Is it plausible? Does your idea violate any known physical laws? Are you asking for a new light bulb that has 110% efficiency? If it passes the plausible test, we go to the next phase:

Is it feasible? Can your product be made, given a reasonable budget in a reasonable time frame? Often the ‘existence proof’ of a similar technology or device can help us answer this with minimal effort. Or we build a prototype to test the most tricky aspect – this becomes an object that can be tested, used, and evaluated, which brings us to the third phase:

Here it is. It’s tangible. A prototype, a ‘proof of principle’, a first example that shows what kind of performance and cost can be expected. Something you can use. This is a way to find the ‘slap on the forehead’ mistakes, and find them early in the project. Before you waste a lot of time, effort money on tooling, marketing, etc.

In the next weeks I’ll be writing about more specific examples of this process flow.

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.

A few prototype motion stages getting outfit with precision encoders.

Nixie tube tester by David A. O' Brien

Recently we started working with David O’ Brien, whose list of accomplishments is too long for this single posting. One of his many devices is this great nixie tube tester – runs on a single 9V battery. You can see more of his fabulous work here: David O’ Brien’s website

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.

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.

Based upon Motchenbacher and Connelly’s excellent book ‘Low Noise Electronic System Design‘. Click on the link for the pdf or on the image for a JPEG version.

pdf: Build-your-own-wideband-attenuator

Build your own wide bandwidth frequency compensated attenuator to test amplifiers.

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.

No, it’s not those flanges (though that seems obvious at first). In a way it’s a kind of ‘closed loop servo system’ or a self-correcting mechanism. In mechanical design I’ve heard this idea called ‘self-help’ where loads get distributed evenly by a careful design (see the book The Elements of Mechanical Design by James Skakoon for more on this topic).

This shows a microcosm of the job we do working with clients, solving problems – we always care about how things really work, and this video shows an example where the first ‘obvious’ idea may not be quite right.

In any case, it’s always fun to hear Feynman explain stuff. There’s a load of footage of him at youtube, explaining fire, light, uncertainty. Enjoy.

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