Here's one way ...

Code:
Packet_Size   CON 3
Old_Packet    VAR BYTE[Packet_Size]
New_Packet    VAR BYTE[Packet_Size]
Idx           VAR BYTE
LIGHT         VAR BYTE

FOR Idx = 0 TO Packet_Size - 1
    IF (Old_Packet(Idx) ^ New_Packet(Idx)) > 0 THEN LIGHT = New_Packet(Idx)
    Old_Packet(Idx) = New_Packet(Idx)
NEXT Idx