Measurement device, method of measurement, and method of manufacturing devices

A measurement device for measuring a substrate in a lithographic apparatus, the measurement device comprising:

• an optical sensor configured to make a measurement of a surface of the substrate by emitting a radiation beam;
• and
• a fluid supply configured to supply a conditioning fluid traversing the beam and flowing in a slit between the measurement device and the surface of the substrate,
• wherein the optical sensor is configured to perform a first measurement process while relative movement is provided between the measurement device and the substrate at a first speed, and to perform a second measurement process while relative movement is provided between the measurement device and the substrate at a second speed higher than the first speed, and
• wherein the fluid supply is configured to supply the conditioning fluid at a first flow rate during the first measurement process, and to supply the conditioning fluid at a second flow rate higher than the first flow rate during the second measurement process.