Liquid crystal cells – elements and systems – Particular structure – Interconnection of plural cells in series
Patent
1995-12-22
1998-10-20
Sikes, William L.
Liquid crystal cells, elements and systems
Particular structure
Interconnection of plural cells in series
349 73, 349 75, 349 96, 349 81, G02F 136
Patent
active
058254412
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to liquid crystal welding glass shields having improved optical angular properties, and then more specifically to filter constructions according to claim 1.
2. Description of the Related Art
Liquid crystal protective glass shields have been known to the art for many years. These shields are comprised of a combination of crossed polarization filters and layers of liquid crystals and the protective glass is able to change from an opaque state to an elevated transparent state in response to a change in an electric influence, this influence being controlled by the light that falls on a detector when welding is in progress and/or by electromagnetic detection. This enables a welder to perform a welding operation and also to perform tasks outside the welding area without removing the protective shield, because the shield will be clear or transparent when not subjected to the glare of a welding arc and will darken immediately when exposed to such glare.
The filters are normally mounted in a face mask. One particular problem is that the visible area is darkened unevenly due to the fact that the density-filter effect is dependent on the angle of incidence of the ambient light.
An improvement in this respect is taught by U.S. Pat. No. 4,398,803, whereby the liquid crystals are made relatively thin with the product of layer thickness and optical anisotropy (i.e. the difference in refractive index .DELTA.n between the refractive index of ordinary and extraordinary light rays) reaching to between 150 and 600 nanometers.
In this context, a typical cell construction consists of a twisted nematic (TN) type liquid crystal cell inserted between two mutually crossed polarization filters, where the defining walls are treated with a plastic layer which has been brushed/rubbed in specific directions (the so-called alignment directions), so that the structure in the liquid crystal defining surfaces will force the nematic molecules to each take specific angular positions and so that the molecules will be twisted mutually through 90.degree. between said defining surfaces. Other surface treatment methods which have corresponding effects are also known to the art. In an electrically non-activated state, the polarization plane will be rotated through 90.degree. as light passes through the filter, so as to compensate for the effect of the crossed polaroids and the cell becomes transparent. This rotation of the nematic molecules can be stopped to a greater or lesser extent, by applying an electric field and therewith obtain a filter effect that can also be controlled. However, a cell of this kind has a relatively strong asymmetry in its dark, electrically activated state, with varying absorption of light that is incident at angles other than a right angle, this asymmetry being further amplified by the fact that the nematic molecules nearest the surface, bound by the surface effect, still give rise to a residual optical activity. Thus, when the angles of incident light increase in relation to the normal (i.e. the perpendicular), the filter in the two bisectrix directions between the alignment directions will be more transparent and relatively constant in relation to the directions of the crossed polaroid filters along the direction of one bisectrix while darkening along the direction of the other bisectrix.
OBJECTS AND SUMMARY OF THE INVENTION
It is known to compensate for the asymmetric effect by combining two TN cells which twist through 90.degree., such that the "weak" bisectrix of one TN cell will coincide with the bisectrix of the other "strong" bisectrix, and vice versa. However, despite this compensation, the field of vision is still uneven, which is troublesome to the user. An object of the present invention is to provide an improvement in this respect.
One particular object of the invention is to provide an improvement with regard to low absorption in the transparent state of the glass. Another object is to provide a protective welding glass
Hornell Ake
Palmer Stephen
Ngo Julie
Sikes William L.
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