Application note Fast focusing for machine vision applications With Optotune s focus tunable lens EL C

Transcription

1 Application note Fast focusing for machine vision applications With Optotune s focus tunable lens EL C Page 1 of 8

2 Introduction Flexible and fast focusing solutions with electrically tunable lenses are highly beneficial for machine vision applications, like e.g. quality control, packet sorting, bar code reading, etc. In traditional setups, the adjustment for objects at different working distances is realized mechanically. In contrast, with Optotune s tunable lenses, refocusing can be achieved in milliseconds without any mechanical translation involved, enabling compact, robust and dust-free optical systems. Besides focusing solutions, focus-tunable lenses can also be used for adaptive illumination with highly flexible lighting parameters reducing the number of light sources and the overall power consumption. The C-mount version of the EL lens, the EL C, is compatible with standard industrial cameras. Furthermore, an offset lens can be integrated in the lens to adjust the focal length range to the desired values. With a positive power offset-lens, the doublet system can achieve short focal lengths and can be used directly in combination with standard industrial cameras. With a negative power offset-lens, the focal range includes (zero diopters), such that the EL C can be placed in front of existing optics as a close-up lens for applications requiring high optical quality. Various configurations for different specifications are described in more detail in this application note. Offset lens In the EL C, the protective cover glass can be replaced by an offset lens of 12-mm or 12.7-mm diameter. This allows shifting the focal length range to any desired value. For example, the EL C-VIS-LD with a normal planar cover glass has a standard focal range of +200 mm down to +80 mm. The integration of an offset lens with f = -150 mm will yield a focal length range of about -600 mm to to +170 mm (1/f res = 1/f EL /f offset ). The design of the EL C is optimized for good alignment of the tunable lens and the offset lens. The maximally allowed thickness of the offset lens is 4.7 mm. Page 2 of 8

3 Close-up configuration: EL C in combination with fixed focal length lens The EL C with an integrated plano-concave offset lens can be used as a close-up lens in combination with a fixed focal length lens to achieve an electronically controllable focusing unit with a high optical quality. Figure 1 shows a comparison between a traditional focusing setup and a combination including a lens with a variable focal length. With the traditional approach with a fixed focal length lens only, one or several lenses have to be mechanically translated to achieve focusing for different working distances. In contrast, in combination with a tunable lens, all the elements stay at a fixed position. Only the curvature of the tunable lens and hence the focal length is varied to achieve focusing for different working distances. Optotune, EL C Fixed focal length lens Fixed focal length lens only Sensor WD 200 mm: f EL : 107 mm WD : f EL : 200 mm Figure 1: With a tunable lens refocusing for different working distances (WD) is achieved by only changing electronically the focal length of the lens, without any translational movement An example of such a configuration is shown in Figure 2. The cover glass of the EL C is replaced by a plano-concave offset lens with a focal length of mm and orientated with the flat side facing the tunable lens. The EL C is mounted to the fixed focal length lens. Depending on the filter thread of the fixed focal length lens, an adapter is required between the EL C with C-mount thread and the fixed focal length lens. Offset lens Tunable lens Camera sensor ½ Sensor camera (IDS UI-3580CP-C-HQ) 9 cm Fixed focal length lens (Edmund Optics, NT85-358) Thread adapter EL C Fixed focal length lens Tunable lens Figure 2: Example of a combination EL C with an integrated plano-concave offset lens, a fixed focal length lens (focal length 25 mm) and a camera (1/2 sensor) for an electrically controllable focusing unit 3 cm Page 3 of 8

4 Such a combination can exhibit a high optical quality. Figure 3 shows a comparison between images taken with the fixed focal length lens only (mechanical focusing) and in combination with the EL C showing that the combination with the electrically tunable lens achieves the same results as the fixed focal length lens only. The field of view of this configuration is 14.5 (for 1/2 sensor sizes) and the f-number is 4. Fixed focal length lens only With EL C Working distance 58 cm Working distance 17 cm Figure 3: Close up configuration: comparison between the optical qualities achieved with a fixed focal length lens only (Edmund Optics, NT85-358) and in combination with the EL C for different working distances To prevent vignetting of the outer fields, it is recommended to use a fixed focal length lens with a short distance between the stop position and the front of the lens (see Figure 4). Vignetting is also an issue in case of standard wide field lenses with short focal lengths and a large field of view >20 (for ½ sensor sizes) (e.g. Edmund Optics, NT54-857, FOV: 19.6, focal length: 18 mm). However, there are extraordinary compact wide field lenses available which are ideally suited for combinations with the EL C (e.g. Lensation, Lensagon B5M12028, FOV: 32.8 (for ½ sensor), focal length: 12 mm), see Figure 5. Stop Stop EL C Suited fixed focal length lens Not suited fixed focal length lens Figure 4: Long distances between the stop and the front of the fixed focal length lens are unfavorable because of vignetting Page 4 of 8

5 ½ Sensor camera C- to S-Mount adapter (Lensation, AD04M) Fixed focal length lens (Lensation, B5M12028) Tunable lens 3 cm Working distance 47 cm 6.5 cm Figure 5: Very compact wide field configuration with the EL C and a compact fixed focal length lens (Lensation, Lensagon B5M12028) with a field of view of 32.8 (for ½ sensor) and high optical quality In Table 1, recommended fixed focal length lenses with different field of views ranging from 6 up to 33 are listed, which are suited to combine with the EL C lens. The field of view corresponds to the horizontal full field of view for a standard 4:3, ½ sensor format. For certain configurations (e.g. with Schneider-Kreuznach, ), the EL C can be directly mounted to the filter threading of the fixed focal length lens. In case of compact fixed focal length lenses (S-Mount, M12 lenses), the EL C can be mounted to the housing of the camera (e.g. with Lensation, B5M12028). It is indicated in the overview table if a customized thread adapter is required to mount the EL C. Page 5 of 8

6 Field of view* Product 32.8 Lensation, B5M12028 B5M12040 B5M Qioptiq, Qioptiq, Tamron, 23FM25SP 14.5 Schneider- Kreuznach, Edmund, NT NT NT NT Edmund, NT Edmund, NT NT NT Qioptiq, Edmund, NT NT NT Edmund, NT Qioptiq, Qioptiq, Edmund, NT Tamron, 23FM50SP 6.2 Edmund, NT Focal length Working distance** 12 mm 30 mm mm 25 mm - 25 mm 30 mm - 25 mm 70 mm mm 50 mm - 25 mm 20 mm - 25 mm 20 mm - 25 mm 60 mm mm 27.7 mm 40 mm - 35 mm 50 mm - 35 mm 50 mm - 36 mm 45 mm mm 50 mm - 35 mm 90 mm - 50 mm 90 mm 50 mm 100 mm - *Horizontal FOV on standard 4:3, ½ sensor format Aperture (f/#) up to up to up to up to up to closed 5 up to 32 5 up to 22 Adapter Lensation AD04M Edmund, and Customized thread adapter required Maximal sensor size no 1/2 no 1/1.8 no 1/2 yes 1/2 **Distance between object and front of the tunable lens Table 1: Recommended fixed focal length lenses for different field of views, focal lengths and f-numbers Page 6 of 8

7 Doublet configuration: EL C for camera objectives The EL C with an integrated plano-convex offset lens can be used as a doublet system directly in combination with standard industrial cameras. Focal lengths as short as 16 mm and reasonably good optical quality are achieved for sensor sizes up to ½. Figure 6 shows the setup with recommendations on offset lenses for different distances between the EL C and the camera sensor. An aperture defining the stop of the object is added in front of the lens. The cover glass is replaced by a plano-convex offset lens orientated with the curved side facing the tunable lens. By changing the current applied to the tunable lens, refocusing for different working distances from 100 mm to can be achieved. Aperture Retainer ring Offset lens Tunable lens Sensor D: Distance to sensor Distance D Focal length offset lens 12.5 mm 20 mm 17.5 mm 30 mm 25 mm 42 mm Offset lens Edmund Optics, NT Edmund Optics, NT Edmund Optics, NT Field of view* ¼ : 11.2 ½ : 22.3 ¼ : 8.8 ½ : 17.4 ¼ : 6.6 ½ : 13.1 *Full horizontal field of view on standard 4:3 sensor format EL C with integrated plano-convex lens Figure 6: Doublet system with EL C with recommended planoconvex offset lenses for different distances between EL C and sensor Resulting focal length range 16 to 18 mm 22 to 26 mm 28 to 35 mm Working distance 54 cm Working distance 17 cm Working distance 7 cm Figure 7: Doublet configuration: images for different working distances with the EL C with an integrated plano-convex offset lens (focal length 30 mm), with an f-number of 5 and a C-mount camera with a ½ sensor size Wavefront quality dependent on orientation and focal length range In principle, Optotune s focus tunable lenses exhibit a spherical lens shape (the nominal parameters can be found in the ZEMAX package, which is available for download on As the membranes used are elastic, the lens shape is influenced by gravity. With the lens lying horizontally (optical axis vertical) the RMS wavefront error of the EL C Series lenses is currently in the order of 0.1 λ (measured at 525 nm). With the lens standing upright (optical axis horizontal) a Y-coma term must be added. Therefore, for high quality imaging, using the EL C with optical axis vertical is recommended. Furthermore, as can be seen in the graph below, the lens exhibits a smaller wavefront error when it is less curved corresponding to longer focal lengths. Therefore, it is recommended to set the position of the fixed Page 7 of 8

8 focal length lens such that the tunable lens can be operated in the long focal length range corresponding to low current values. RMS error [λ] Gravity induced Y-coma of about 0.2 λ EL C-VIS-LD, Optical axis horizontal EL C-VIS-LD, Optical axis vertical Figure 8: Wavefront measurement of a typical EL C-VIS-LD (at 525 nm and 80% of clear aperture, defocus, tilt & sphere excluded) The gravity induced Y-coma term depends on the size of the lens, the density of the liquid and the mechanical properties of the membrane. While it is insignificant with lenses of apertures below 5 mm, it accounts for about 0.2 λ for the EL C. Stronger membranes can be used to reduce Y-coma, however, this is at the expense of total focal tuning range. Optotune is working on improved materials and lens designs to reduce the dependence on gravity. Response time The rise time on a current step is about 2.5 ms. However, it takes about 15 ms until the lens has fully settled. Please see EL C datasheet for detailed information including a graph with the optical response of the EL to a current step. For faster response times, please also consider the smaller electrical lens EL-6-18, which is about 50% faster. Coating options The deformable surface of Optotune s focus tunable lenses cannot be coated with standard processes as it needs to remain flexible. This means about 3-4% of reflection at the air/membrane interface must be expected. The cover glasses, however, can be coated adequately. The EL C is available with two standard broadband coatings for VIS and NIR, but also narrow band coatings are available (e.g. for 355 nm or 1064 nm, see EL C datasheet for detailed transmission curves). Controlling the EL C Focal length [mm] The EL C is current controlled (0 300 ma). Please refer to the EL operating instructions for advice on available drivers. For simple focusing applications, a calibration using a look-up table is sufficient to achieve good repeatability. If the heating of the lens is significant, then sensing the temperature is recommended to calculate more precise look-up values. The EL C has a temperature sensor and 256 bytes of memory built into the lens. Page 8 of 8