If the voltage is really too low, the diode wont even light.
But after that point, the lighting is progressive and linear.
1.8v for a 3.2 does seem low. You might need to bring in some more power using transistors for instance
Edit : i am talking about them really bright leds. Not the ones that just indicate ON/Off for instance. Cant really remember, but I think you are going to have to get the full voltage, otherwise they wont light up at all, or will be barely noticeable.