How do you erase an "old school" EPROM chip? (has a glass window on top of the chip)

You erase a windowed (UV) EPROM by exposing the silicon die to ultraviolet light through the quartz/glass window on top of the package. The UV photons give trapped electrons on the floating gates enough energy to leak off, returning all cells to the erased state (all 1s / 0xFF).

Practically:

• Use a dedicated EPROM eraser containing a UV lamp at ~254 nm wavelength (UV-C, the mercury line), placed close to the window.
• Typical exposure is ~10–30 minutes (depends on lamp intensity/age and distance; datasheets specify a minimum dose in W·s/cm²). Erase is bulk only — the whole chip erases at once; you cannot selectively erase bytes.
• After erasing, cover the window with an opaque sticker/label in normal use, because ambient sunlight and fluorescent lighting also contain UV and will slowly corrupt the contents over months/years.

Cautions: UV-C is harmful to eyes and skin — never look at the lamp or expose skin; use the enclosed eraser. Ordinary visible light and most window glass won't erase it quickly (and standard "blacklight"/UV-A is far less effective than 254 nm UV-C). Note this only applies to true EPROMs with a window; OTP EPROMs in windowless plastic packages cannot be erased.