Photoresistor callibrate

const int ledpin = 5;
const int photopin = A0;
unsigned long timeValue = 0;
int minVal = 120;
int maxVal = 0;
int photoValue;

void setup()
{
pinMode (photopin, INPUT);
pinMode (ledpin, OUTPUT);
Serial.begin (9600);
digitalWrite (ledpin, HIGH);
timeValue = millis();
while(millis()-timeValue<5000)
{
  photoValue = analogRead (photopin);
  if (photoValue<minVal)
  {
    minVal = photoValue;
  }
  if (photoValue>maxVal)
  {
    maxVal = photoValue;
  }
}
digitalWrite (ledpin, LOW);
}

void loop()
{
photoValue = analogRead (photopin);
photoValue = constrain(photoValue, 0, 120);//makes min and max values 40 and 120
photoValue = map (photoValue, 0, 120, 255, 0);//maps 255-0 along 40-120
Serial.println (photoValue);
analogWrite (ledpin, photoValue);
  // put your main code here, to run repeatedly:

}