Low Pressure Sodium Lamps

To be able to demonstrate the black flame experiment, the most important component is the source of monochromatic yellow light that is produced by a low pressure Sodium (LPS) lamp.

For additional details on the spectral characteristics of the low pressure sodium lamp, head over to the spectroscopy section where we have additional images from our spectroscope and also a spectrograph from our B&WTek spectrometer.

A high pressure sodium lamp will not work as effectively due to is broader emission spectrum that is produced by the addition of Mercury. Many streetlights today are either high pressure Sodium (a whiter hue of yellow light) or high pressure Mercury (greenish-white light).

I have procured a small collection of low pressure sodium (LPS) lamps and LPS spectral lamps including the Philips BY22d 35W and 55W "SOX" lamps and three Russian DNaS-18 Spectral Lamps.

The DNaS-18 Russian spectral lamps operate at a nominal voltage of 20V across the lamp at 1.05A. Normally the lamp would have operated off a magnetic ballast, which also provides the inductive kick-back to start the lamp, very similar to a fluorescent lamp ballast, however sourcing these is becoming somewhat of a challenge.

For the first DNaS-18, I semi-successfully was able to operate this off a CFL driver, however as it was under driving the lamp, this had caused significant sputtering of the electrodes, and darkening of the internal wall of the discharge envelope.

In researching a better way to feed the lamp with correct current and voltage, I ventured across scientific apparatus that use very similar spectral lamps, and I understand that these also use the magnetic ballast.

I continued my search, and I happened to come across a small dedicated spectral lamp driver, its output of 20V 1A (20W) matched very well with the DNaS-18 spectral lamp.

At the same time, I also had found a 35W high-frequency driver for the BY22d SOX lamps which also ordered from the UK together with the SOX lamps themselves.

Low Pressure Sodium; - "Black Flame" Experiment

Now that we have successfully got an operating low pressure sodium lamp, we can now demonstrate the black flame experiment.

For the burner, its best to use an alcohol, such as methylated spirits or isopropyl alcohol as this will burn with a near invisible flame.

We now need to introduce sodium ions into the flame by way of a normal non-iodised salt (sodium Chloride). The salt is dissolved into some warm water and then either introduced by dripping in the alcohol burner or as I did, saturating a small piece of paper.

Normally, sodium will burn with a bright yellow flame, however, when we illuminate the flame under the sodium lamp, a rather interesting thing happens, we now see the flame burning black.

Another very interesting observation, the black flame casts a shadow.

So, why does the flame appear black?

The reason is to do with the absorption of energy. The Sodium electrons in the flame will produce the same colour (wavelength) of light (energy by way of photons), therefore are also very capable of absorbing the same colour of light.

A shadow is cast as most of the light is absorbed and blocked from going through the flame.

- Flavio Spedalieri -
Written: 9th August 2021

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