This image shows a polarizer: enter image description here

It's only blocking certain light waves.

Is there a filter which blocks all light waves (like a shutter) but which is "controlled" (by manual rotation) like the polarizer ?

Imagine glasses that could be turned dark just by "rotating" the filter that is attached in front of them.

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    \$\begingroup\$ Maybe not. There are two common implementations of variable ND filters. One is just like this, using two rotating polarizes. The other is two use two fixed polarizes and a layer of LCD to rotate the light electronically. Maybe you can find a variable ND filter that both polarizes can rotate, so you can use the front to get the reflection you want, and use the rear one to get the brightness you want. \$\endgroup\$ Nov 21, 2017 at 15:04

1 Answer 1


Yes, a variable Neutral Density (ND) filter will do that.

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    \$\begingroup\$ A variable ND filter is still a polarizer, though. It still blocks "certain" light waves while letting others pass. Rotating the ring changes the angle between the polarized plates, changing the range polarization angles of light that's blocked. \$\endgroup\$
    – Caleb
    Nov 21, 2017 at 15:03
  • \$\begingroup\$ Two of them actually. \$\endgroup\$ Nov 21, 2017 at 15:04
  • \$\begingroup\$ @Caleb If the two plates are exactly 90° apart they should, in theory, block all light. \$\endgroup\$
    – Michael C
    Nov 22, 2017 at 0:56

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