Monochromatic sodium light source
Sodium is an interesting element in that, when its atom are energised in a suitable manner it produces a bright yellow light. This is commonly seen in the yellow street lamps. This yellow is also seen when lots of materials burn because sodium is contained in small quantities in many materials.
What is useful about in metrology is that, if the spectrum of the light produced is examined using a spectrometer it will be seen that almost all of the light is not only yellow but consists of two wavelengths that are so close to each other that for the purposes here the light can be seen to be monochromatic.
When using interference patterns to compare lengths then the fringes produced are sharper the more monochromatic the light being used.
Sodium lamps are made in two forms. There is the low pressure form. This produces the required yellow light. Then there is the high pressure form. This is used because it produces a light that is not yellow. This is thought to look better than the yellow lights.
A sodium light is a bit like a fluorescent tube but instead of the gas that is in a fluorescent tube that produces the light the sodium tube uses sodium which is normally a solid. Like a fluorescent tube the sodium light needs some control gear to get it started and keep it running properly.
What is needed is a low pressure sodium tube (available on ebay) a low pressure set of control gear, not available on ebay. However a low pressure sodium tube will work with the high pressure control gear. This is probably less efficient which is an important point when lighting streets but not here.
When the power is applied the tube goes pink. This is due to the ionisation of gases that are used to get it started. After a few minutes it will go yellow. The light comes from the inner tube. The outer tube is needed in order to insulate the inner one so it stays hot. The light is very intense so it is necessary to fit it in an enclosure so the tube cannot be seen directly at all.
The whole system is housed in a box. In the top is the sodium tube and the electronics to drive it. Underneath the tube is a piece of white perspex sheet to provide a uniform lighting.
This light shines down onto whatever is underneath.
If an optical flat is placed onto of a flat reflective surface below an interference patter is produced.
The tube used here produced such a bright light that it left an impression on the eye after looking at the image for too long.