Q. If someone has a tattoo or for that matter anything written or drawn with ultraviolet ink or similar designs that will show, glow or change when exposed to u.v. light on his skin, his clothing or utensils he is handling, can he get close to the u.v. light source on Shabbos or Yom Tov?
A. Horav Shlomo Miller’s Shlit”a opinion is that the above is permitted, since the changes are only temporary while the subject reflects the u.v. light and are not visible anymore when he moves away. He compared it to an image in a mirror, that is obviously permitted.
Similarly, Minchas Shlomo (1: 10 : 6) Rav Moshe Feinstein zt’l (often quoted) and other Poskim also permit the use of gray-glasses that dim on the sunlight, see Piskei Teshuvos (320 :fn. 38.)
Today, armbands or wristbands that are coated with reflective materials that glow in the dark when a car’s headlights shine on them directly are highly recommended by all specially when walking outside on Shabbos nights. However, see question above in regards to strip-thermometers, where some Poskim expose differences and prohibit their use
See also question 220 in this forum in regards to wearing on Shabbos a raincoat that has a hidden pattern of colored flowers that become only visible when the coat gets wet in the rain, and disappears when it dries. Horav Shlomo Miller's Shlit"a opinion is that if you wear the coat without specific intention of the fabric changing or showing hidden designs when it becomes wet, it is permitted to use on Shabbos.
See also question 23 in regards to why it is permitted to use during Shabbos diapers with an indicator that changes color when wet.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a
Important Numbers
