lens would also allow imaging of the surrounding scenery without distortion if multiple exposures are taken and the avalanche photo diode is replaced with a small image sensor. The second important factor to consider is the price of the sensor and related optics. Because the omnidirectional lenses presented in this work are roughly 40 to 50 millimetres in diameter and are made of plastic to allow for easier serial production of this type of optics. In serial productions when the production volume approaches hundreds of thousands of pieces per year, the price for a single omnidirectional lens is around several cents. In Minifaros project, the omnidirectional lens is used in a laser scanner application (LIDAR) to prevent and mitigate the consequences of vehicle accidents. Acknowledgements MiniFaros is part of the 7th Framework Programme, funded by the European Commission. The partners thank the European Commission for supporting the work of this project. References [1] Hrabar, S., Sukhatme, G., Omnidirectional Vision for an Autonomous Helicopter, Proceedings of IEEE International Conference on Robotics and Automation, , [2] de Souza, G., Motta, J., Simulation of an omnidirectional catadioptric vision system with hyperbolic double lobed mirror for robot navigation, ABCM Symposium Series in Mechatronics, Volume 3, , [3] Lima, A., et al., Omni-directional catadioptric vision for soccer robots, Robotics and Autonomous Systems, Volume 36, , [4] Fernando, C. et al, Catadioptric Vision Systems: Survey, Proceedings of the Thirty- Seventh Southeastern Symposium on System Theory, , [5] Yagi, Y., Yokoya, N., Omnidirectional Vision: A Survey On Sensors and Applications, Transactions of Information Processing Society of Japan, Volume 42, 1-18, [6] Gal, E. et al, Self-contained panoramic or spherical imaging device, USP B2, [7] Agurok, I. et al, Passive Electro-Optical Tracker, USP A1, [8] Ge, Z., Mohcitate, S., Omnidirectional Imaging Apparatus, USP A1, [9] Gal, E., et al, Omni-directional imaging and illumination assembly, USP B2, [10] Ito, M., Murayama, O., Omni directional photographing device, USP A1, 2008.

6 Mika Aikio Kaitoväylä 1, P.O.Box OULU Oulu Finland Keywords: omnidirectional vision, catadioptric lens, laser scanner