A rubber hood is an accessory that allows the instrument to make contact based measurements. This is useful when calibrating any monitors, TV screens, etc. It is designed to provide shielding from ambient light.
Measurement spot diameter on contact measurements is 8 mm for a unit that has a 2 degree aperture and 14 mm for a unit that has a 1 degree aperture.
The rubber hood is accompanied by a screw-on adapter that can be installed to attach the rubber hood on the lens for contact measurements.