Neon calibration lamp

Neon lamps are great devices to calibrate a spectrometer, because they have well defined peaks, they are cheap, easy to get and have a long lifetime.

Their voltage requirement is not quite so great and their form factor is not ideally suited to couple the light into a fiber. This device addresses both issues.

Mechanics

The case is built from two 3D printed shells. It contains a tiny integrating sphere with a SMA905 port, a DCDC converter and a USB-C connector:

Everything is designed with OpenSCAD, if you want to change something. I printed the shells with a 0.4 mm nozzle, a layer height of 0.2 mm and an extrusion width of 0.4 mm. The quality is OK, but more fine settings or a more fine nozzle would have been better.

Optical SMA 905 connectors are very popular, but also very expensive for hobbyists. The subtypes SMA 905 A/B use 4 mm for the ferrule, like the metal part of electrical SMA connectors. Most common is SMA 905 C using a ferrule of 3.175 mm, also called F-SMA I. Devices have female connectors and cables have male ones.

You can save money by buying a female electrical SMA connector and pushing out its inside. The result is a connector with the right thread and flange, but it is typically 9.4 mm long, which lets the 12.0 mm long ferrule look out by 2.6 mm. It is also too wide at the end. This design uses the length to let the ferrule end up at the surface of the integrating sphere. The sphere port is narrow enough to center the ferrule. The SMA connector has 2.6 mm diameter holes and is made to be mounted with 2-56 UNC-2B screws. That is not recommended for M2.5 and M2.2 is rather uncommon, but M2 works as well. I designed a M2 core hole and simply cut a thread into the print. Adjust the connector to be centered to the sphere port.

The integrating sphere is painted white after mounting the SMA connector. The tip of the neon bulb is fixed with a small drip of epoxy glue so that the electrodes stay centered to the sphere port.

If you never used OpenSCAD before: Install it, load the part-* files, click on render and export them as STL.

Electronics

The DCDC converter has a shutdown line. Contrary to many seller specifications, this line must not stay open (see MAX1771 datasheet). I connected with it with some wire to the GND pin next to in on the PCB.

The high voltage ground must be connected to the lower electrode of the neon lamp, which sits near the SMA port, because only that one glows.

I used a 100 kOhm resistor in series with the lamp, but there is some flexibility. My neon lamp is specified for 1.26 mA and I run it at 1 mA. The DCDC voltage is adjusted until there is a voltage drop of 100 V at the resistor: 100 V / 100 kOhm = 1 mA. My neon lamp has a voltage drop of 73 V at that current. It was sold to be used with 150 kOhm at 230 V and specified as 0.29 W (which includes the resistor): 0.29 W / 230 V = 1.26 mA.

I could not find much information on the effect of DC current other than some kind of polarization happening over time. It makes sense, because with DC, sputtering and wear occurs for one electrode only. I expect the life time to be halved compared to AC. The slight current reduction compared to AC probably extends the life time and it still delivers plenty light. At 100 mW, a 1/4 W resistor is plenty, but if you use more powerful lamps, watch out for the resistor power consumption.

There are many kinds of neon lamps, even with the same mechanical dimensions. Make sure you get some specs from the seller. Isolate the lamp wires with heat shrink tube. You don't want 170 V getting in contact with anything.

The DCDC power supply is soldered to the USB-C breakout board. The solder spots on the underside must be fairly flat, otherwise the board no longer fits in the case.

Optics

Coupling the light into the fiber is tough. I placed the neon lamp near the fiber entry, yet only a small fraction of the light will couple in directly and the rest goes in all directions. The electrodes are anything but point sources. My solution is a tiny integrating sphere, where some light may eventually hit the sphere port. It works, but there may be better solutions. Fortunately, it delivers enough light.

Neon glow lamps show a characteristic spectrum that is easy to identify: A very bright orange peak (shown yellow by the camera, but really looks orange) starts a series of peaks towards longer wave lengths that begins with two pairs of lines, then a single one, followed by two more narrow pairs. The green peak is faint and was visually invisible, but the camera caught it.

This spectrum is great for calibration, because you can easily identify the wavelengths from the literature. The lack of peaks in the blue region does not yield a known precise calibration of the full spectrum, though. HDR shows very weak blue lines, but there is no satisfying single shot. This image shows the visual spectrum with 1 nm resolution.

The dominant lines are (data from NIST Strong Lines of Neon) in nm:

540.05618
576.44188
585.24879
588.18952
594.48342
597.46273
597.55340
602.99969
607.43377
609.61631
614.30626
616.35939
621.72812
626.64950
630.47889
632.81646
638.29917
640.2248
650.65281
653.28822
659.89529
667.82762
671.70430
692.94673
702.40504
703.24131
717.39381
724.51666

This spectrum was acquired as a single shot of 16 ms exposure time with an Ocean Optics USB 2000+, which calibration has long expired (data set). Note the asymmetry in peaks towards the right end, which supposedly is coma from that type of folded light path. That asks to check the spectral PSF.

This spectrum is a single shot of 250 ms exposure time with a 100 µm slit, 1000 lp/mm transmission grating, magnification 0.5 and a Canon EOS 350D. The higher resolution is obtained by a much smaller range and the need for more light.

BOM

Lower and upper case shell: 3D printed from OpenSCAD files
SMA connector female: Reichelt but much cheaper from Ebay. Press the inside out with a pin.
2x M3x8 mm screws with countersunk head or 2x M3x6 mm screws with cylinder head
2x M3 threaded insert
4x M2x5 mm screws with cylinder head
Neon lamp 14.8 mm x 5.8 mm (often sold as 16x6 mm or 0.55" x 0.22")
100 kOhm 1/4 W resistor
Heat shrink tube isolation
White paint
Clear glue, e.g. epoxy to fix the tip of the neon lamp
USB-C breakout board
MAX1771 DCDC converter 150–220 V