INCAS BULLETIN, Volume 13, Issue 3/ 2021, pp. 113 – 122 (P) ISSN 2066-8201, (E) ISSN 2247-4528 Post-Processing of Schlieren Images Emilia PRISACARIU* ,1 , Tudor PRISECARU 2 , Valeriu VILAG 1 , Cosmin SUCIU 1 , Cristian DOBROMIRESCU 1 , Marius ENACHE 1 , Razvan NICOARA 1 *Corresponding author 1 COMOTI – Romanian Research & Development Institute for Gas Turbines, B-dul Iuliu Maniu 220D, Bucharest 061136, Romania, emilia.prisacariu@comoti.ro*, valeriu.vilag@comoti.ro, cosmin.suciu@comoti.ro, cristian.dobromirescu@comoti.ro, marius.enache@comoti.ro, razvan.nicoara@comoti.ro 2 Politehnica University of Bucharest, Splaiul Independentei No. 313, Bucharest 060042, Romania, tudor.prisecaru@upb.ro DOI: 10.13111/2066-8201.2021.13.3.10 Received: 11 June 2021/ Accepted: 12 July 2021/ Published: September 2021 Copyright © 2021. Published by INCAS. This is an “open access” article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Abstract: In general, the Schlieren visualization method is used to qualitatively describe phenomena. However, recent studies have attempted to convert the classical Schlieren system into a quantitative method to describe certain flow parameters. This paper aims at analysing pictures from a qualitative and a quantitative point of view. The post-processing of images for both situations is described based on different applications. Real examples are used and both methodologies and logical schemes are explained. The article focuses on image processing, and not on the studied phenomena. Key Words: calibrated schlieren, quantitative image processing, qualitative image processing, data acquisition 1. INTRODUCTION Schlieren is a visualization method and can be defined as visible streaks produced in a transparent medium (such as air, water, gas, etc.) as a result of density variation which produces changes in the refractive index. The simplest configuration that a Schlieren system can have is attributed to Auguste Toepler. This configuration is based on the properties of ray distribution of a point-like light source on a parabolic mirror. There can be many optical path variations, but as a functioning principle, all can be reduced to the single mirror setup. Other optics can be added in order to increase system sensitivity and contrast or to expand and modify the testing area, adapting the system to the experiment. The versatility and simplicity of the system are the main properties that give it an advantage over other visualization techniques. It can be adapted to almost all applications with optical access and transformed into a quantitative measuring system. In the recent decade, many ways to quantify information from the images have been described, but all implied altering the system by introducing different elements into the optical setup. The most popular method is Schardin’s calibrated Schlieren, which uses a small diameter lens with a very large focal distance (placed in the testing area) to compare the