Ultraviolet is a specific wavelength of the electromagnetic spectrum that takes on the form of light but is beyond human perception. It can however react with the photo emulsions on film. 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, creating the effects of fluorescent colors or optical brighteners.
The term undivided back generally refers to all forms of postcards printed before March 3rd, 1907 when postal regulations reserved a card’s entire back for the mailing address and postage. It can also specifically refer to postcards within the same period, but only those that were printed after December 24th, 1901, when postal regulations ended the use of private mailing cards. These were the first privately printed cards that were authorized to use the words Post Card.
Universal Postal Union
The General Postal Union was created by the Bern treaty on October 9, 1874 in order to form a single 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 and come with 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. It required multiple transparencies making it a difficult and expensive technique so it never gained widespread commercial use. This method was used to produce color postcards published under the Uvachrom name, and book illustrations for the Union of Color Photography (Farbwenphotographie).