In-process measurement can provide feedback for control of workpiece precision in terms of size, form error, and roughness. It can be used for precision control of a machining process, therefore to improve productivity and reduce defect rates. Optical measurement methods have greater advantages in the in-process form profile measurement systems due to their non-contact type and high precision. However, use of opaque coolant in precision machining will introduce an opaque barrier problem, making the workpiece surface y(x, z) inaccessible to the optical measurement devices. To solve this problem, several approaches, including the water beam approach, the single air beam approach, the multiple air beam approach, and the dual coolant displacing medium method, are examined in terms of coolant removal capability, level of acceptance to surface complexity, flow rate and velocity of the coolant displacing media, and also errors induced by the coolant removal methods. The results of the study show that the multiple air beam method and the dual coolant displacing media method are more advantageous than the other methods.