1. Field of Invention
This invention relates generally to the monitoring of ultraviolet light, and more particularly to a device for monitoring cumulative exposure to ultraviolet light.
2. Description of Prior Art
Sunburn is a common illness, especially in the summer time, yet dangerous or even fatal, condition. Because of the popularity of sun tanning, consumers are continually exposed to the risk of sun burn. Therefore there is a need for a device that allows the consumer to easily determine the amount of cumulative ultraviolet (UV) exposure. This would enable the consumer to strike the balance between acquiring enough sun exposure to develop a tan yet avoid overexposure that leads to sunburn and other related medical problems.
Approximately five percent of the sunlight reaching the earth's surface is composed of ultraviolet radiation (200-400 nm). The near ultraviolet spectrum can be divided into three sub-regions: ultraviolet A (320-380 nm), ultraviolet B (280-320 nm) and ultraviolet C (200-280 nm). UV radiation has both positive and negative effects on the human body that may be felt either immediately or only decades after the occurrence or irradiation. The UV B (290-320 nm) region is responsible for most of the related biological effects.
The biological effects of UV light primarily effect the skin, eyes and immune system. Early damage includes dermatitis, conjunctivitis and keratitis. Delayed damage may include premature skin aging, skin cancer, exacerbation of skin diseases, cataracts, weakening of the immune system and skin cancer.
Sun tanning, especially if there is overexposure, can lead to many of the health consequences described above. The strength of sunlight is influenced by various factors including the seasons of the year, hour of the day, and geographical conditions.
The quantity of UV peaks between 11 AM and 2 PM. Because UV rays reach the earth's surface even on overcast days, it is difficult to estimate the quantity of UV from the brightness of the day. Therefore, cloudy weather represents a relatively high risk of sunburn because the warning effect of high temperature is missing. For those who desire sun exposure to obtain skin tanning for cosmetic purposes a more accurate method is needed to regulate the amount of exposure other than simple guessing. Of prime importance is that such sensing means be responsive to UV B radiation.
Chemical means have long been understood to be a method to measure sunlight and light exposure in general. An early example is the invention by Plishker U.S. Pat. No. 2,046,409, Jul. 7, 1936 that teaches a pocket sized light meter based on chemical means. This invention utilizes a housing in which contains a light sensing element. This said sensing element is exposed through a window in said housing during the period of light or sun exposure. The method disclosed in U.S. Pat. No. 2,949,880, Fromer, Aug. 23, 1960 teaches a sun tan control device based on a similar method. Both of these embodiments suffer from the necessity of complex housings and movable parts in order to utilize the invention. Chemical dip-sticks have also been developed to measure sun intensity. Such a method is disclosed in U.S. Pat. No. 5,589,398, Krause, Dec. 31, 1996. Such dip-sticks also suffer from the need of a housing or holder to properly position the dip-stick.
There are also a number of ultraviolet dosimeters based on label technology. This much more closely meets the needs of the consumer in so far that it can be worn while at the beach or performing other activities that involve exposure to the sun. Three of these devices utilize multi-layer polymeric label structures to measure the total dose of ultraviolet radiation and involve color change in response to increasing time in the sun.
There is the system of Zweig, U.S. Pat. No. 3,903,423, Sep. 2, 1975 that utilized a three or four layer label with a plastic film top layer with printed color scale. A similar multi-layer device is utilized by Trumble in U.S. Pat. No. 3,787,687, July 1974. Yet another multi-layer device is utilized by Wright in U.S. Pat. No. 4,788,433, November 1988. The methods disclosed by Zweig, Trumble and Wright, however, are relatively expensive to produce due to the requirement for a multi-layer polymeric system to house and support the photochromic material. Additionally, neither of these chemistry systems produce a color change that has the desirable tan coloration.
There is also the invention by Mullis, U.S. Pat. No. 5,436,115, Jul. 25, 1995 that discloses an ultraviolet sun sensor based on pH changes in a hydrogel. In commercial practice this invention suffers from the need to precisely control the amount of water in the hydrogel in order to get a pH change that is directly proportional to the amount of sun light exposure. In order to solve this limitation the commercial reduction of practice of the Mullis art involves a complex multi-layer polymeric system similar to Zweig, Trumble and Wright. Despite this increased complexity, this embodiment is still subject to shelf-life issues apparently due to the hydrogel drying out in the product over time. I have found that the relatively commercially unattractive characteristic of the prior art ultraviolet sun dosimeters may be overcome by incorporating a photochromic material that is active in the solid state directly into a printable emulsion. I have unexpectedly and surprisingly found that by incorporating ammonium dimolybdate into a vinyl acetate or poly(vinyl)alcohol emulsion, a label based dosimeter is created. Further, I have discovered that this emulsion can be screen printed with conventional techniques.