sorry for replying late, the model of LED's I used are also from Vishay, they are tiny LEDs, sorry I forgot the model number, I got them as a few samples by asking Vishay's office in China, I'm sure you can also find it out from Vishay's office in your country?
You are right, using more batteries or higher voltage batteries are much easier, I was just thinking 3 or more batteries would be too heavy for a pen, but it's better than a complicated circuit.