Anti-retroreflection filter

A filter configured to reduce retroreflective effects of an optical viewing lens arrangement that is capable of causing retroreflection in use and that defines a field of view thereof and that defines an optical axis thereof passing substantially centrally through an inlet thereof, the filter comprising:

• a light blocking arrangement, that is adjustable to be stably switched between each of a group of pre-defined arrangements, with respect to at least one wavelength band, the group of pre-defined arrangements comprising:
• fully open, such that in use, the inlet is substantially free of obstruction across its field of view,
• partly restricted, such that in use:
• the light blocking arrangement blocks between 50% and 99% of incoming light entering the inlet, at least with respect to a forward viewing direction, and;
• the light blocking arrangement is arranged to block any optical path having an axial inlet path through an inlet region of the inlet, and an axial exit path through an exit region of the inlet that is diametrically opposed to the inlet region with respect to the optical axis of the inlet;
• wherein the filter comprises means to aid a user in switching the light blocking arrangement to the pre-determined partly restricted arrangement, and to retain it stably there, and
• wherein in the partially restricted arrangement, the light blocking arrangement covers a region of the inlet, that region comprising all of the area of the inlet that is at the optical axis or to one side thereof, with respect to a direction that is perpendicular to the optical axis.