Optical design of a high resolution vision lens Paul Claassen, optical designer, paul.claassen@sioux.eu Marnix Tas, optical specialist, marnix.tas@sioux.eu Prof L.Beckmann, l.beckmann@hccnet.nl Summary: The design of a high resolution, multi-color vision lens is complicated for a large part due to the dispersive nature of glasses for different colors of light. Chromatic aberrations such as focal shift for different illumination wavelengths become dominant. Sioux CCM designed a telecentric high definition vision lens with 5 µm resolution, 9 mm field of view, a long working distance of 75 mm, low image distortion of <0.5% and fully corrected for 3 LED wavelengths: blue, green and red (457, 525 and 622nm respectively). This lens is currently produced by Sioux CCM and successfully used in a vision application. Keywords: Vision lens, High resolution, Color corrected, Zemax Introduction Modern inspection applications in e.g. printing, flat panel or life science require small feature detection. Therefore high definition lenses are available from a number of suppliers with a resolution of 5 µm. The objects in many applications are specular, i.e. the substrate reflects light like a mirror rather than diffuse. In this case a ring or dome light is less useful and coaxial illumination is applied. The new lens design includes coaxial multi-color illumination based on LED. The lens is fully color corrected for 3 wavelengths: blue 457nm, green 525nm and red 622nm. The illumination wavelength can be switch between red, green or blue without a focus adjustment or loss of image quality. The illumination design is optimized for uniform illumination. No dark corners are present in the field of view. Further, the lens is telecentric which makes the magnification constant at slight defocus. Telecentricity is required when distances are measured. The free working distance of 75 mm from the object to the first lens element makes integration in the machine convenient. Finally, the magnification can be changed from 1.5x to 0.75x for a larger field of view by changing the adapter lens in front of the camera. Lens performance specifications Requirements Configuration 1.5x Configuration 0.75x Resolution 5 µm 10 µm Object tilt < 2 deg < 2 deg Numerical aperture 0.064 0.032 Magnification 1.5x 0.75x Field of view ø9 mm ø20 mm Free working distance 75 mm Distortion 0.5% Telecentric 0.25 Camera mount C-mount LED wavelength 457 / 525 / 622 nm or white 2017 Sioux Page 1 of 10
Imaging result Typical applications of high definition telecentric lenses are found in the printing, flat panel and life sciences where flat specular substrates such as PCB s, solar cells and microscope slides require inspection. A flat field of view with uniform coaxial illumination and low grid distortion facilitates the post processing of the images in software. The use of different colors of illumination is useful to enhance the contrast for specific features on the substrate. In the figures below, two example images are shown of an inkjet printed etch resist mask on a copper substrate for a printed circuit board and a resolution test pattern on a glass plate. The image is sharp from the center to the corners of the field up to camera pixel level which indicates low curvature of field. The lens is designed for 15% contrast at 160 lp/mm. Further the illumination is very uniform, no dark corners are observed. Such a high quality image is the starting point of a reliable and successful inspection. O.D. Printed etch mask from Mutracx 0.2 0.3 0.4 0.5 Lp/mm: 256 192 168 128 96 80 64 128 0.6 48 40 0.7 32 24 20 96 Figure 1 Example images with the CCM telecentric lens Resolution test pattern Line pairs / mm 2017 Sioux Page 2 of 10
Lens design The lens assembly consists of 3 groups: the Front lens, the Adapter lens and the LED illumination unit. Front lens Adapter lens Camera LED Figure 2 Optical layout of the telecentric lens design Full color lens optimization is needed in order to use the 3 LED wavelengths without mechanical focus adjustments. At the start of the design, different configurations were evaluated for the front lens with a long free working distance: standard achromat lenses, Tessar, Heliar and Double Gauss lens types. The standards achromat lens can be optimized for 2 wavelengths only. So a more advanced lens design is needed. The Tessar, Heliar and Double Gauss are well known photographic lens designs in which color correction is a necessity. The optical performance of these photographic lens types fitted well in our application. The Double Gauss was the lens type that fulfilled all the optical requirements. The 2 adapter lenses are of telephoto and Heliar type and also full color corrected Achromat Tessar Heliar Double Gauss Figure 3 Different lens types evaluated for the front lens in the 3 color corrected telecentric lens The LED illumination unit is realized with Fresnel lenses since aberrations are less important in the illumination optics. The lens design and optimization is done in Zemax, including the performance prediction and tolerance analysis of optical components. First, the Zemax results for the 1.5x magnification configuration are shown. At this magnification the maximum numerical aperture is used for which the aberrations are most pronounced. In a second section, the Zemax results are shown for the 0.75x magnification configuration. 2017 Sioux Page 3 of 10
Configuration 1: Magnification 1.5x The MTF graphs below show the contrast (Modulus of the OTF) in the image as a function of the resolution in the image (Spatial Frequency in cycles per mm) at nominal focus. The higher the contrast, the better is the imaging performance. Note that the resolution in the image can be translated into the resolution at the object by multiplying with the magnification. The black line on top of the other lines is the theoretical best result of a lens (Diff. limit). white blue green red Figure 4 The MTF performance of the 1.5x magnification lens for different illumination sources at a fixed focus position. The resolution performance for blue is almost on the theoretical diffraction limit. For green and red wavelengths, the contrast is slightly lower. This is understood from the small focus offset between colors which is still present in the design, as is observed in the Field Curvature graph below. 2017 Sioux Page 4 of 10
The best focus plane for red wavelengths clearly has a small offset with respect to blue and green. Figure 5 The calculated field curvature and grid distortion for different illumination wavelengths. In our case the lens was optimized for best performance per color allowing a small focus offset of +/- 0.1 mm for each specific color. In the MTF graphs below it is shown that the lens performance is again on the theoretical diffraction limit when a small focus correction is applied per color. Defocus -0.1 mm for green Defocus +0.1 mm for red Figure 6 The calculated MTF performance when applying a small focus correction for green and red wavelengths. It can be concluded that the lens shows excellent performance per illumination color and very good performance when different colors are used simultaneously on the same object. The second type of chromatic aberration is the spot XY-displacement in the image plane for different wavelengths. This is called the lateral chromatic aberration and causes rainbow effects on blackwhite transitions when white light is used. The following graph shows that the lateral color error is negligible and well within the Airy spot. Hence, the lens shows no scaling differences when using different illumination colors. 2017 Sioux Page 5 of 10
Figure 7 The calculated lateral color error. Finally, the grid distortion is only 0.18% max, mainly in the corners of the field of view. Such small grid distortion makes grid calibration usually unnecessary. Figure 8 The calculated grid distortion error. It can be concluded that the lens performance is excellent for vision purposes using different illumination colors for optimal detection of specific features. Configuration 2: Magnification 0.75x The only difference with the previous configuration is the adapter lens. The image and object plane are located on the same position, just as the front lens. As a result of the lower magnification the field of view increases 2 times, and the numerical aperture at the object side decreases 2 times. Hence the optical resolution at the image side of this lens will be equal to Configuration 1, but on the object side it will be lower. 2017 Sioux Page 6 of 10
The MTF graph for white light and other colors shows equal performance as Configuration 1: close to the theoretical diffraction limit. white Figure 9 The calculated MTF curve with the image contrast on the vertical axis and resolution on the horizontal axis. The lateral color error and grid distortion are even better than in Configuration 1. Only a small astigmatism effect is observed in the field curvature plot. But this amount of astigmatism is within the depth of field and will not be visible in the image. Figure 10 The calculated lateral color error and grid distortion. 2017 Sioux Page 7 of 10
Lens qualification results The measured test results of the telecentric lens with highest resolution (1.5x magnification) are presented in this section. Resolution The main requirement of the lens is resolution. The lens design has a resolution limit according to the Rayleigh limit of 0.61*λ / NA = 4.4 µm for blue 457nm illumination. This corresponds to a contrast of 9% at 230 line pairs per millimeter (lp/mm). For longer wavelengths, the resolution scales linearly. The resolution qualification is based on measured MTF functions at different positions in the field. For measured MTF curves, see Figure 13. The optical performance of the lens is close to the diffraction limit, but some margin is reserved for aberrations and production tolerances. This explains the difference between the Requirement and the Diffraction limit in the following figure. Figure 11 The measured resolution from the contrast in the measured MTF curve for several illumination wavelengths. The bar represents the median value in the field of view; the uncertainty range is the 5%-95% range of measured values throughout the field. It is concluded that the measured optical resolution is close to the diffraction limit in the full field of view. Focus shift The measured focus shift for red, green and blue is limited to only +/-40 um, which is well within the theoretical depth of field λ / NA 2 = 111 µm for blue 457nm illumination. See next figure for measured result. Figure 12 The measured focus shift between the illumination wavelengths. 2017 Sioux Page 8 of 10
The figures below show the measured MTF curves for the different illumination wavelengths in the center and the 4 corners of the field. The theoretical diffraction limit has label Ideal. white blue green red Figure 13 The measured MTF curves for the different illumination wavelengths. The measured resolution of the lens is close to the expected performance. Other optical properties Sioux CCM built in total 15 lenses up to today. The optical properties of all lenses are validated before delivery to our customers. In the following graphs, the prediction from the Zemax optical model is indicated by and the measured 3*STD variation between the lenses is indicated by -- at the end of the bar. 2017 Sioux Page 9 of 10
Figure 14 Several optical properties measured for 15 lens HDO 1.5x magnification assemblies. The worst case values in the field per lens are depicted, including the 3*STD of the worst case values. The observation is that the realization of the optical lens assemblies performs close to the prediction from the Zemax optical model. Conclusions A high definition telecentric vision lens has been designed with an optical resolution of 5 µm for green LED illumination. The lens is fully corrected for red, green and blue illumination wavelengths, i.e. no focus shift and resolution close to the diffraction limit. Assembly and qualification is done by Sioux CCM before delivery to the customer. The lens is successfully applied in a high resolution inspection application. 2017 Sioux Page 10 of 10