The ratioed image film thickness meter

Abstract

A technique for measuring the thickness of a fluorescent oil film is presented. Incident light is cast upon the oil film and the intensity of the luminescent signal from the fluorescent dye is ratioed with the intensity of the incident light which is scattered from the surface of the model. The quotient is independent of the intensity of the incident light and proportional to the film thickness. Experiments are presented supporting that for sufficiently thin films the ratio is independent of the intensity of the incident light as well as independent of the angle from which the experiment is imaged and the angle from which the incident light is cast.

Feasibility of skin-friction field measurements in a transonic wind tunnel using a global luminescent oil film

“…where is the normal vector to the model surface, and camera is the vector from the model surface point to the optical center. It should be noted here that, according to Husen et al (2018), the resolution of the oil-film thickness increases when P increases, provided that the signal-to-noise ratio of the image corresponding to the quotient of the emission and excitation intensity distributions is the same.…”

“…(1). The measured luminescent intensity I, emitted from an optically thin oil film, is proportional to the oil-film thickness and to the excitation light intensity (Liu and Sullivan 1998;Husen et al 2018), i.e. :…”

“…3 are given by the used oil, by the spatial resolution of the camera system, and by the time interval between the GLOF images, respectively. The calibration parameter h * is determined according to the ratioed image film thickness method (Husen et al 2018), which is based on the oil-droplet volume method. In practice, the unit thickness h * is estimated using measurement results according to the following equation:…”

“…Similar to OFI measurements, the surface angle to the camera position should be considered for the measurement of the oil-film thickness at each location on the surface (Naughton and Liu 2007). The oil-film thickness normal to the model surface is (Husen et al 2018;Lee et al 2020c): where I is the image intensity, I dark is the dark current image (which accounts for the influence of residual light and camera electronic noise), and I exc is the excitation light distribution image. The angle P between the surface and the camera position is calculated as:…”

“…For the application of the GLOF method (Liu et al 2008;Husen et al 2018), a luminescent dye is dissolved into the oil to be applied as a film to the wind-tunnel model surface. When the oil film is illuminated by incident light at the dye-excitation wavelength ex , it emits light at em > ex (Stokes shift), which can be recorded by a photosensitive device, such as a Charge-Coupled Device (CCD) camera.…”