Absolute interferometry for fast and precise radius measurement

Abstract

A novel radius of the curvature measurement method for optical spherical surfaces using absolute interferometry is proposed. A measurement setup is designed and built around a common-path Fizeau interferometer. The cavity length (volume of air between reference and tested surfaces) can be measured by the absolute wavelength tuning interferometry. An interconnection of data from three different tunable laser diodes (central wavelengths 780, 785 and 852 nm) allows us to measure the cavity length with uncertainty from tens to hundreds of nanometres. Once the reference radius of curvature is known/measured/calibrated, the radius of surface under test can be computed applying the value of the cavity length. The radius of curvature is measured directly in confocal position of the interferometer with relative precision of about 10 ppm. Moreover, unlike standard radius measurement by interferometry, the uncertainty of the introduced method can be optimized by selecting a suitable transmission sphere. In the paper, the method is described, tested, and verified by measuring several specimens featuring different radii of curvature. The results are analysed and furthermore compared to other measurement device.

Two-wavelength interferometry for measurement of transonic airflow in a compressor blade cascade

“…Examples may be shock waves that show a step change in air density. For the measurement of phenomena with high gradients, where the undersampling of the fringe pattern occurs, it is possible to use multiple wavelength interferometric approaches [4][5][6]. This paper presents interferometric technique for research of compressible fluid flow in a wind tunnel facility [7].…”

Accurate Measurement of Surface Irregularities for a Standard Sphere Using Fizeau Laser Interferometer

“…2b; the challenge is testing high-quality spheres of larger diameters with automatic rotation. Some improvements were performed by previous works such as; mounting a convex lens of 260 mm diameter, but it not applicable for spheres and nonrotatable [21], a 3-pin mount to test a bulk sphere of 50 mm diameter, but it non-rotatable [22], a 3-pin mount to test a sphere of 93 mm diameter, but it non-rotatable [23], a rotating mount with vacuum chamber to test a sphere of 93 mm diameter, despite of high-accuracy, the high cost made this a problematic modification [24], a mount to test a sphere of 48 mm diameter, but it non-rotatable [25], a mechanically-rotating mount with spherical cavity has been developed to test a sphere of 93 mm diameter, but it needs an airtight vacuum, which makes it an expensive modification [26], a hollow mount made from Acrylic is employed to test a sphere of 93 mm diameter, but it non-rotatable [27], a mount to hold concave lens of 120 mm radius, but it not applicable for spheres and non-rotatable [28], a mount to test lenses and mirrors, with either concave or convex surfaces of diameters up to 1038 mm, this feature is not only applied for absolute wavelength tuning Fizeau interferometer but is also non-rotatable [29]. tested sphere at 360° around the y-axis, to avoid the errors caused by the manual rotation and improve the measurement repeatability.…”