Optical surface accuracy is crucial for achieving the desired performance of optical components, marking the difference between the intended and actual shapes of surfaces.
This parameter, commonly quantified in terms of fringes or waves corresponding to the laser wavelength used in interferometry, is vital in preventing additional wavefront distortions. These distortions can lead to aberrations and compromise the efficacy of the system.
Measurement Techniques
Optical surface accuracy is measured using various methodologies, including different techniques such as test plates, laser interferometers, contact profilometers, laser interferometers integrated with computer-generated holograms (CGHs), and coordinate measuring instruments.
Traditionally, the test plate method predated interferometry, which involved placing a transparent plate next to a polished surface to assess its flatness. Using monochromatic light, interference fringes appear, and accuracy is determined by measuring the fringe count and irregularities.
The number of rings represents the difference in radius between the two surfaces, known as power, and the spacing between identically colored fringes represents a height deviation of half a wavelength of the light used, which indicates irregularities.
Computer-Generated Holograms (CGHs)
Computer-generated holograms (CGHs) act as null lenses, enhancing interferometric testing of intricate surfaces like aspheric and freeform shapes. CGHs are often combined with laser interferometers and serve an important role in monitoring the surface accuracy of aspheric lenses during the polishing process.
Typical CGH Test Configuration. Image Credit: Shanghai Optics
Contact Profilometry
Contact profilometry is a popular and economical method that includes measuring the sag values of the test surface (while a mechanical stylus traverses the element) and then comparing them to the nominal surface shape to determine accuracy.
Laser Interferometers
Laser interferometers use complex algorithms to precisely determine optical surface accuracy by splitting a light beam into two paths of unequal length, allowing for their subsequent interference.
Upon recombination, the resultant intensity modulation correlates with the amplitude and phase of the colliding beams, resulting in a pattern of light and dark bands known as interference fringes. These fringes aid in the detection of power errors and irregularities over the optical surface under consideration.
Zygo 6″ Verifire XP Interferometer. Image Credit: Shanghai Optics
Coordinate measuring tools reveal the topography of optical surfaces by point-wise scanning and detection. Surface accuracy is assessed using modern data analysis software by comparing the obtained surface data to the nominal surface profile.
This information has been sourced, reviewed and adapted from materials provided by Shanghai Optics.
For more information on this source, please visit Shanghai Optics.