Archive for category metrology
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/).
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:
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.
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.
We like to measure things – and NIST is our source of the best measurement advice. We knew that time has been measured with astonishing accuracy, almost 1 part in 10^15.
Given how accurately time is measured, we wondered ‘how is the kilogram calibrated?’
We learned that, according to NIST,
“The magnitude of many of the units comprising the SI system of measurement, including most of those used in the measurement of electricity and light, are highly dependent upon the stability of a 131-year-old, golf ball-size cylinder of metal stored in a vault in France”.
And over the years the International Prototype Kilogram, the IPK, and its copies that are distributed around the globe, have ‘gained weight’. There’s more here at wiki: http://en.wikipedia.org/wiki/Kilogram
Here’s the graph to show it:
NIST further explains:
“Mass drift over time of national prototypes K21–K40, plus two of the IPK’s sister copies: K32 and K8(41).[Note 9] All mass changes are relative to the IPK. The initial 1889 starting-value offsets relative to the IPK have been nulled. The above are all relative measurements; no historical mass-measurement data is available to determine which of the prototypes has been most stable relative to an invariant of nature. There is the distinct possibility that all the prototypes gained mass over 100 years and that K21, K35, K40, and the IPK simply gained less than the others”.
Uncertainty, it seems, is here to stay.
However, there’s an improved kilogram standard being worked on, called the Watt Balance, which measures the electrical power used to null the weight of a one kilogram test specimen. To get accurate results, NIST must establish the gravitational force accurately. Here’s a link to a NIST article describing how they do it:
Which reminds me of a Jim Williams article from Linear Tech, where he describes a VERY accurate electronic weigh scale, capable of measuring your heartbeat – since all the blood pumping and flowing changes your weight, a little bit. He called it a ‘ballisto-cardiogram’. Here’s where you can read more, in AN43 that Jim wrote for Linear Tech, about bridge circuits – including how to make a scale that can resolve 0.01 pound at 300 pounds full scale, or about 33 parts per million. It uses a clever circuit to achieve balance quickly and accurately, check it out:
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.
We notice the assertion that A/D converter quantization noise is equal to ADU/SQRT(12), where ADU is the quantization unit or LSB. We saw this in Hobbs’ excellent book Building Electro-Optical Systems, Making It All Work.
So, we decided to derive this. Took us a while to get the ‘trick’, and to remember how to perform calculus, to get that pesky root-mean-squared function.
Think of the quatization error as a sawtooth function that repeats. Then work out the RMS noise of that sawtooth wave (it happens to be the same as a triangle wave). And, yes, it does work out to that value.
Now the next part is Hobbs’ assertion that this quantization noise is not a Gaussian distribution. Get to work.