How to fix vtkFollower size ,when zooming the camera

I am use vtkFollower add a text.I want to fix the font size. It does not change when zooming the camera

    vtkSmartPointer<vtkVectorText> atext = vtkSmartPointer<vtkVectorText>::New();
	atext->SetText("Origin");
	vtkSmartPointer<vtkPolyDataMapper> textMapper =
	vtkSmartPointer<vtkPolyDataMapper>::New();
	textMapper->SetInputConnection(atext->GetOutputPort());
	vtkSmartPointer<vtkFollower> textActor =// vtkVectorActor vtkFollwer;
	vtkSmartPointer<vtkFollower>::New();
	textActor->SetMapper(textMapper);
	textActor->SetCamera(m_renderer->GetActiveCamera());

	//textActor->SetScale(0.2, 0.2, 0.2);
	textActor->AddPosition(position.x(), position.y(), position.z());
	textActor->GetProperty()->SetColor(0, 0, 1);
	textActor->PickableOff();