Q. If you can not use a strip thermometer that changes colors on Shabes, why can you use sun glasses that turn darker on the sunlight?
A. The use of photosensitive eyeglasses that turn dark on light is permitted on Shabbos because the color change is done indirectly, without intention, and it is only temporary. Rav Moishe Feinstein ZT”L ( O’CH 3-45) equates it to placing a coloured item on top of the glass, that is obviously not Tzoveah (colouring).
On liquid crystal thermometers or plastic strip thermometers as they are commonly called, the opinions vary. If only the colour changes, many Poskim permit their use (Piskei Teshuvot 340, 6) but some forbid because the change is done intentionally. If numbers appear as the temperature changes, many Poskim prohibit utilizing them (Rav. S.Z. Auerbach, Shmirat Shabbat Kehilchatah 40, footnote 8, responsa Tzith Eliezer 14-31.) Some permit their use if the numbers or letters are present at all times, and they only get highlighted, when they reach that degree of heath specified .(ibid.).
Horav Shlomo Miller Shlit”a is of the opinion that when no liquid containing thermometer is available one can use the plastic strip where only the colour changes and no numbers are created.
Rabbi Abraham Bartfeld as revised by Horav Shlomo Miller Shlit”a
Important Numbers
