1.6 seconds or 1.6 second

I would go with seconds. Because it is more than one second, it is plural.

'1.6 seconds'-- it is more than one second.

'1.6 seconds' sounds right to me.

Also I wouldn't use '1 second'; I'd replace it with 'one second' or 'a second', for example "The correct exposure duration is one second" or "it happened a second ago".

what about this--> 0.4 second or 0.4 seconds? how do you explain that.

Here's my explanation of most of these cases in one rule, for what it's worth.

"One", and only "one", is singular. Anything else is plural. We mark with the sign of plurality anything that is not singular.

0.453 secondS; 5.364 secondS; 1.0012 secondS; 1 seconD; 0 secondS, no secondS at all.

This is why the tendency of an enormous number of speakers is to make "n