Which is the Right Photo Sensor for an Application

Keyence do a huge range of opto-switches, some with fibre optic light guides with lenses to get into very tight corners. The also have laser types. A bit costly, but very good.

I'm sure that they have something suitable.

http://www.keyence.co.uk/

They have offices worldwide.

Usual disclaimers.

Your sensor uses an emitter-receiver pair. I'll assume that the emitter is an LED and the receiver is the photosensor itself. I'll bet your problem is the emitter LED beam spread is too wide and some of the light shines into the adjacent rollers. You need a simple collimator to ensure the light from the LED only reaches the sensor, and likewise that the sensor only sees the LED.

Take 2 pieces of thin-walled aluminum tubing that you can slip over the ends of both the receiver and the emitter. The tubing should extend outward from the LED and sensor as far as possible without getting in the way of the cardboard as it moves. Both pieces should fit snugly. Before installing the tubing permanently, lightly sand the inside and outside of the tubes, then spray paint them with a good optical-quality flat black paint. The black paint will absorb unwanted scattered light. Then install the tubes, bonding each in place with a good epoxy or using hardware if there is a means of attaching it with screws or some type of fasteners.

You might have to experiment with aligning the tubes a bit to get them collimated, but it should be possible to do this so that the LED light shines only onto the receiver and so that the receiver only sees the LED. Something like this: