GB1353776A - Indication of changes in ambient conditions - Google Patents

Abstract

1353776 Polorizing apparatus S B ELLIOTT 19 April 1971 [12 March 1970] 23640/71 Heading G2J [Also in Division G1] A hygrometer comprises light transmitting layers 1, 4, Fig. 1, polarized at right angles to each other to produce a dark field, and a chemical crystal layer 3 deposited on a translucent substrate 2 which layer 3 is birefringent (double refracts light) at a first relative humidity but is non-birefringent at a second humidity. The layer 3 is formed of crystals containing unstable hydrates or alkali metals, many examples of which are given, and may split an incoming light beam into two rays vibrating at right angles to one another with a different velocity to give a phase difference therebetween, the beam then being generally referred to as elliptically polarized. The two components are resolved by the layer 4 into one plane polarized beam again but a phase difference has been introduced between its two parts. White light passing through will emerge from 4 either as brilliant colours or brilliant white, depending on the crystal size, but in either case the dark field now glows brilliantly. In a variation of Fig. 1 the layers 2 and 3 are in a roll form which may unroll close to a heated moving stylus in a recording instrument. The heated stylus forms a narrow line of non-birefringent crystals and traces a curve. In Fig.2 (not shown) the layers 1, 4 are formed by folding a single sheet at 45 degrees to its polarizing axis and the layer 3 may if required be deposited on 1 and 4 instead of upon 2. In Fig.3 (not shown) the layer 1 is perforated to allow air to stream therethrough and its humidity to be checked by viewing the colour of layer 4. In Fig.4 (not shown) one of the outer layers reflects light back to the first linear polarizing layer which then acts as an analyzer. In a variation the first layer may circularly polarize so that when the layer 3 is non-birefringent no light will be reflected through. The hygrometers thus described may be buried in materials to be tested for moisture content e.g. grain and allowed to remain there to equilibrate with the grain, or they may be sealed into a container with the apparatus to be checked. The layer 2 of glass or isotropic plastic may be sandblasted or etched to prevent the layer or layers 3 in its liquid state from flowing off. To further control drainage, the layer 3 may be deposited in areas e.g. by rough spraying to provide a dot formation or by filling the interstices between glass beads covering and stuck to a plate 2. In Fig.5 several different coatings 3, 3', 3" are applied to etched areas on 2, the viewer 6 scanning the system through analyzer 4 to determine whether uniformity of birefringence exists. The chemical of the layer 3 may be wetted before application, or the crystals of 3 may be dissolved by the moisture in the air to provide another visual indication of the humidity level. Various thickeners, materials to increase sensitivity to humidity, and dyes may be added to the crystals of the layer 3.