„Experiments with Polarized Light” változatai közötti eltérés
| 24. sor: | 24. sor: | ||
===90<sup>o</sup> Twisted Nematic LC Cell=== | ===90<sup>o</sup> Twisted Nematic LC Cell=== | ||
| − | In the | + | In the 90<sup>o</sup> twisted nematic (TN) cell shown in Fig. 2, the LC director of the back |
| − | surface is twisted | + | surface is twisted 90<sup>o</sup> with respect to the front surface. The front local director is set |
parallel to the transmission axis of the polarizer. An incident unpolarized light is converted | parallel to the transmission axis of the polarizer. An incident unpolarized light is converted | ||
into a linearly polarized light by the front polarizer. | into a linearly polarized light by the front polarizer. | ||
| − | When a linearly polarized light traverses through a | + | When a linearly polarized light traverses through a 90<sup>o</sup> TN cell, its polarization |
| − | follows the twist of the LC directors (polarized light sees | + | follows the twist of the LC directors (polarized light sees $n_e$ only) so that the output beam |
| − | remains linearly polarized except for that its polarization axis is rotated by | + | remains linearly polarized except for that its polarization axis is rotated by 90<sup>o</sup> (it’s called |
| − | the polarizing rotary effect by | + | the polarizing rotary effect by $n_e$; similarly we can also find polarizing rotary effect by $n_o$). |
| − | Thus, for a normally black (NB) mode using a | + | Thus, for a normally black (NB) mode using a 90<sup>o</sup> TN cell, the analyzer’s (a second |
polarizer) transmission axis is set to be parallel to the polarizer’s transmission axis, as | polarizer) transmission axis is set to be parallel to the polarizer’s transmission axis, as | ||
| − | shown in Fig. | + | shown in Fig. 3. However, when the applied voltage V across the LC cell exceeds a critical |
| − | value | + | value $V_c$, the director of LC molecules tends to align along the direction of applied external |
electrical field which is in the direction of the propagation of light. Hence, the polarization | electrical field which is in the direction of the propagation of light. Hence, the polarization | ||
guiding effect of the LC cell is gradually diminishing and the light leaks through the | guiding effect of the LC cell is gradually diminishing and the light leaks through the | ||
| − | analyzer. Its electro-optical switching slope | + | analyzer. Its electro-optical switching slope $\gamma$ is defined as $(V_{90}–V_{10})/V_{10}$, where $V_{10}$ and |
| − | + | $V_{90}$ are the applied voltages enabling output light signal reaches up to 10% and 90% of its | |
maximum light intensity, respectively. | maximum light intensity, respectively. | ||
</wlatex> | </wlatex> | ||
A lap 2017. szeptember 17., 05:47-kori változata
Investigation of Liquid Crystal Displays
Liquid Crystals
Liquid crystal (LC) is a state of matter that is intermediate between the crystalline
solid and the amorphous liquid. The nematic LCs are organic compounds consisting of
long-shaped needle-like molecules. The orientation of the molecules can be easily aligned
and controlled by applying an electrical field. Uniform or well prescribed orientation of the
LC molecules is required in most LC devices. The structure of the LC cell used in this
experiment is shown in Fig 1. Rubbing the polyimide film can produce a well-aligned
preferred orientation for LC molecules on substrate surfaces, thus due to the molecular
interaction the whole slab of LC can achieve uniform molecular orientation. The local
molecular orientation is called the director of LC at that point.
The LC cell exhibits the so-called double refraction phenomenon with two principal
refractive indices. When light propagates along the direction of the director, all polarization
components travel with the same speed 
, where 
is called the ordinary index of
refraction. This propagation direction (direction of the director) is called the optical axis of
the LC cell. When a light beam propagates in the direction perpendicular to the optical axis,
in general, there are two speeds of propagation. The electric field of the light polarized
perpendicular (or parallel) to the optical axis travels with the speed of (or
, where 
is called the extraordinary index of refraction). The birefringence
(optical anisotropy) is defined as the difference between the extraordinary and the ordinary
indices of refraction 
.
90o Twisted Nematic LC Cell
In the 90o twisted nematic (TN) cell shown in Fig. 2, the LC director of the back surface is twisted 90o with respect to the front surface. The front local director is set parallel to the transmission axis of the polarizer. An incident unpolarized light is converted into a linearly polarized light by the front polarizer.
When a linearly polarized light traverses through a 90o TN cell, its polarization
follows the twist of the LC directors (polarized light sees only) so that the output beam
remains linearly polarized except for that its polarization axis is rotated by 90o (it’s called
the polarizing rotary effect by ; similarly we can also find polarizing rotary effect by ).
Thus, for a normally black (NB) mode using a 90o TN cell, the analyzer’s (a second
polarizer) transmission axis is set to be parallel to the polarizer’s transmission axis, as
shown in Fig. 3. However, when the applied voltage V across the LC cell exceeds a critical
value 
, the director of LC molecules tends to align along the direction of applied external
electrical field which is in the direction of the propagation of light. Hence, the polarization
guiding effect of the LC cell is gradually diminishing and the light leaks through the
analyzer. Its electro-optical switching slope 
is defined as 
, where 
and
are the applied voltages enabling output light signal reaches up to 10% and 90% of its
maximum light intensity, respectively.
