Archive for category optics
This custom built instrument will allow Dr. Rony Sayegh at the MEEI (Mass Eye and Ear Infirmary, Boston, MA) to improve the sight of his patients. Dr. Sayegh has developed a cornea implant called the Boston KPro (here’s a link to the wiki article about it: http://en.wikipedia.org/wiki/Boston_keratoprosthesis). In order to continue improving this design, we supported Dr. Sayegh with this instrument. Using a carefully aligned LED source and a rotational camera mount, in a light tight enclosure, the glare of various new cornea implant designs can be tested and evaluated.
We were able to deliver a custom instrument for reasonable cost and effort by using a combination of custom parts (mostly laser cut plexi from Danger Awesome in Cambridge, a link to their website here: http://dangerawesome.co/ ) and off the shelf mechanical/optical components from Thor Labs (http://www.thorlabs.com/).
At an early stage in this project, Pete provided measurements and guidance about the most effective LED optics for the jaundice cure for new born babies. Full project description can be found at the Design That Matters website here. Uniform light with the fewest LEDs, to reduce cost and insure effective treatment.
Pete has been working with some folks at the MIT Media Lab, in particular the Camera Culture Group of Ramesh Raskar. Using the display on the smart phone, and some pinholes and plastics, you can interact with the phone and measure your prescription and astigmatism. Link to project description at MIT Media Lab
There is a new start-up spin off company called Eye-Netra, that can be found here.
Here’s Pete demonstrating the clumsy kit that Eye Netra will replace:
Here’s a couple of our clients puzzling over some lens test data. This was an exciting day, to see this huge lens tested! The puzzling was over by the end of the day, but this is a better action shot.
And here’s a rotary stage we use to test corneas and LEDs – to determine the angular spread of light or scatter:
Recently we designed and built a precision motion stage capable of focus adjustments to about 0.5um. Yes, that is 500nm, or about the wavelength of green light. It’s stable, has smooth travel of a few mm. Based on a flexure design, so there’s no bearings to ‘rumble’ or cause misalignment.
This project shows how we combine off the shelf parts and custom parts to get a job done. Our goal was helping our client specify their unusual custom lens, so that it could be manufactured. Their custom lens had to work with a range of existing medium format photo lenses and with cinematography ‘prime’ lenses. Since the catalog information about these medium format and cinema lenses was not detailed enough for our client’s design needs, we needed to make some measurements.
The common image quality metric is the Modulation Transfer Function, MTF. The MTF function describes the image modulation vs. spatial frequency – perceptually, it’s equivalent to contrast. Good looking images have high contrast in the mid range spatial frequencies. (For more info, check out this excellent tutorial on this topic, Norman Koren tutorial about MTF ).
We used some parts from Thor labs, our PointGrey camera, the excellent ImageJ software, and our own flexure stage design.
We needed an imager with 0.5 micron pixels, so we used a 10X microscope objective in front of the PointGrey camera (which has about 5um pixels) to get the line pair per mm resolution we needed for the tests.
The custom lens design for our client requires an image modulation consistent with the cinema/large format lenses used in the application. The MTF data is infrequently given in these commercial camera lens data sheets, we realized we needed to measure the MTF ourselves. We found an excellent method to calculate MTF from an image of an optical step, which was purposefully slightly misaligned to the imager pixel array. Intuitively, each pixel acts as a pinhole, the array of pixels effectively scans the pinhole, and the resulting data are like scanning the edge of the image bar. The computer program calculates the MTF from the “scanned edge” profile.
We needed an imager with 0.5 micron pixels, so we applied the idea of a scanning micro-densitometer. The micro-densitometer uses microscope optics to magnify the image structure in the film for subsequent analysis. So we reached into our lens bin for a 10X microscope objective.
We found rotating the focus ring on the camera lenses was inadequate for precision focus, and our work was compounded by a lens without a focus ring. The camera this lens is designed for has a translating lens mount, with a bellows for eliminating stray light. Our lens bench does not have the precision required for focus either.
The problem was solved by combining a flexural translation stage with a differential adjusting screw from Thor Labs. This screw advances 250 microns per revolution with the external threads, and the internal differential screw advances 25 microns per revolution. At first use, it was apparent the design was adequate for the task.
We verified the performance of the 10X objective by getting crisp images of a Roncii ruling. Then we measured the on axis MTF of the photographic lenses of interest to learn that part of lens performance.
Our first, hand made, stage was not robust enough for laboratory conditions. We decided to design and build a robust version that interfaced easily to commercial optical bench components. The attached photos show the device almost ready for work.
Here’s a pdf version of this document, which includes Thor Labs part numbers for the lens tube parts needed, MTF-Testing-071510.
Somehow, a mirror ‘knows’ to swap left and right, but not up and down.
Ok, stand in front of the mirror and it swaps left and right – you wiggle your right hand, the left hand wiggles in the mirror. But, up and down don’t get swapped, the top of your head is still at the top. More puzzling – when you lie down, the mirror still ‘knows’ when you right hand moves, and ‘shows you’ a left hand moving. Asking a few good questions leads to the real answer. Here’s another Feynman video. Enjoy.
Improvements in machining precision, testing and simulation make the use of aspheres available to improve optical system performance.
Most lenses are spherical, in that each curved surface is some part of a sphere (usually a big radius compared to the lens glass diameter). Lately we’ve been working on some systems that require the use of lenses that have an ‘aspheric’ curve. These are more unusual, but if you can solve a problem that is otherwise unsolvable, ‘unusual’ is a good answer. Ok, maybe since I’m the electronics guy, I’m impressed with the precision of these optics and their measurement – I think you’ll be too, when you look into it.
We’ve found some references about designing and testing these asphere elements. Start with the article by Jay Kumler, and then read the other two about some fancy gear to test these aspheres.
Jay Kumler, Designing and Specifying Aspheres for Manufacturability, by Jay Kumler of Jenoptik-Inc
Interferometric Measurement of Rotationally Symmetric Aspheric Surfaces, by Michael Kuechel of Zygo
Subaperture stitching interferometry of high-departure aspheres by incorporating configurable null optics, by Andrew Kulawiec, Markus Bauer, Gary DeVries, Jon Fleig, Greg Forbes,
Dragisha Miladinovic, Paul Murphy of QED Technologies.
We were looking at various microscope objectives – those lenses on the turrets that aim toward the slides. Or, if you’re like me, the expensive silver thing that just went ‘crunch’ on the slide while I was trying to focus the image.
Pete noted that there seemed to be a parabolic curve fit – better NA, numerical aperture, better light collection, and the more expensive the objective lens gets. Here’s the curve, and the supporting data.