I wanted to know if one could use invisible ink on shabbat or Yom Tov? I know writing is melacha, but what about invisible ink? Is there a responsa on this? Could one write with it on Shabbos to read (with ultraviolet light) after shabbat?
R Yirmiyohu Kaganoff answers your question here
There is no Torah violation in writing with ink that will disappear within a few hours. This halacha has major ramifications when dealing with the needs of a Jewish hospital. Recording data is necessary, but if disappearing ink is used until after Shabbos, the prohibition is only mi-derabbanan. Poskim rely on this, if a hospital cannot employ sufficient non-Jewish staff.
Note this can only be relied on in case of great need, which is why he mentions hospitals.