We chose a diffuser polished with 1500 grit (Part Number: DG10-1500 www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1132&pn=DG10-1500&YVI=23 ). In our demonstration, the source provided a collimated 635 nm laser beam with a 3 mm diameter. Lenses with 75 and 150 mm focal lengths provided focal spots estimated to be between 20 and 40 microns in diameter. We chose this diffuser because the resultant speckle was nicer in appearance, over a wider range of beam diameters, than the speckle provided by a diffuser polished with a coarser grit. There are a lot of variables to consider when selecting a diffuser. If you would like assistance choosing a diffuser for an application, please contact Tech Support (techsupport@thorlabs.com).
@onnobo7859 The orientation is the same in both. We were careful to make sure the beam was incident on the ground glass side, since it is so important to the accuracy of the measurement. Check out the view at 4:07, where it is easier to see that the ground glass face, and not the retaining ring, is flush with the front surface of the mount.
@Thomaszhang3115 The speckle pattern will change as the distance between the diffuser and the viewing screen changes, but using the technique demonstrated in this video does not require placing the viewing screen at a particular distance. However, adjusting the distance to the viewing screen might make it easier to see where the lens placement is optimized. This is because the average speckle size depends on the distance to the viewing screen. When the viewing screen is farther away, the average speckle size is larger.
The method also works for other wavelength of light. However, light of different wavelengths may focus at slightly different distances from the lens due to a lens effect called chromatic aberration. Achromatic lenses are designed to have reduced chromatic aberrations.
