What is the intensity sensor? (NXT)

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Hello,

I was doing the tutorials for NXT robots. In the second tutorial, you are proposed to create a model robot, with just some sensors and motors. One of the sensors they mention is Intensity Sensor.

This makes me feel a bit confused. I do not see this sensor in the list of sensors of LEGO NXT Mindstorms. The only ones that I think it could be are the Light Sensor and the Color Sensor. At the beginning I thought it was the light sensor, but it did not work.

So, what is exactly the thing they call Intensity Sensor?

Thanks