I can give you a non-scientific answer. And that is that some orchids grow in full sun, such as in the tops of trees, or in an open field. Examples: Vanda, Mule Ear Oncidium. And some orchids grow somewhere in the middle of the trees where they get dappled light. Example: Phalaenopsis. While others are all the way down on the forest floor, completely shaded from bright light. Example: Jewel Orchids.