# How do I determine the capacitor to use when converting current to voltage?

I'm trying to figure out how to use an ALS-PT19-315C/L177/TR8 phototransistor (or something like it).

The datasheet has this sample circuit for converting current to voltage so that I can measure it with an IC.

What does CL do? How do I select the appropriate value for it? I've also seen similar circuits where there is no CL so I assume it's optional and for filtering/smoothing maybe?

The datasheet further describes the relationship with the capacitor:

Vout(max.) = IPH(max.) × RL ≦ Vout(saturation)＝Vcc－0.4V

RL × CL ≧ 0.5 (empirical data)

What does the part about RL x CL ≧ 0.5 mean?

If there were to be a step change in illumination (for example, the light is turned on in a dark room), the circuit will respond with an exponential curve toward the final value given by $$\I_{ph}R_L\$$ with a time constant given by $$\R_LC_L\$$.