The MINI retroreflectometer instrument has an external light beam.
That is,the light shines out of the instrument housing and on to the roadway away from the instrument. The external beam instruments are useful for the measurement of the wet condition.
The external beam instrument is essential for measuring retroreflectivity in the falling rain condition ASTM E2832.
Here a spray of water is directed over the subject section of road during measurementand this is only practicably possible with an open beam retroreflectometer like the new MINI for road markings.