Holographic interferometry features , advantages and disadvantages
Holographic interferometry is a technique that allows static & dynamic displacements of the objects with optically rough surfaces to be measured to the optical interferometric precision ( i.e. to fractions of a wavelength of light ) , These measurements can be applied to stress , strain & vibration analysis , as well as to non-destructive testing & radiation dosimetry .
Holographic interferometry can be used to detect the optical path length variations in transparent media , that enables , for example , fluid flow to be visualized and analyzed , It can also be used to generate the contours representing the form of the surface .
Holography is the two-step process of recording the diffracted light field scattered from the object , and performing the image rendering , This process can be achieved with the traditional photographic plates or with the digital sensor array , in digital holography .
If the recorded field is superimposed on the live field scattered from the object , the two fields will be identical , If the small deformation is applied to the object , the relative phases of the two light fields will alter and it is possible to observe the interference , This technique is known as live holographic interferometry .
It is possible to obtain the fringes by making two recordings of the light field scattered from the object on the same recording medium , The reconstructed light fields may then interfere to give the fringes that map out the displacement of the surface , This is known as frozen fringe holography .
The fringe pattern form is related to the changes in surface position or air compaction , Many methods of analyzing these patterns automatically have been developed in the recent years , Applications of holographic interferometry such as Laser vibrometry and Laser Doppler imaging .
Laser Vibrometry by the holographic interferometry has become commonplace , The local vibration amplitude can be assessed by fringe counting , By sequential imaging of frequency side-bands , the issue of fringe counting has been alleviated .
The side band order is a marker of the local amplitude of sinusoidal out-of-plane motion , Multiplexed measurements of optical side-bands enable quantitative measurements of out-of-plane vibration amplitudes much smaller than the optical wavelength .
Laser Doppler imaging
Holographic interferometry is very sensitive to allow wide-field , laser Doppler imaging of the optical fluctuations in amplitude & phase , either with the slow or fast camera , The slow (e.g. video rate) camera will record time-averaged holographic interferograms which will result in low-pass filtering of the optical fluctuation signal .
When you shift the frequency of the reference beam , the low-pass filter becomes the band-pass filter centered at the detaining frequency , Selective narrow-band detection & imaging can be performed .
This method permits micro-vascular blood flow imaging & wide-field measurement by detection of out-of-plane tissue motion , The wide temporal bandwidth of a high throughput camera can allow the wide-band detection & analysis of the optical fluctuations , It can be used for pulsatile blood flow imaging .
Holographic Interferometry advantages
Holographic interferometry allow to record the whole thermal or concentration field by using the holographic interferometer , The observed area is not disturbed by the sensors or sensing heads to detect the local displacements , The light beams can be considered totally free from the inertia that enables to record the immediate events .
Holographic interferometry allows to visualize the distribution of thermal & concentration fields in the shape-complex objects , The holographic interferometer is cheaper than the classical interferometer because it does not put high requests on the optical components ( lenses & mirrors ) quality .
The interferometric measurements enable the non-contact measurements , the deformation process is conditioned exclusively by the object characteristics , its embedding (fastening) & its loading , Holographic interferometry does not overtax the quality of recorded object surface , So , we can carry on the measurements on the investigated object whereas it is not necessary to produce any physically similar models .
Through the methods of the holographic interferometry , we can compare the shapes of already non-existing objects & analyze them , The holography helps obtain a three-dimensional display of the object , that allows to determine three components of the displacement vector from one holographic plate .
Holographic Interferometry disadvantages
The equipment for the holographic interferometry is rather complex , expensive ( cheaper than in classical interferometers ) and limited by laboratory conditions , The dimensions of the investigated object are limited by the size of the objective viewing field , Larger deformations lead to formation of a non-distinguishable interference structure .
The method of holographic interferometry is possible to apply mainly in the laboratories to ensure the stability of the holographic equipment ( with the exception of the holographic interferometry in the impulse mode) .
The experimental equipment for the object investigation should satisfy the specifications of the holographic interferometer respecting its dimensions & construction , If we use the real interferometric equipment , we must keep in view the deviation from the ideal interferometric system & rectify it by using the corrections .
In the interpretation of the holographic interferogram , it is necessary to measure the value at the edges by the sensors or determine them from the experiment character , That is the case of the holographic interferometry of diffusively reflecting objects interpretation , i.e. the loading force direction is not possible to determine .
During the interpretation of the holographic interferogram of diffusively reflecting objects , larger deformations lead to formation of a non-distinguishable interference structure , When looking through the hologram , it is not possible to tell if the phase difference is positive or negative .