I have a ring flash conected to an optical trigger, both Yongnuo. The trigger is triggered by my E-M10's small onboard flash. The trigger worked OK in heavy shade, but not in bright shade or sunlight. My solution was to tape white paper over the sensor aperture. With 4 layers of paper it now works ok in bright shade. I will add more paper to see if it will operate in sunlight as well as bright shade.
Is there a more sophisticated way of doing this?
I note that the trigger has two tiny switches under a clear cover, which I first mistook to be the sensor.