I recently installed six optocouplers in a two-channel push-pull circuit. I checked the LEDs with an near-infrared viewer before applying high voltage and noticed there s a lot of light coming out of the clear housings. It seemed like a significant amount, compared to ambient light. Will painting the outside of the devices black, or wrapping them in electrical tape cause problems? (electrical breakdown). The concern is the potential for optical cross-talk while operating at 6kV preventing the “off” arm from completing turning off. I also noticed that there are two small holes in the housing (not near any leads) Do these need to be left uncovered?
Cross-talk can be an issue with these devices. Because they are high voltage, many end-users will encapsulate the optocouplers (and surrounding circuitry) with an epoxy or coating to further insulate the optocouplers. This epoxy is usually opaque, so it eliminates the cross-talk.
You can paint or otherwise coat and protect the optocouplers, but you will want to do some testing after the parts are coated to ensure the coating does not cause any arcing around the part at your applied voltage levels. Some customers paint the outside of optocouplers with a white paint to have more complete internal reflection in the optocoupler, increasing the gain of the device.
The two holes you mentioned are just depressions in the epoxy that are there to assist in centering the LEDs during molding. They are not significant to the operation of the optocoupler. They can be covered up if you like.