I ended up sending these invisible ink pens back because two of the four built in UV lamps didn't work. Sometimes they barely lit or flickered, but never fully turned on and didn't burn. Shame as the ink pen part worked perfectly and if it weren't for the faulty lightbulbs they would have been fine. Also keep in mind that these pens have button cell batteries for the lights and it's not that hard to get into the compartment that holds the batteries. Therefore, these pens are dangerous for households with very young children who can put batteries in their mouths and choke.