From: Ethnography of Goa, Daman and Diu (first published in Portuguese in 1923) by A.B. de Bragança Pereira (translated by Maria Aurora Couto), Penguin Group, 2008.
p 85
…Since the Hindu house is very low, daylight can only enter through the door and a few small windows supported on wooden balusters. In some houses, window panes are made of conch shells instead of glass. These conch shells are called karepa (a translucent conch shell of oyster placuna placenta which filters a soft and muted light)…

From: Living Heritage, Shared Space by Angelo Costa Silveira (translated by Maria Flavia Ribeiro), Yoda Press, 2008.
pp 96-99
Carepas are undoubtedly one of the most enigmatic elements of construction in Goa. They have been used practically from the time of the arrival of the Portuguese. Their use was motivated by the need to illuminate and at the same time preserve the interiors of the house where social events could take place during the day…Carepas came to replace the system of wooden shutters and balusters which was very common in the early period…From the early days this system impressed travellers who frequently made references to it. Pyrard de Laval says in 1616: ‘(in Goa) they have no glass windows, instead very fine and straight oyster shells are used, inserted into wooden frames, allowing the light to come in as if it was of paper, as they are not as transparent as glass.’…