Read the Lip
If we want to track what a person speaks, we normally think of having a microphone. We can also have a camera and the lip movements can be decoded to figure out what the person speaks. There can also be an automated program that can decode the movements to actual sound. This is indirect method but both of these devices need to be in the vicinity of the person. The article here explains an alternate approach to track the movement of the object(here the lips) from a distance.
When an electro-magnetic wave passes from one medium to other, part of the wave gets relected at the boundary and part transmitted. The reflecting surface electro-magnetic property determines the amount of reflection.
For a Z-directed wave with Electric intensity E in x direction and magnetic intensity in Y direction, the phasor fields are given by
Ex = E0e-jkz + E0’ejkz
Hy = (h)-1[E0e-jkz - E0’ejkz] -> (1)
After applying the boundary conditions, we obtain the reflection and transmission coefficients (i.e the ratio of reflected wave or transmitted wave to the incident wave]
E0’/ E0 = h” – h/h”+h
E0”/ E0 = 2h”/ h”+ h -> (2)
This is for the normal incidence of the wave. For oblique incidence the angle between the incident wave and the normal to reflecting surface, will have impact on the above equation.
Eq (1) can be rewritten as
Ex = E0 [e-jkz + kejkz]
Hy = (h)-1 E0 [e-jkz - kejkz]
With the electro magnetic wave of constant amplitude incident, the changes in electric intensity dEx/dz measures the movement of object from reference position. If we consider light as the electro magnetic wave, then what we have described is similar to the camera, but the source of the light wave is not the camera itself.
Since we are considering dE/dZ, the object of concern can be nth medium with all other (n-1) medium thro which the wave has passed has no displacement.
...
- Puyel
[The
above abstract is just an idea of the author and the validity is not
ascertained]
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