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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?

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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.

Of course, consult your rabbi before implementing anything you learn here.

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    Invisible ink is not disapearing ink
    – Naftali Tzvi
    Commented Oct 10, 2023 at 5:19

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