Bubis E.L., Gusev S.A., Zheglov A.V.
Institute of Applied Physics
RAS, Institute of Microstructure Physics RAS, Russia
Phase-contrast technique with photo-thermal cell Zernike
This
paper describes a method of phase-contrast imaging of transparent objects using
a layer of weakly absorbing optically thick media, located in the Fourier plane
of the system as a cell Zernike (photo-thermal cell) [1-4]. The required level
of the technique corresponds to the initial stage of thermal blooming in the
medium and in the liquid filter is about milliwatts. Experimentally recorded
increase in the edges of rendered images of transparent objects. Demonstrated
the feasibility of the method for monitoring and visualization of optical
inhomogeneities in liquid glycerine.
For small quantities raid phase distribution of light intensity in the
image plane in the phase-contrast technique is as follows:
, (1)
where I0 – intensity of light, before object, j(x, y) – distribution of the wave phase immediately after transparent
object, Q – phase shift between zero and the higher space frequencies, produced by the filter Zernike.
Focusing the beam in the optical path length (infinite) weakly absorbing
medium, because of its heat, it acquires an additional phase shift. In the
nonstationary case, it is equal to:
(2)
Here 
– the energy of the beam 
– the critical energy of thermal blooming. In the
nonstationary case the phase shift is a linear function of time (with a
rectangular pulse) and the desired shift 
is achieved only to
the end.
For
example, for water in the visible range spectrum 
and the required
energy can not exceed several tens millidzhouls.
When
using a continuous laser radiation (
>> 
, 
– the radius of a
Gaussian beam in terms of 
, 
– thermal
diffusivity; æ - coefficient of thermal conductivity) among the steady
temperature distribution, and approximate estimate the required capacity (for
not too keenly focused beams) is determined from the expression [6]:
(3)
Using
tabulated data for water (æ 
and taking 
get 
. For alcohol (æ 
and other values that
and is satisfied in this
case.
In the
experiment, a technique for the construction of single images with photo-thermal
unit, located in the Fourier plane (see [1-3] and the references cited therein).
On the screen, located in the image plane (at a distance of 10 meters from the
lens) rendered an image of the transparent objects photographed digital camera.
Fig. 1b, c) are rendered images of optical inhomogeneities (Svilen) in liquid
glycerin, which takes place after mechanical perturbation [5]. Case 2a)
corresponds to put forward a filter from the Fourier plane and hence the lack
of visualization.
Fig. 1
We have studied the quality of rendered images of transparent objects.
Found that the visualization of large-scale objects with sharp boundaries
(stripe phase) has a slight edge enhancment [5], earlier in the process
predicted theoretically [3]. For objects with smooth edges and smaller limits
increase flagging. Figure 2a) shows a photograph of the visualized image of the
phase stripes, and Fig. 2c) of its transverse distribution of brightness.
Fig. 2
1. Trevino-Palacios C.G. et al
// Appl.Opt., 2003, Vol.42, No.25, pp.5091 – 5095.
2. Yelleswarapu C.S.et al
//Applied Physics Letters. 2006. V. 89.
P. 211116-1.
3. Bubis E.L., Matveev A.Z. //
Proc.SPIE, Vol. 6729, p.82, 2007.
4. Áóáèñ Å.Ë. // Ïðèáîðû
è òåõíèêà ýêñïåðèìåíòà, 2009, ¹1, ñ.119-120.
5. Iizuka K. // Engineering Optiks», Springer-Verlag, 1983
6. Chenming Hu and J.R.Whinnery// Appl.Opt. 1973, Vol.12, No.1, p.72 –
79.
7. Stebnovskii S. V. // Technical Physics, Vol. 47, No. 11, 2002,
pp. 1369–1372.