How could I predict how much of a voltage drop the phototransistor will produce by reading its datasheet?

[j3m]

I've wired a phototransistor and a 10kOhm resistor in series. Then I connect the A0 pin to a point between the phototransistor and the resistor, and I measure a different voltage depending on the light level in the room. I'd like to know how I could predict what the voltage will be just be inspecting the datasheet of the phototransistor (assuming I know exactly what the intensity of my light is).

My understanding is that the phototransistor induces a current in the circuit depending on the light level, so if I knew that current I could work out the resistance of the phototransistor and calculate my voltage drop. So I'd expect to see some kind of graph mapping lumens to amps or something in the datasheet, but I don't see anything like that.

Instead I see the words "linear output conforming to illuminance", and a variable called "collector photo-current" with a "test condition" of 10 lux and a typical value of 60 microamps. What I am assuming this means is that when you hit it with 10 lux, the PT sets the current in the circuit to 60 amps (I'm assuming it sets it to 60, not that it adds 60 to whatever current was there before or something). Since the output is linear, I guess this means it will produce 0A at 0lux, 120µA at 20lux, etc.

If I know the current in the circuit is I, then I must have V = 5 = I(r + 220), where r is the resistance of the PT. So I can work out r and from that determine the voltage drop across the PT. Is this all correct?

Datasheet: https://www.arduino.cc/documents/datasheets/HW5P-1.pdf

[jremington]

Transistors act to a limited extent like a constant current source (or sink). So, the voltage drop across a 10K resistor, with 60 uA collector current flowing at 10 lux illumination, would be about 0.6V. The constant current relationship will break down when the voltage drop approaches the supply voltage.

"lux" is not a very precisely defined unit, as it is an approximation of the human eye response, and depends on the spectrum of the illumination source. You don't know what illumination the manufacturer used for the tests.

To make accurate measurements, all sensors must be calibrated against a known standard.