As the quartz window is expensive to make, OTP (one-time programmable) chips were introduced; here, the die is mounted in an opaque package so it cannot be erased after programming - this also eliminates the need to test the erase function, further reducing cost. OTP versions of both EPROMs and EPROM-based microcontrollers are manufactured. However, OTP EPROM (whether separate or part of a larger chip) is being increasingly replaced by EEPROM for small amounts where the cell cost isn't too important and flash for larger amounts. A programmed EPROM retains its data for about ten to twenty years and can be read an unlimited number of times. The erasing window must be kept covered with an opaque label to prevent accidental erasure by sunlight. Old PC BIOS chips were often EPROMs, and the erasing window was often covered with a label containing the BIOS publisher's name, the BIOS revision, and a copyright notice. The practice of covering the BIOS chip with a label is still commonly seen as of today, even though current BIOS chips are actually EEPROMs or NOR flashes, with no erase windows.
Erasure of the EPROM begins to occur with wavelengths shorter than 400 nm. Exposure time for sunlight of 1 week or 3 years for room fluorescent lighting may cause erasure. The recommended erasure procedure is exposure to UV light at 253.7 nm of at least 15 W-sec/cm2 for 20 to 30 minutes, with the lamp at a distance of about 1 inch.
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