θˈæliəm aɪ ˈaɪədˌaɪd wʌz ɪnˈɪʃəli ˈædᵻd tə mˈɜːkjʊɹi ˈɑːɹk lˈæmps tʊ ɪmpɹˈuːv ðɛɹ pɚfˈoːɹməns ðə lˈaɪt pɹədˈuːst wʌz mˈeɪnli ɪnðə blˈuː ɡɹˈiːn pˈɑːɹt ʌvðə vˈɪzᵻbəl lˈaɪt spˈɛktɹəm lˈiːst ɐbsˈoːɹbd baɪ wˈɔːɾɚ, sˌoʊ ðiːz hɐvbɪn jˈuːzd fɔːɹ ˌʌndɚwˈɔːɾɚ lˈaɪɾɪŋ.
Thallium(I) iodide was initially added to mercury arc lamps to improve their performance The light produced was mainly in the blue green part of the visible light spectrum least absorbed by water, so these have been used for underwater lighting.