The critical angle of total reflection splits the incident beam on the prism surface into two beams, one totally reflected and the other refracted into the product. Then an optical image is projected on the digital CMOS sensor with a bright reflected zone and a shadow. The position of the shadow edge indicates the concentration of the product. 

For a probe refractometer, flow forces will bend the probe. It makes most probe refractometers flow sensitive, because the probe bending causes an offset ∆ on the image sensor.

In contrast, flow induced bending does not influence ThinProbe measurements, because it uses an image conduit to transport the optical image to the CMOS sensor. The image conduit is a bundle of optical fibres that transports the pixels one by one, eliminating any offset caused by bending.