Ultraviolet is a specific wavelength of the electromagnetic spectrum that takes the form of light. While beyond the ability of human perception, it can react with most photo emulsions. This wavelength is very energetic, and with larger photons than in the visual spectrum, ultraviolet light can more easily break chemical bonds. This damage causes some colorants to fade, especially dyes which have smaller thus weaker molecular structures. It can also cause the small molecules in dyes to become very reactive, causing fluorescent colors and optical brighteners to radiate more light than they take in.
The term undivided back generally refers to all forms of postcards printed before March 3rd, 1907 when U.S. postal regulations reserved a cardŐs entire back for the mailing address and postage. It can also refer more specifically to postcards within this same period, but only those that were printed after December 24th, 1901, when regulations ended the use of private mailing cards. These were the first privately printed cards in the United States that were authorized to use the words Post Card. Most other countries ended the use of undivided back postcards at earlier dates.
Universal Postal Union
The General Postal Union was created by the Bern treaty on October 9, 1874 in order to form a single international postal territory for the reciprocal exchange of correspondence between member post-offices. Within a year they had adopted most of the principals previously set down by the International Postage Association and the Paris Postal Conference of 1863 in regard to uniform mail rates and regulations. Prior to this agreement every country had to make an individual treaty with each other nation to regulate international mail. The initial signatories of this treaty were Germany, Austria-Hungary, Belgium, Denmark, Egypt, Spain, France, Great Britain, Greece, Italy, Luxembourg, Norway, the Netherlands, Portugal, Romania, Russia, Serbia, Sweden, Switzerland, Turkey, and the United States. In 1878 they changed their name to the Universal Postal Union, and they became a special agency within the United Nations in 1948. All but a handful of nations belong to it today.
A Uranotypie is a trade name for a type of glossy real photo postcard published by the New Photographic Society in Berlin at the beginning of the 20th century. These cards are hand colored with added gilding.
A Uvachrom is the product of a subtractive mordant dye tone process for the printing of color photographs, patented by Arhur Traube in 1916. The technique required the use of multiple transparencies, which made it too difficult and expensive for widespread commercial use. This method was used to produce book illustrations and color postcards published under the Uvachrom,/i> name by the Union of Color Photography (Farbwenphotographie).