Archive for category tech notes

Lens testing – our precision MTF test jig

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.

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.

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Low noise x1000 amplifier

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!

pre production prototype of x1000 low noise amp

Here's the amp on the bench, next to a 9V battery. Click on image to enlarge it.

view of the internal PCB of the x1000 low noise amp

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.

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Frequency compensated attenuator tests amplifiers

We’re working on a low-noise amplifier product, and to test it we need some attenuation, to generate clean low-level signals. The pdf shows a schematic and describes some test results. It’s a little more tricky than just a couple resistors because with every resistor you get (free!) parasitic capacitance that tends to distort your signals, unless you compensate the network.

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.

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Prototype, progress, and the first digital camera

We make prototypes. Sometimes it’s hard to imagine how a product will evolve from the first (clumsy?) prototype into a sleek product, once it gets ‘real’ and becomes a manufactured (perfected?) item.

Since we all know what digital cameras look like now, here’s a photo (ironically a digital photo) of an early Kodak prototype digital camera, to show how things change and improve: (note that we don’t need the cassette tape to store images any more).

The full article can be found here: link to story about first digital camera at Retrothing

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Low noise, low drift techniques

This presentation has a set of clear, well thought out images describing how chopper techniques can reduce 1/f noise, reduce drift, and even how to cancel the nasty charge injection of FET switches. It shows how modulation can reduce noise in a sensor amplifier system.

link to pdf of Kofi Makinwa\’s presentation, Dynamic Offset Cancellation Techniques

I first learned of Kofi Makinwa’s excellent work through the recent IEEE Solid State Circuits magazine, Winter 2010, Vol. 2, No. 1. He demonstrates a clever accelerometer that uses a small air volume as the ‘proof mass’. The Wheatstone bridge has been around a long time, but it’s clear it can be taught some new tricks. This is the first I’ve heard of a ‘nested chopper’ architecture. Great stuff. Check out Makinwa’s other publications at the IEEE.

I’ve spent some time trying to squeeze good data from MEMS sensors, and I know how difficult it can be. These articles show why adding some switches and circuit complexity can really pay off. And it’s only CMOS and FETs, so we get ‘em for free from Moore’s law, right?

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Microscope objectives – NA, cost, and parabolas

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.

Microscope-objective-NA-cost

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SPIE Photonics West 2010

We’ll be teaching a ‘short course’ at the Photonics West 2010 show this January.

Titled ‘Fluorescent Detection: System Design and Tradeoffs’, it’s based on work we did to help develop a hand-held chemical detection instrument for Cogniscent Inc., (homepage for Cogniscent Inc.) who have granted us permission to show off some of the things that went into that product’s design.

We also discuss various design options that were not selected for the Cogniscent system, and what motivated those decisions.

Here’s where you can find more information, at the SPIE website:

link to course description at SPIE website

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Radiometry explained by NIST

The measurement of light is complicated by a variety of units and concepts that are not used in other fields. For example, the ‘light level’ could be measured in units appropriate to the sensitivity of our eyes (lux), or by the power level (Watts) – but that’s confounded by the wavelength (nano-meters, but sometimes Angstroms) and you need to think in steradians, etendue must be conserved … you get the idea.

We’ve written about some of these issues in earlier posts, but this is one big, complete reference manual – a kind of ‘everything you wanted to know about light, but were afraid to ask’ – and it’s from NIST. They call it a ‘Self-Study Manual’ and it’s a clearly written tutorial on optical radiometry.

And it’s a free download. Enjoy. The test is Tuesday.

link to NIST Self Study Manual on Optical Radiation

The official title is The Self-Study Manual on Optical Radiation Measurements, edited by Fred Nicodemus

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Quantization noise

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.

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Precision Resistors, low tempco

Here’s a couple sources for precision resistors.

One source seems to provide a great price/performance trade, the other is really the best quality resistor you’ll be able to buy. BUT precision low drift resistor arrays require careful inspection of the data sheet. Here’s a couple examples.

Recently I noticed that Maxim IC, (famous makers of 5V powered RS232 interface chips), have begin selling a pair of precise resistors in a SOT23 package, at an attractive price.

This family of parts, the MAX5491, claims to have 2ppm/degC resistor to resistor drift of the pair in the same package. Take care – sometimes the large print giveth, and the small print taketh away – the absolute tempco is spec’d as 35ppm/degC, so if you compare two different individual SOT23 units with each other, they may drift more that that nice small 2ppm value at the top of the spec sheet!

link to the Maxim-IC precision resistor array MAX5491 data sheet

For better matching, the VFCD1505 parts made by Vishay, also available at DigiKey, are spec’d for 0.2ppm/degC drift, or 10X better than the Maxim parts (and, well, about 5.7X the price, at $20/qnty 1, vs the $3.50 qnty 1 at DigiKey).

link to the Vishay VFCD1505 precision resistor data sheets

I like that Vishay also details subtle values such as ‘voltage coefficient’ and the current noise (or sometimes called ‘excess noise’, noise that is beyond the thermal noise of the resistor value).

The specifications of the Vishay parts are about where the state of the art is for actual resistors you can buy. Better resistors can be found only at NIST.

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