Tag Archives: flatness

Monochromatic Light Box for determining flatness optically

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.

Newton's Rings

The following descriptions and photos discuss the why and how of building a source to measure flatness.

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Determining TEC Flatness with Newton’s Rings – part 1

With a 340W, 50x50mm TEC (thermoelectric cooler) arriving at work this week for a project we’re working on, Steve asked me to look at using Newton’s Rings to determine if this new TEC was flat enough to use as it was, or if we’d need to lap it with diamond lapping paste to improve the contact between it and the water cooled copper block it will be clamped to.

A large part of the problems I had when writing my dissertation for my degree came down the the flatness of the thermoelectric cooler (TEC) I was using and the clamping force I could achieve on it. Without both being as flat as possible, the thermal contact between the TEC and the heat sink was poor, and so the TEC didn’t heat or cool as well as when both were lapped flat.

Newton’s Rings provide a way to determine the flatness of two surfaces optically. With a piece of glass you know is flat, you can tell whether another surface is flat by looking at the interference patterns between the two surfaces. With a monochromatic light source and optical flat on order, Steve and I armed ourselves with our impatience, a telescope mirror blank (maybe almost flat), this excellent PDF and the TEC and copper block to see what we could see.

Newton's Rings

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