Searching for a proximity sensor

Hi everybody! I am looking for your support for a "simple" advice.

I am discovering the power of Arduino since few months now and I am blocked for a new project.

I would like to have a cheap proximity sensor that detect people only if they wear a special armband for example. It would be like an infrared barrier which would be activated only of this armband enters the area (between 20 and 90cm from the sensor I would say).

Does this kind of sensor exist? Would it be possible to design it if the armband contains a special metal, or is in a specific color?

Thank you all in advance for your advices!

For an example with a cat:

Medium range RFID could do this, with the tag in an armband http://www.thingmagic.com/rfid-basics Otherwise, use a camera module with built-in image processing software, like the very inexpensive PIXY http://charmedlabs.com/default/?page_id=211

Thank you for the answer! PIXY module is indeed interesting but maybe too high end for the purpose.

However I haven't though about RFID and I think I will investigate this way.