Nature of project: experimental, experimental
Available to students on full-time physics degree schemes only.
Optical instruments manipulate light with reflection or refraction at optical surfaces which can have a variety of shapes including flat, spherical and aspheric. In order to achieve a satisfactory performance, these optical surfaces must be accurate to at least a quarter of the wavelength of the light being manipulated. This means that for optics intended for visible light, the optical surfaces must be accurate to around 100nm or better. This level of accuracy cannot readily be achieved with mechanical measurements and so a variety of optical tests have been devised to measure the shape and accuracy of optical surfaces.
The first reliable optical test was the knife edge test devised by Leon Foucault in 1858 which allowed him to produce better telescopes than those before him. Numerous more sophisticated optical testing techniques have been devised since Foucault’s time including the Ronchi test and interferometric measurements. Laser interferometers are now routinely used for commercial optical testing as they are quick and very accurate but commercial instruments are complex and expensive.
Very high levels of accuracy can be achieved in optical testing with relatively simple apparatus – amateur telescope makers still use the Foucault knife edge test, the Ronchi test and even simple laser interferometers to characterise the figure of their home-made optics. In this project you will build an optical testing setup and use it to characterise the surface shape and accuracy of an optical component. You will measure focal length, surface shape and surface accuracy and you will determine the accuracy with which the technique employed is able to measure these values.
A successful project will develop beyond the above in one/some of the following directions:
As described above and in the references, there are numerous methods of optical testing and each as advantages and limitations. More than one optical testing technique may be tested in order to compare and contrast the results.
Many of the techniques can be used to measure the figure of several different surface shapes such as spherical and aspheric – the apparatus can be used to test a variety of optical surfaces.
When considering where to take your project, please bear in mind the time available. It is preferable to do fewer things well than to try many and not get conclusive results on any of them. However, sometimes it is useful to have a couple of strands of investigation in parallel to work on in case delays occur.
Additional scope or challenge if taken as a Year-4 project: Optical test results can be simulated for ideal surfaces as an aid to the analysis of the experimental measurements. A numerical simulation should be developed to determine the output of the measurements of the chosen optical surface(s), and the results of the simulation and experimental measurements compared. Any discrepancies between the theory and reality should be explained.
Please speak to Matt Gunn if you consider doing this project.
Initial literature for students:
The project difficulty will be dependent on the chosen optical test - the Foucault knife edge test is relatively quick and easy to set up whilst an interferometer will be more demanding. The project would be best suited to an experimentally minded individual / group.
|milestone||to be completed by|
|Project outline||end of November|
|Apparatus design complete and build started||end of February|
|Apparatus set up and measurements underway||mid-March|
|Complete measurements and data analysis||Easter|