1. Field of the Invention
The present invention is related to an apparatus and a process for characterising samples. Such samples can be gaseous or liquid volumes or even solids, such as a refractive surface or a transparent volume.
A particular application of said apparatus and process concerns the measurements of the refractive index gradients in samples such as liquids. Other applications such as shape measurements, optical property measurements, non destructive testings can be envisaged.
2. State of the Art
Since the end of the 19th century, the Schlieren technique is commonly used in order to identify and quantify density or temperature gradients which may occur in a sample or experimental volume.
To summarize, the Schlieren technique consists in measuring relight beam deviations such density or temperature gradients induce. A conventional Schlieren arrangement comprises a light source associated with a slit, a filter known as the “Schlieren filter” and located in front of a camera, for taking an image of a sample, said sample being placed between the light source and the Schlieren filter. Any refractive index gradient in the studied sample will deviate the light beam by a deviation angle α and the spot on the Schlieren filter, which is preferably a knife-edge, will be displaced, thereby resulting in a modification of the light intensity reaching the camera.
The success of said technique, which consists in measuring light beam deviations such gradients induce, lies on its high sensitivity, its low cost and its easiness of implementation.
The main drawback of this technique lies in the necessity to calibrate the complete measurement apparatus to extract quantitative data from it.
Furthermore, the resolution of measurement is generally low.
Aims of the Invention
The present invention aims to provide an apparatus and a method that do not present the technical drawbacks of the state of the art.
More precisely, the present invention aims to provide an apparatus and a method for characterising properties of samples. Such samples can be gaseous or liquid volumes or even solids.
The present invention also aims to provide an apparatus and a method which are adapted to provide high resolution measurements.
Another aim of the present invention is to provide an apparatus and a method which are easy to use and of low cost.