OPK test strips?

Ok so I've been using Wondfo OPK test strips this cycle. So far all neg tests and I'm on CD 24 already. However, my cycles have been known to go well into 40 days before so I guess not all hope is lost on ovulating this month?
​So my main question is this... Does anyone know if the line on the test strip should get slightly darker in color the closer I get to my LH surge? Or is it simply if the surge is there the dark line is there, and if the surge is not there it's a faint line?