Contact angle analysis can correct for curvature in the sample surface where the curvature appears in the drop profile. The FTA software first measures the curvature of the sample and approximates it as a circle of some radius and center. The drop profile is compared against this circle rather than a straight line across the drop base. You take the following steps to make a measurement with a curved baseline:
First measure the curvature of the sample surface before the drop is placed on the surface. This can be at the beginning of a movie or in a separate SnapShot taken before the drop is dispensed. Analyze the image of the dry surface exactly like it was a wet drop. That is, the surface is regarded as a drop. The baseline can be anywhere but it is best for the "drop" to be high so the curvature is accurately determined. In the following image, the profile of a 12ml vial (end-on view) is measured as if it were a sessile drop.
Now click the "Save this Curve as Baseline" checkbox on the Contact Angle tab. You will get a confirming message that the curvature has been saved. Do this with the "Correct for Curved Baseline" checkbox unchecked so the measurement you are making is not affected by any previous curved surface stored in the system.
Dispense the drop and take a SnapShot or Movie. Check the "Correct for Curved Baseline" checkbox before your contact angle analysis. The contact angles reported will be corrected for the curved baseline. However, the baseline drawn in the current software will be straight across, from one three-phase point to the other. Future versions may draw the curved baseline instead. In the following image, the curvature of the baseline has been subtracted from the "original" value to give the reported value. Without the corrected, the reported value is 88 degrees, with the correction it is 70.2 degrees.
You can see the effect of the baseline correction by unchecking "Correct for Curved Baseline" and re-analyzing. This will be the nortmal, or uncorrected contact angle.
It is unimportant where you set the baseline in the original, dry image. It need not match where the wet baseline actually falls. This correction or calculation is made automatically. However, it is important that the dry baseline accurate reflect the curvature that is present under the wet baseline.