tensorflow.python.framework.ops.Tensor to numpy

I am using the map function to do some preprocessing on the dataset and one of the steps is to get the median of the gray images so I can get rid of defected ones I tried to use .numpy but it did not work and then I found that the tensor type was <class 'TensorFlow.python.framework.ops.Tensor'> and I can not convert it to NumPy so my question is why we can't convert <class 'TensorFlow.python.framework.ops.Tensor'> to NumPy ?