Archive for category prototype
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:
We’re working on some energy harvesting projects – and we learned about this amazing project – a radio powered only by the sound of the voice of the person talking at the transmit end.
This was able to transmit 150 miles with NO batteries at all. And no ‘super caps’ either. Just the transient power of his voice. Michael Rainey provides a schematic here: Schematic of voice powered transmitter, El Silbo.
The trick for using a speaker as a microphone to absorb sound wave power is to get a good step-up transformer. The speaker has current, but little voltage, even into a high impedance. Of course, you can get more power output using the tin can approach, demonstrated in this video:
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:
the kind of arms to pick up things, not like the puny arms that a T-Rex has. That’s the word we hear from Bec Conrad, who’s taking a break from her work with animatronic dinosaurs (see this link here: http://www.dinosaurlive.com/funstuff/).
Bec is helping us make this motion stage, based on a salvaged DVD player, a platform for some new precision products. With an encoder capable of 15um resolution from Austria Micro, and controls theory support from Bec, we look forward to using this prototype in our shop to improve our optical measurements.
We’re using some Arduino boards for a variety of projects – to run the motion stage (25um resolution), to test LED colors, to run an oven for testing temperature compensation electronics. The add on boards (‘shields’ – since that means something else to me, I have trouble with that name) are a big help. We’ve been using SparkFun as our Arduino parts vendor, and we like ‘em.
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.
Here’s the specs: Voltage noise 2.5nV/√Hz, Gain 1000, bandwidth 0.3Hz to 500kHz. I measure 600nVpp from DC to 1kHz at the output.
Here’s the story: Designing and debugging a high precision A/D stage, you will want to know how quiet the voltage reference really is. When I worked on a 20 bit A/D board, I found this amp a great way to prove that a simple voltage regulator was not good enough as a reference voltage (measured data trumps an any preconceived notions). It was a big help getting the shielding and grounding debugged too!
Since most ‘scopes have about 5mV/Div at their highest gain, this AC-coupled amp allows you to ‘see’ to 5uV/Division. For a 5V full scale A/D that’s 1ppm per division on your ‘scope. Trust me, you’ll see things there. (I read that Pasteur freaked out his dinner hosts, using his microscope to look for germs on his food. He was ‘debugging’ in his day too, with his new favorite toy).
Here’s a pdf of the schematic – it’s based upon the design in the book Low-Noise Electronic System Design, by Motchenbacher and Connelly (a highly recommended text). Since the text came without the amp, I had to make one myself.
I also have a SPICE model for it, using Linear Tech’s LT-SPICE and I’ll send it to you, if ask.