SUN HEAT RADIATION SENSOR

Abstract: A sun heat radiation sensor is provided having a plurality of display areas each of which comprises a different, heat activatable liquid crystal composition. The liquid crystal compositions are selected so that the display areas provide a quantitative visual indication of the sun heat radiation sensed by the sensor. The sensor may be applied to a container for sun-screening substances, e.g. typically to a lid of the container.

Description: This invention relates to a sun strength indicating device of the kind according to the precharacterising portion of claim 1. The invention also relates to a sun tan factor meter for measuring the instant sun tanning strength of radiation received from the sun. Electromagnetic radiation emitted by the sun covers a wide range of the spectrum. However a spectrum analysis of the emitted electromagnetic radiation shows that, on a clear day with little or no filtering cloud cover, there is a precise relationship between the proportion of ultra violet (u.v.) radiation emitted and the proportion of infra red (i.r.) radiation emitted. Thus a measurement of the temperature of a body heated by the sun (i.e. heated almost entirely by the i.r. radiation of the sun) will give a proportionate measurement of the u.v. radiation emitted by the sun on that body. Since the "burning" or sun tanning of the skin is caused, on the whole, by u.v. radiation emitted by the sun, it will be appreciated that a measurement of the heat radiation from the sun received by a body can be used to provide an accurate measurement of the strength of the u.v. radiation from the sun. It is known to provide complicated and expensive devices which electronically sense the temperature of solar radiation. It is also known to provide devices employing liquid crystal compositions which display temperatures over relatively narrow ranges _ e.g. from 35°C to 40°C for a fever scope or from 10°C to 40°C for a room thermometer. U.v. dosimeters employing liquid crystal compositions are also known from US-A-3903423 and US-A-4212535. However these known dosimeters provide a measure of the cumulative u.v. radiation received over a period of time from the sun and do not provide a measure of the instant sun-tanning strength of radiation received from the sun. Finally it is also known from US-A-3829980 to provide a lid of a container for spraying sun tanning lotion with a device for assisting the user of the lotion. In this latter case the device is a gnomon of a sun dial for converting the lid into a sun dial so that the length of time a user is exposed to the sun can be measured. None of the known devices provide a relatively cheap device for providing an indication of the instant strength of the heat radiation from the sun _ e.g. to assess the sun-tanning power of the sun. An aim of the present invention is to provide a relatively cheap sun strength indicating device capable of providing a visual indication of the instant sun-tanning strength of radiation received from the sun. According to one aspect of the present invention there is provided a sun strength indicating device as claimed in claim 1. The device may include further filter means to obscure some, but not all, of the heat radiation from the sun. However the use of such an i.r. or heat radiation filter is not preferred. The reason for this is that, in use of the device on a sunny day, the thermochromic liquid crystal compositions will be heated both by the ambient air temperature and by the heat radiation from the sun. When the heat radiation from the sun is not blocked or filtered it will normally be capable of heating the liquid crystal compositions to far higher temperatures than the ambient air. In order to minimise the effect of the ambient air temperature on the activation of the liquid crystal compositions, it is therefore preferable not to provide the device with an i.r. or heat radiation filter. In this preferred case, the thermochromic liquid crystal compositions are preferably selected so that said wide temperature range includes the range from at least 25°C, e.g. 30°C, to at least 80°C, e.g. 90°C, and is preferably at least 50°C, e.g. 60°C, wide. Conveniently the liquid crystal compositions are arranged to be heated relatively quickly when exposed to heat radiation from the sun _ i.e. the compositions have a low "thermal inertia". In practice this is achieved by employing thin layers of the liquid crystal compositions. For example microencapsulated inks of the thermochromic liquid crystal compositions may be printed in layers of less than 25 µm, e.g. 15 µm. Preferably the thermochromic liquid crystal compositions employed have a low temperature dependence of colour but have different clearing points, the liquid crystal compositions displaying substantially the same colour (e.g. light green), when viewed in sunlight and when at temperatures below their respective clearing points. By careful design, the thermochromic liquid crystal compositions may be applied, e.g. printed, and selected in such a manner as to display a sun screening "factor" for different strengths of heat radiation sensed. In this respect, skin-applicable sun-screening substances (i.e sun tan lotions, oils, creams, etc.) normally have a numerical sun-screening protection factor ranging from 1 up to 16 (although in the United Kingdom the range is generally from 1 to 8), the higher the factor, the greater the filtering or sun-screening strength of the sun-screening substance. Conveniently, therefore, the device may be designed to display different "factor" numbers over different narrow temperature ranges within said wide temperature range in dependence on the heat radiation sensed from the sun. In conjunction with liquid crystals having various colour/temperature responses the background to the device can be of a colour to influence the thermal efficiency of the system, e.g. black for a high absorber and grey for a low absorber. The system can therefore be tuned for various background/liquid crystal formulations as a function of heat radiation sensed from the sun. According to another aspect of the present invention there is provided a sun tan factor meter as claimed in claim 8. According to a further aspect of the present invention there is provided a container for skin-applicable sun-screening substances in combination with a sun tan factor meter according to said another aspect of the present invention. Preferably the sun tan factor meter is applied to the container. In the case of the container having a lid or cap (i.e. when the container is a bottle or aerosol-type container), the sun tan factor meter is suitably applied thereto. According to a still further aspect of the present invention there is provided in combination a first container containing sun screening lotion in concentrated form, a second container containing a diluent for diluting the concentration of the concentrated sun screening lotion to enable adjustment of the sun tan factor number thereof and a sun tan factor meter as claimed in claim 8. The sun tan factor strength of the sun screening lotion can thus be adjusted, by mixing the correct proportions of the contents of the two containers, in dependence on the strength of the heat radiation from the sun sensed by the sun tan factor meter. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figures 2 to 9 are schematic views showing the various layers of different thermochromic liquid crystal compositions arranged in front of a light absorbing layer, illustrated in Figure 1, of a sun tan factor meter according to the invention; Figures 10 to 18 are schematic views illustrating the different displays of the sun tan factor meter at different operating temperatures; Figure 19 is a perspective view of a container for a skin-applicable sun-screening substance having a sun tan factor meter on its lid; and Figure 20 is a perspective view of two containers, one containing a sun-screening lotion in concentrated form and the other containing a diluent for diluting the concentrate and provided with a sun tan factor meter on its lid. A sun tan factor meter according to the invention is made by printing a plurality of different thermochromic liquid crystal compositions onto a light absorbing, e.g. black, backing or carrier 1 (see Figure 1) of sheet material, such as PVC film, polyester, heavily plasticised PVC (cling film) or paper. A protective transparent front covering layer (not shown), e.g. of PVC film, is then applied over the printed carrier 1. Alternatively, the liquid crystal compositions may be printed onto the inner surface of the front covering layer and a light absorbing layer, e.g. of black ink, printed over these liquid crystal compositions. A backing layer is then adhered to the printed surface of the front covering layer. The thermochromic liquid crystal compositions are characterised by having different "clearing points" and by having a low temperature dependence of colour _ i.e. they display a small variation of their iridescence with temperature and appear virtually the same colour, e.g. light green, when viewed in incident white light, over a wide temperature range immediately below their respective "clearing points". Thermochromic liquid crystal compositions having a low temperature dependence of colour are described in an article by Dr. D.G. McDonnell entitled "Thermochromic Liquid Crystals" published by BDH Chemicals Limited. Such a composition can be prepared by mixing liquid crystal compounds. For example, the composition may comprise a mixture of chiral nematic liquid crystal compounds mixed in proportions to adjust the clearing point of the composition and a nematic liquid crystal compound mixed in a proportion to adjust the colour of the composition in its cholesteric phase. Typically the chiral nematic liquid crystal compounds comprise TM74 and TM75 (available from BDH Chemicals Limited), and the nematic liquid crystal compound comprises ME35 (also available from BDH Chemicals Limited). The "clearing point" of a thermochromic liquid crystal composition is the temperature at which the liquid crystal composition undergoes the transition from a liquid crystal phase, e.g. the cholesteric phase, to the isotropic state. However, it will be appreciated that other liquid cyrstal types, e.g. cholesteric formulations, may be used instead of chiral nematics. The thermochromic liquid crystal compositions are microencapsulated and applied as "inks" in a number of printing operations. Figures 2 to 9 show a typical sequence of overprinting operations in which the compositions G1 _ G9 are printed in thin layers, e.g. below 20 µm thick onto a light absorbing backing. However, as previously mentioned, the compositions can be printed in the reverse order and in mirror images of the designs shown in Figures 2 to 9, directly onto a transparent front sheet, e.g. of plastics material. In this case the light absorbing backing layer would be printed over the liquid crystal compositions in a final printing operation. By way of example, the various thermochromic liquid crystal compositions have the following clearing points: The completed sun tan factor meter is comparatively thin and has a low thermal inertia. Figures 10 to 18 illustrate the various displays that the meter is capable of displaying when exposed to direct sunlight. The heat radiation from the sun heats the various thermochromic liquid crystal compositions and, as the various clearing points are passed through, roman numerals (from I to VIII) are displayed. Thus if the strength of the sun is sufficient to heat the temperature of the meter to 43°C, the meter will rapidly switch from displaying black numerals "I", then "II" and finally "III" against a completely light green background as the liquid crystal compositions are heated by the sun. Since the layers of the thermochromic liquid crystal compositions are thin, and have low thermal inertia, the numeral "III" will be displayed in a relatively short length of time as the various clearing points are passed through. If the meter is heated in excess of the clearing point of G3, e.g. to 48°C, the transition through the G3 clearing point will result in the top half of the display switching from a liquid green background to a black background and the numeral "IV" being displayed in light green. The particular numeral "I" to "VIII" that is displayed when the meter is exposed to heat radiation from the sun, gives a quantitative indication of the strength of the sun. In particular the numeral displayed gives an approximate indication of the "strength" or "factor" of skin-contacting sun-screening material (e.g. sun tan lotion, cream, foam or oil) that it is advisable to use to cover the skin in any particular sun condition. Of course it will be appreciated that the meter described measures i.r. or heat radiation from the sun. However there is a direct relationship between the i.r. and u.v. radiation emitted from the sun, and thus sensing of the i.r. radiation received from the sun provides a fairly accurate measurement of the sun-tanning radiation (basically u.v. radiation) of the sun. Since u.v. radiation degrades thermochromic liquid crystal compositions, a u.v. radiation filter (not shown) is applied as a protective layer over the printed liquid crystal compositions. The particular meter described above with reference to the drawings is quite sophisticated since it "switches" to give different displays at eight different temperatures. However less sophisticated meters may be designed having fewer, e.g. three, different types of thermochromic liquid crystal compositions typically having clearing point of 30°C, 40°C and 60° C. Alternatively more sophisticated meters can be designed having more than eight different thermochromic liquid crystal compositions. The meter described above may be used in conjunction with a container for a skin-contacting sun-screening substance. For example the meter may be applied, e.g. adhered, to an outer surface of the container _ such as, for example, a container top (e.g. a screw-top of a bottle or a snap-on lid of an aerosol container). In such an application, a fairly basic meter, e.g. only displaying three "factors", would typically be employed although, of course, more sophisticated meters could be employed. Figure 19 shows a sun tan factor meter 10 adhered to a snap-on lid 11 of an aerosol, spray or foam container 12 containing a sun-screening substance. In another application a sun tan factor meter may form part of a cosmetic kit for enabling a user to prepare his or her own strength or "factor" of skin-applicable sun-screening material. For example the kit might include a first bottle 20 (see Figure 20) containing sun-screening lotion in a concentrated form and a second bottle 21 containing a diluent. Contents of the two bottles would be mixed together in the desired proportions to produce a sun-screening lotion of the desired "factor" based on a measurement of the sun strength by a sun tan factor meter 22. In Figure 20, the meter 22 is shown applied to the lid of bottle 21, although it could, of course, be attached to the lid of bottle 20 or be separate from the two bottles. The invention has been described almost exclusively in relation to the application as a sun tan factor meter. However, the invention may have other applications where it is necessary to indicate the sun-tanning strength of radiation from the sun. Furthermore it is envisaged that a workable device could be designed using other types of thermochromic liquid crystal compositions _ e.g. those displaying "colour play" (i.e. which change colour through the spectrum when subjected to different temperatures). Such a device would employ a number of different compositions whose colour plays from start of red to start of blue in the electromagnetic spectrum extend over overlapping temperature ranges covering the entire wide range of temperatures that the device could be heated to when subjected to heat radiation from the sun. It will be appreciated that throughout this specification the term "thermochromic" when applied to liquid crystal compositions is intended to mean liquid crystal compositions which display different colours over a specific temperature range, which display two different colours whether above or below a specific temperature or which display a specific colour below a "clearing point" and which clear or become tansparent at temperatures above the clearing point.

Claims: 1. A sun strength indicating device for providing a visual indication of the instant sun-tanning strength of radiation received from the sun comprising carrier means and a plurality of discrete display areas on the carrier means each comprising a different heat activatable liquid crystal composition arranged to receive heat radiation from the sun, characterised in that u.v. radiation filter means is arranged in front of said liquid crystal compositions to prevent or reduce degradation of the liquid crystal compositions by u.v. radiation on exposure of the indicating device to radiation from the sun and in that the liquid crystal compositions are selected to be reversibly activated to change their appearance within different relatively narrow temperature ranges in a series of narrow temperature ranges within a wide temperature range, the wide temperature range covering the full range of temperatures that the liquid crystal composition would be expected to be heated to when the device is exposed to heat radiation from the sun, whereby a quantitative visual indication of the instant sun-tanning strength of the sun at any instant of time is provided by the appearance of the display areas in dependence on the temperature that each liquid crystal composition is heated to by the sun at that instant of time within said wide temperature range. 2. A device according to claim 1, characterised in that further filter means is provided to obscure some, but not all, of the heat radiation from the sun. 3. A device according to claim 1 or 2, characterised in that the liquid crystal compositions are selected so that said wide temperature range includes the range from at least 25°C, e.g. 30°C, to at least 80°C, e.g. 90°C, and is at least 50°C, e.g. 60°C, wide. 4. A device according to any one of the preceding claims, characterised in that the liquid crystal compositions are encapsulated and are printed as inks in layers of less than 25 µm. 5. A device according to any one of the preceding claims, characterised in that the liquid crystal compositions have a low temperature dependence of colour but have different clearing points, the liquid crystal compositions displaying substantially the same colour when viewed in sunlight and when at temperatures below their respective clearing points. 6. A device according to claim 5, characterised in that the liquid crystal compositions are applied and selected in such a manner that the display areas display different sun screening "factors" for different strengths of heat radiation sensed. 7. A device according to any one of the preceding claims, characterised in that the device is provided with a background of a colour to influence the thermal efficiency of the system. 8. A sun tan factor meter for measuring the instant sun-tanning strength of radiation received from the sun, the meter comprising a plurality of discrete display areas on carrier means, each display area comprising a different reversible heat activatable liquid crystal composition arranged to receive heat radiation from the sun, characterised in that the liquid crystal compositions have u.v. radiation filter means in front of them to prevent or reduce their degradation by u.v. radiation on exposure of the meter to radiation form the sun, the liquid crystal compositions being selected so that the display areas display different sun tan factor numbers for different sun-tanning strengths of heat radiation received from the sun in dependence on the temperature that each liquid crystal composition is heated to by the sun in order to provide a quantitative visual indication of the instant sun-tanning strength of the sun at any instant of time. 9. A container for skin-applicable sun-screening substance in combination with a sun tan factor meter according to claim 8. 10. A combination according to claim 9, in which the sun tan factor meter is applied to the container, e.g. to a lid or cap of the container. 11. A combination of a first container containing sun screening lotion in concentrated form, a second container containing a diluent for diluting the concentration of the concentrated sun screening lotion to enable adjustment of the sun tan factor number thereof and a sun tan factor meter according to claim 8.

Inventors: RICKSON, COLIN DAVID
Applicants: LIQUID CRYSTAL DEVICES LIMITED
Application year: 1988
Main classification: G01J1
IPC codes: G01J5, G01J1