-Can an Arduino card takes an intensity as an input or must I convert it into a voltage ?
The Arduino reads voltage. The default range is 0-5V and it's a 10-bit analog-to-digital converter which means it reads from 0-1023 with in increments of about 5mV. There is an optional built-in 1.1V reference which results in readings in approximately 1mV increments.
A photodiode puts-out a (small) current proportional to light intensity. You generally need an op-amp circuit to convert that current to a usable voltage. Look for some example circuits.
The 1st thing to do would be to look-up the specs on the photodiode and if you know the light intensity range, you can estimate the current from the photodiode. Then, build the amplifier circuit and measure the actual voltage output and adjust the amplifier circuit to give a useable voltage range.
A phototransistor would put-out more current and you might be able to get-away without the op-amp circuit. But, I'm not an expert on this and it will depend on the light intensity and the sensitivity of the phototransistor.
-Should i use a Raspberry instead of an Arduino ?
I've never uses a Raspberry Pi, but the Arduino the input/output should be easier on with the Arduino. If you want to directly connect a monitor & keyboard, the Raspberry would be the better choice. But I believe the Raspberry's ADC is optimized for audio and I'm not sure how well it works with DC voltage.
Finally, the proportionality factor depends on a variable (the wavelength), can the user input it on his computer, then the Arduino card calculate the factor, divide the voltage
Sure. You can do math in your program ("sketch"). But of course, you'll have to know the wavelength and the relationships, and/or you'll need to some calibrate the setup with some already-calibrated equipment.
and send it via USB to the user ?
Yes. Although you may have to write a program for the computer if you want something user friendly on the computer side of things. The Arduino IDE (Integrated development Environmnet) includes a [u]Serial Monitor[/u] so it's super-easy to send any information you want (in text form) to the your computer screen while running the IDE.
