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  Home  >   Product Information   >   Optical Films for LCDs   >   Polarizing Films
 
Polarizing Films
 

Taking advantage of the features of being thin, lightweight, and energy efficient, liquid crystal displays (LCD) have come to be used for the flat panel displays needed for the current age of electronic communications.
Polarizing films are optical filters that render LCD displays visible to the human eye. If the polarizing film is removed from an LCD, the display appears merely as a bright screen, with neither text nor images discernible.

Polarizing films are normally used at the front and rear of LCDs. The brightness attained when the two polarizing films overlap in parallel is the maximum brightness of an LCD. The dark tone arising when the two polarizing films overlap at right angles corresponds to black on the LCD display.
In other words, LCD optical characteristics, such as brightness and contrast, are greatly influenced by the characteristics of polarizing films.


Fig. 1 Function and construction of polarizing films.

Fig. 1 shows the function of the polarizing films. The polarizing film changes natural light into linear polarized light. This function allows only one element of the intersecting polarized light to pass. The other element is screened out by absorption (or reflection/scattering). Most of the LCD polarizing films made practical for mass production are made to express absorption dichroism by dying/absorbing dichroic material such as iodine or organic dye in base film (polyvinyl alcohol, or PVA), and then drawing and orienting it. Optical characteristics of polarizing films can be expressed as the three parameters of transmittance (T), degree of polarization (P), and hue. Because they are directly related to brightness and contrast of the LCD, the T and P values are particularly important characteristics, and it is essential to obtain sufficient T and P values in the visible wavelength range.


Fig. 2 Optical properties for iodine polarizing film.

Fig. 2 shows the correlation of T and P characteristics for a typical polarizing film. Because it is based on dichroism, it is understood that theoretical values of T = 50% and P = 100% are the limit. With a fixed manufacturing method (orientation), transmittance (T) is determined by amount of coloring of iodine (dichroic material). As amount of coloring increases, transmittance decreases, and degree of polarization (P) gradually approaches 100%.
 
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