Xavier Remington 
If a light emitting device is blessed it’s actual light, aka photons, may be holy for a short time depending on the power, and faith of the priest along with the deities they worship. We’re talking about a matter of seconds, or perhaps a few minutes at the most. Even then the effectiveness of those holy photons would wear down with distance. There is actually an expiration date on holy items. The fresher the blessing the more effective the item is at expelling evil.
Candle wax, wood, oil, or other fire fuel sources can be blessed creating holy flames that will last until the flames have extinguished. For reasons not fully understood holy flames will give off holy light the entire time they’re burning unlike light bulbs. Although it’s effectiveness as a deterrent would be minimal unless it was of substantial size like a bonfire. Again with distance the holy photons would lose their evil repellant power.
Basically blessing something doesn’t mean that everything the object creates is holy. 
Especially considering the trillions of photons that may come from a light source. Each one must be blessed for the light to be considered pure, and holy in nature. This is somewhat the same reason why a Priest, or even a group of Priests wouldn’t have the power to make an entire ocean holy water, or make the entire Earth holy thereby repelling, and expelling all entities of darkness from it. It would be great if we could carry around holy flashlights that would be pointed at dark vampires, and other unsavory holy light sensitive creatures to extinguish them to ash instantly. Unfortunately that would be too easy! We still hold out hope for the future when we see the day of the holy laser gun, and lightsaber.
