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Optimized Wedge Illuminator For A Liquid-Filled Passive Display

IP.com Disclosure Number: IPCOM000048877D
Original Publication Date: 1982-Apr-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 3 page(s) / 35K

Publishing Venue

IBM

Related People

Wood, JC: AUTHOR

Abstract

A thin wedge-shaped light guide, illuminated at its narrow face, is employed to illuminate an electrochromic or other liquid-filled passive display module having light-scattering properties produced, for example, by surface roughening of electrodes. The wedge is in optical contact with the module liquid or index matched with the module top. Optimum wedge angles are calculated so that, for as wide a range of incident light angles as possible at the input face of the wedge, light within the wedge is predominantly totally internally reflected from its upper face and exits from the wedge only via its lower face to enter the module. Light reflected from the display returns through the wedge and is viewed directly or projected.

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Optimized Wedge Illuminator For A Liquid-Filled Passive Display

A thin wedge-shaped light guide, illuminated at its narrow face, is employed to illuminate an electrochromic or other liquid-filled passive display module having light-scattering properties produced, for example, by surface roughening of electrodes. The wedge is in optical contact with the module liquid or index matched with the module top. Optimum wedge angles are calculated so that, for as wide a range of incident light angles as possible at the input face of the wedge, light within the wedge is predominantly totally internally reflected from its upper face and exits from the wedge only via its lower face to enter the module. Light reflected from the display returns through the wedge and is viewed directly or projected. Some upward escape of light which has not entered the module, from the tip of the wedge, is prevented by truncating it at an angle and silvering the truncated portion.

The path of a ray of light entering a wedge parallel to the plane of the module is shown in Fig. 1. It can be seen that a succession of internal reflections take place and that the angle of incidence, 0, is reduced by the wedge angle, delta, at each reflection until the critical angle is reached, whereupon refraction out of the module occurs, i.e., (See Original). In the case where the wedge is made of an acrylic material, the medium above the wedge is air and that below the wedge is a solution of viologen, the critical angle at the air interface is 42.16 degrees, whereas that at the solution interface is 63.2 degrees. For wedge angles of less that about 20', it can be seen that after any reflection at the bottom face, the ray cannot be refracted at the subsequent top face encounter. This is illustrated for a parallel ray in Fig. 1 which shows three total internal reflections taking place before the critical angle is exceeded and the first refraction from the bottom face occurs at an angle of emergence, Beta, and at a distance, x, from the wedge tip.

If the wedge is illuminated with divergent light from an uncollimated or partly collimated source, such as a lamp having an ellipsoidal reflector, multiple reflection light paths are only maintained for rays within a certain angle of incidence, Gamma, (Fig. 2). As long as the angle of incidence is such that total internal reflection occurs at the first interface, the only difference between upwardly and downwardly inclined rays is an extra reflection of the latter at the lower face of the wedge and a displacement in the point at which they emerge, since the first reflection of downwardly inclined rays at the bottom face renders them parallel to the upwardly inclined ones. It is essential, for most practical examples, to maintain a light path with three or more reflections, as otherwise the angle of emergence, Beta, is close to 90 degrees and emergent rays may miss the display area of the underlying module entirely. Thus, e...