Testing response of Photodiode to different light colors

So we have this experiment that "tries" to measure the response of photodiodes with different colors of light. We had a reverse biased photodiode in series with a resistor. The voltage is measured across the resistor with 10M ohm resistance.

Using a multimeter, there is some noticeable difference in voltage readings. However, using the Arduino, we got this: 0.04 0.00 0.00 0.00 0.00 0.00 0.62 0.63 0.56 0.26 0.46 0.14 0.03 0.13 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.59 0.72 0.64 0.57 0.63 0.52 0.41 0.36 0.37 0.39 0.07 0.03 0.05 0.04 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.59 0.65 0.43 0.38 0.31

the program we used is this:

/* ReadAnalogVoltage Reads an analog input on pin 0, converts it to voltage, and prints the result to the serial monitor. Attach the center pin of a potentiometer to pin A0, and the outside pins to +5V and ground.

This example code is in the public domain. */

// the setup routine runs once when you press reset:

void setup() { // initialize serial communication at 9600 bits per second:

Serial.begin(9600); }

// the loop routine runs over and over again forever: void loop() { // read the input on analog pin 0: int sensorValue = analogRead(A0); // Convert the analog reading (which goes from 0 - 1023) to a voltage (0 - 5V): float voltage = sensorValue * (5.0 / 1023.0); // print out the value you read: Serial.println(voltage);

delay(500); }

I've read that the input is expected to fluctuate when there the pin is not connected to anything. But I am sure that ours was connected to measure the voltage across the resistor. What could be wrong?

What could be wrong?

Besides the multiple cross posts and the lack of a schematic or code tags?