Write the EPROM with known and simple pattern. In this case the data written HEX value of "5555" . This value is selected since the alternative bits are '1' 

HEX 5555 =  0101 0101 0101 0101 in binary

So the Whole of the EPROM, it contains 524288 Bytes. All the Bytes are written with the same value of "5555"

The erased or blank EPROM will have all bits changed to '1' or "FFFF" is the destination value. So we are only writing half of the bits with '0'. 

So it is only 50% work load. Worst case DATA is "0000". Changing all the bits to '0' is the 100% work to flip all the 0's to 1's.  But some times it sucks. 

After writing the "5555" data into all of the EPROM, read the content of the EPROM and VERIFY the content of the EPROM.

READ and VERIFY the content. The programmer will report Verify operation are OK.

Now Turn ON the UV-C LED  and wait to 5 to 9 minutes. 

Do not change anything. There are multiple parameter governs the time taken to flip 1 bit in the whole EPROM.

Die size, Manufacturer, process( cell size nm), etc

Verify the EPROM periodically. We can just verify at an interval of 60 seconds. After the first bit change, the number of bit changes across the EPROM will grow very quickly. We can keep verify till the 50% of locations changed it's original values.

This tells how many bytes changed any of it's original content of "55" or "0101 0101" 

To count the Erased bytes, we need to FILL the BUFFER with "FFFF". That is the target value which means the erased byte or blank byte.

If any of the Byte changed it's value from "5555" to "FFFF", then the count will reflect while we verify with the value of "FFFF"

ERASED BYTES = 524288 - Reported Value

Just READ the EPROM and after few seconds VERIFY the content.

This will report the BYTES changed it's value between READ and VERIFY operation.

If we fill the buffer with 5555 and verify, it will tell how many bytes changed its value or ERASED partially.

If we fill the buffer with FFFF and verify, it will tell how many bytes are completely erased and changed the content value to FFFF or BLANK

We can do both the check to see the status of the EPROM erasure.

When we fill the buffer with FFFF and keep verifying, the number of mismatch bytes comes down for every verification.

The change is non-linear and initially it will be small and after a while it will be very large and then finally the rate at which erasure occurs comes down.

There are few last bytes takes longer time to erase. We can see those bytes are located in some specific locations.

When we fill the buffer with "FFFF" and after all the bytes were erased and changed it's value to "FFFF", the Verification will PASS. But do not turn OFF the UV-C LED immediately. Give another 10 to 15 minutes for complete erasure. 

The moment the Verification is successful with "FFFF" does not guarentee complete erasure. The reason is digital logic level. 

It just passed the threshold between 0 to 1. If you turn OFF the UV-C immediately, after some time there may be few bits flip back to 0.

So exposing for additional time increases the threshold and all the bits will become '1' with more margin.