Infrared phosphor and material having latent images and optical reading system using said phosphor
 The invention provides a phosphor excited by infrared rays to emit infrared rays which has a nonacicular particle shape and a particle size of 4 .mu.m or smaller after being synthesized. The phosphor preferably comprises an oxoacid salt-containing compound including at least one element selected from the group consisting of Nd, Yb and Er. The invention further provides a material having a latent image pattern comprising said phosphor and an optical reading system which use the material having a latent image pattern. 
Hitachi Maxell, Ltd.
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C09K B41M C09K G06K C09K Y10S G06K C09K C09K C09K
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Toshio Ryuzo Hisao Yoji
Oshima Fukao Kanzaki Takeuchi