An occulder is a simple object with one simple function. It's purpose is to occlude or obstruct the vision of one eye in a patient in order to test the vision in the other eye.
An occluder is a simple object. It's very straightforward. Since it's sole purpose is to obstruct the vision in one eye, almost any object that could be used for that purpose can become an occluder. You could use your hand or the hand of the patient as an occluder. However we seldom do that because it would be too easy to cheat and peek between the fingers.
Most offices have special plastic objects, shaped somewhat like a spoon that are used to occlude the vision in one eye. This is what I use in my office. The problem with these devices is that they seem to break easily. I don't really know why. They are actually made from fairly sturdy, rigid plastic. However, some patient's seem the have the mistaken notion that the purpose of the occulder is not only to cover up one eye, but also to create an indentation in the skull bones around the covered eye. Since occulders aren't specifically made for creating indentation in bones, they will frequently break when used for this purpose.
Some patient's have difficulty following the simplest directions when it comes to testing the vision in one eye, as well. At least once a week, I'll hand an occluder to a patient and tell them to cover their left eye. Then I'll turn on the eye chart and ask them to read it, only to discover that they've not only covered one eye, but they've CLOSED the OTHER (uncovered) eye. Needless to say, they seem to have difficulty reading under these circumstances.
I suppose it is at this point that these patients seem to get the idea (from where, I have no clue) that since I obviously don't want them to see the chart, I must want them to make indentations in their skulls with the occulders.
Once the occulders break, I have to find other uses for them. They cost too much to simply throw away. A broken occulder has many uses. You can use it to dig boulders out of the back yard. You can use it as a soup spoon. Or you can use it to slap a patient upside the head when they break one of your new occluders.