Trouble with phototransistor reading low number at first

So everything works fine with my phototransistor drop detection system except I get this when I run the code:

Welcome to the MultiFlow Lab! :smiley:
Please stand clear for your own safety!
Drop detection starting in…
drop detected!

The phototransistor gives me a really low value at first and detects a drop on the first loop iteration?
the phototransistor value is always around 550 and below 500 when a drop falls.

heres the code

int sensorPin = A0; // connect phototransistor to pin A0 on arduino board
int IRLED = 13; // connect the IR LED to pin 13
int sensorValue = 1000; // initialize sensor value
int green = 12; // connect green to pin 12 on arduino board
void setup() {
  // put your setup code here, to run once:
pinMode(sensorPin, INPUT); // input sensorPin
pinMode(IRLED, OUTPUT); // output for IR LED
pinMode(green, OUTPUT); // output for Green LED
Serial.println(" ");
Serial.println("Welcome to the MultiFlow Lab! :D");
Serial.println("Please stand clear for your own safety!");
Serial.println("Drop detection starting in...");

void loop() {
  // put your main code here, to run repeatedly:
digitalWrite(IRLED, HIGH); // turn on IR LED
sensorValue = analogRead(sensorPin); // read the value from phototransistor
Serial.println(sensorValue); // print the sensor value for user to view in serial monitor

     // go to [ tools > serial monitor ]

  if(sensorValue < 450 ){ // 
    digitalWrite(green,HIGH); // if drop interrupts the infrared light photoresistor is recieving then turn LED on
    Serial.println("drop detected!"); // telling user in serial monitor a drop was detected
    delay(5000); // delay to allow time for surface to move and camera to fire
  if(sensorValue >= 450){ // this is the value if no drop is present
    digitalWrite(green, LOW); // turn green LED off

You can sometimes get a more stable result from analogRead() by issuing the command twice and discarding the first result.
You could also experiment with a small delay after switching on the IR led before attempting to sense its status.

It took 18 reads to fix the bug xD

It took 18 reads to fix the bug xD

That sounds rather excessive and would imply that you are misusing analogRead() as a sort of delay() statement. Since you don't switch the IR emitter off in the loop(), why not switch it on in setup() then add a small delay after it.

Maybe also post a schematic. Usually an IR emitter / Phototransistor pair react very quickly.