This article discusses how to build a monochromatic light source to determine the flatness of a surface optically. It uses a high pressure sodium lamp housed within a box to shroud the light source, designed around a diagram from the book Amateur Telescope Making by Albert G. Ingalls. High pressure sodium lamps are the bulbs used in most street lights, they emit visible light within a narrow band of wavelengths.
In my previous post I described observing Newton’s rings using my computer monitor as a light source to show the flatness of a TEC. A flatter TEC has a better thermal contact with its heat sink, allowing faster transfer of heat away from the hot side and hence faster cooling of the cold side. The light I used previously was not monochromatic and so all of the wavelengths of light interfere constructively and destructively at different points, creating rings of different colours blurring into each other. While this allows you to see if it is flat or not, it doesn’t let you do so quantitatively. By replacing the non-monochromatic light of my PC monitor with a monochromatic source, I should in the future be able to determine how flat the TEC is.
The following descriptions and photos discuss the why and how of building a source to measure flatness.