With opto end stops:
One side of the opto is the emitter, and the other side is the sensor. Make sure the emitter side is toward the floor or away from any light source.
Any other light source can trigger optos if it contains the right light frequencies that the sensor is sensitive to. Many of the sensors are sensitive to Infrared, which means sunlight can set it off of the sensor gets exposed to sunlight.
As regards to the opto flags themselves, I recommend painting them with an IR absorbing paint. If you don't, the flag itself can reflect outside light back into the sensor. I've used spray-on cans of "Kettle black" for this purpose, as it tends to absorb IR and it's usually a matt black (so no reflections).
One side of the opto is the emitter, and the other side is the sensor. Make sure the emitter side is toward the floor or away from any light source.
Any other light source can trigger optos if it contains the right light frequencies that the sensor is sensitive to. Many of the sensors are sensitive to Infrared, which means sunlight can set it off of the sensor gets exposed to sunlight.
As regards to the opto flags themselves, I recommend painting them with an IR absorbing paint. If you don't, the flag itself can reflect outside light back into the sensor. I've used spray-on cans of "Kettle black" for this purpose, as it tends to absorb IR and it's usually a matt black (so no reflections).