Archive for category metrology
Lens testing – our precision MTF test jig
Posted by Pete Flowers in how-to, metrology, optics, prototype, tech notes, tools on July 15th, 2010
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.
we recently built this rig to test the MTF of cinema and medium format lenses
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.
Exploded drawing showing the internals of the flexure stage.
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.
Development Details:
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.
Low noise x1000 amplifier (new product)
Posted by Michael Beach in how-to, low noise design, metrology, prototype, tech notes, tools on July 14th, 2010
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!
Here's the amp on the bench, next to a 9V battery. Click on image to enlarge it.
A view inside the prototype x1000 low noise amp. Click on image to enlarge it.
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.
SCHEMATIC-low-noise-x1000-amp-MODEL-101
I also have a SPICE model for it, using Linear Tech’s LT-SPICE and I’ll send it to you, if ask.
Weight gain at NIST
Posted by Michael Beach in NIST, application notes, low noise design, metrology, technical articles on May 5th, 2010
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.
This shows the improvement in clock accuracy at NIST from 1950 to 2000.
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:
Weight change of the standard kilogram and its copies over 100 years.
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.[12] 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:
http://nvl.nist.gov/pub/nistpubs/jres/106/4/j64schw.pdf
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:
http://ece.olin.edu/instrument/LinearAN43.pdf
Testing Optical Aspheres
Posted by Michael Beach in application notes, metrology, optics, technical articles, tools, vendors on April 19th, 2010
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.
Microscope objectives – NA, cost, and parabolas
Posted by Michael Beach in math, metrology, optics, tech notes on December 14th, 2009
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.
Quantization noise
Posted by Michael Beach in books we like, low noise design, math, metrology, tech notes on August 27th, 2009
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.
Light uniformity testing
Posted by Michael Beach in how-to, metrology, optics, photo, prototype, tech notes, tools on July 15th, 2009
For both a clinical test microscope, and a home theater HDTV projection display, the light from the source must be quite uniform.
To test some non-imaging illumination optics, we set up our digital camera, and wrestled with the RAW data files from the camera. Most cameras have some ability to ‘see’ infra-red, so we can also test the pattern from the remote control output, or for other purposes.
Here we test the light uniformity of an LED source using a digital camera and some Thor Labs mounts.
These graphs were generated by ImageJ from the RAW data files of the Nikon D1x camera.
Low noise NMR design
Posted by Michael Beach in low noise design, metrology, research papers, tech notes, technical articles on July 8th, 2009
Careful consideration of all the elements of a system’s design can lead you to some very improved performance. Imagine improving a benchtop NMR system by making it 60 times lighter (120kg to 2kg), 40 times smaller, and yet 60 times more sensitive!
This article, from the IEEE Journal of Solid State Circuits (Vol. 44, No. 5, May 2009), shows an excellent example of how this occurs.
link to IEEE abstract of ‘CMOS RF Biosensor Utilizing Nuclear Magnetic Resonance’ by Sun, Liu, Lee, Weissleder, and Ham
I recommend reading the article – it’s very well written, it describes how NMR works, and it details their systems approach to their improved design. Much can be learned here. The use of a resonant circuit for gain (they call it ‘passive amplification’) is detailed in Figure 8 of the article. (It reminded me of the old ‘regenerative’ type radio receivers, back when a vacuum tube had a power gain of about 12).
Put another way, this article shows that the ‘building block’ approach, when off-the-shelf 50 Ohm compatible RF modules are used, makes it easy to build a system that works – but that it leaves out some great performance improvements that are only possible when you analyze the basic system operation and theory. The design improves when you ask questions like ‘why 50 Ohms’ or ‘where does that noise originate and how can I maximize the signal’ and ‘how can I make this work with a much smaller and lighter magnet’? The article also answers ‘now that I can use a small magnet, can I make a custom CMOS IC that performs the RF detection, and seriously reduce system cost and size’?
Buying as much stuff off the shelf is not bad – it’s a great way to get a proof of principle working FAST, and it demonstrates that an idea or technique can work. Nothing says ‘success’ like working hardware – it allows the investors, managers and engineers to breathe easier.
But that extra performance gain from really digging into the details of how things work can pay off – in this case, it changes a benchtop lab instrument into a battery operated portable clinical test platform – this opens new opportunities and situations where this NMR system can be utilized.
Radiometric Measurements
Posted by Michael Beach in application notes, books we like, how-to, metrology, optics, tech notes on July 6th, 2009
It’s easy to confuse the units of LED light output. Steradians, luminous intensity, etc.
Here’s a link to an application note that explains these well, written by C. Richard Duda of UDT (now part of OSI Inc.). Apertures, intentional and otherwise, are discussed, along with typical test configurations.
Link to pdf of application note titled Radiometripdf of Radiometric and
Please tell us if the link gets broken!
Image Analysis with (free) ImageJ software
Posted by Michael Beach in metrology, optics, tools on June 29th, 2009
Lately we’ve been able to use our digital camera to perform some nice measurements, through the help of a program called ImageJ.
It’s free, was developed at NIH, is open-source, it has a ton of features and plug-ins, and you can write scripting macros, etc etc. It was developed so that the scientific community would have an open standard to process images. (Without an open standard for image number crunching, there’s no good way to independently reproduce an experiment that makes heavy use of images and image processing.)
You can read about it here at Wikipedia:
link to wiki entry for ImageJ program
http://en.wikipedia.org/wiki/ImageJ
It’s available here:
http://rsbweb.nih.gov/ij/
We were turned onto this image analysis program by a couple of our clients. We recommend it. Today the cool thing was to separate the RGB channels, and allow us to ‘see’ an IR LED without being confused by the camera’s ‘grey scale’ clipping algorithm. Very nice.