Archive for category tech notes

Light uniformity testing

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.

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.

These graphs were generated by ImageJ from the RAW data files of the Nikon D1x camera.

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Custom front panels

Here’s a vendor we recommend, and a photo of some parts they made for us.

photo of parts made by Front Panel Express for our Diff Amp project

When you make one or 10 of something it can be difficult to make it look ‘real’, that is, to make it look finished. Sometimes the Dymo-Marker labels are OK, say in a sci-fi movie, for your Custom Flux-Capacitor. But other times, you want the prototype to look clean.

We have enjoyed using the services of FrontPanelExpress to make some custom front panels for our projects. They provide free software, that’s simple and easy to use, and you upload the files to their on line ordering … and you get great parts back. A variety of anodized aluminum options and thicknesses.

They will engrave text, add paint colors into the text -makes a very nice professional looking prototype or short run of parts.

Just imagine – they can easily make a D-shaped hole for the BNC – so it won’t UNSCREW and FALL OUT !!! That alone is worth the price of admission. I can’t find my D-shaped drill bit …

They can do a lot of things that are a pain to do by hand – countersink holes, nice RS-232 type D-cutouts, square holes, etc. Check out their site for some examples.

We also made a Plexiglass panel – this allows us to compensate for the very annoying differences in height of the switches and knobs – and it allows simple paper graphics to be protected. We also considered making a glowing logo, but have not yet done that.

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Low noise NMR design

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.

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Radiometric Measurements

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!

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CCD Cameras, eyes, and physics

This tech note was motivated by the question – how does the response of our eyes

differ from the response of a CCD camera sensor.

Using the data of a particular Hammamatsu CCD camera as an example,

we compared how silicon ‘sees’ to the photopic eye response

and compared both to a Planck black-body curve of a light at a particular

color temperature.

We don’t know what those lumps are in that CCD response curve – maybe some

strange reflection interference??

If you know – tell us!


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Vision response vs. Planck’s Black Body Curve

Color temperature is based upon the idea of a Planck black-body radiator.

Here’s a Tech Note that shows how our eyes respond to the Planck Black-Body radiator.

For a lamp filament at a certain ‘color temperature’ there’s a curve of how our eyes

respond to the lamp. Pete put this into a MathCAD model, and there’s a pdf here

that shows off a few nice graphs.


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attenuators, connectors, etc.

Here’s where to get quality attenuators and connectors

Testing any high gain low noise amplifier requires a nice clean attenuator.

You need to drop the level of your function generator,

or that x1000 gain amp would need to supply 100 V output.

Pasternack Enterprisese sells a nice 30dB atten for about $42.

Their part number is PE7000-30.

If you put 2 of these in series, you have about x1000 attenuation.

(These assume a 50 Ohm load, so buy one of those too).

Here’s a link to their website:

Pasternak weblink

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Silicon Photodetector Units

Our eyes and silicon light detectors see things differently.

AND the units of photometrics differ from units used by normal MKS systems

here’s an Actinica tech note that tries to sort this out, click link for pdf file


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Actinica Book List

Ok, we have a book problem.

Both of us waay like good engineering books. A good explanation, or a great

graph that sums up why that camera ‘sees’ differently than my eyes, etc.

Since we’re always stumbling on more good books, this list will grow.

Drop by later see what’s new.

Here’s some of the books we like, as a pdf file here,


and here’s some more books we like:

  • the Feynman Lectures on Physics, a 3 volume set. Here’s a guy who can explain anything well. Like how sine, cosine and the magic number e all relate to the imaginary number i (square root of -1). He also has a great description of how a ’50 Ohm’ transmission line acts like ’50 Ohms’ no matter how long it is. For a really great puzzle – read his description of how charging a capacitor really involves magnetic fields outside the cap’s plates.

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