Why my light meter's values are blank on Arduino Serial Monitor?
Use Serial.println(x); to print a number as text. Function write() sends the raw byte and Serial Monitor shows the character with that ASCII code, which is not valid or a not visible control character.



For the z = (y, DEC); from Wikipedia:


  In the C and C++ programming languages, the comma operator (represented by the token ,) is a binary operator that evaluates its first operand and discards the result, and then evaluates the second operand and returns this value (and type). 


so z is always 10 (value of constant DEC), which is ASCII code for the new line character