Transfer method for using a transfer device working as a depalletization device for transferring pre-formed layers of products from a pallet on which layers can be put one above the other to form a stack to an external conveyor, said transfer device comprising - a support structure comprising at least one vertical guide column, - a tray, mounted on the support structure and defining a tray zone and receiving the pre-formed layer, said tray being movable in the vertical direction, up and down between the top level of the stack and the external conveyor, while a tool mainly moves in the horizontal direction between the stack and the tray, - said tool, defining a pallet zone and mounted on the support structure, for transferring a layer between the tray and the pallet or the stack, said tool being a gripping head movable in the vertical direction, up and down between the top level of the stack and the tray - an object detector with a wide detection angle, for detecting products at the same vertical level in a wide horizontal field of observation, said detector being positioned so as to be able to detect objects in the pallet zone and to be able to detect objects in the tray zone, said detector being movable in the vertical direction along the support structure, said method comprising - transferring a layer to the tray from the stack or pallet and transferring a layer from the tray to an external conveyor, - detecting objects with an object detector, and moving said object detector in a vertical direction, - after the layer has been taken, slightly lifting up the gripping head, then transversally moving it to the tray zone, and lowering it down so that the layer can be directly released on the tray, and - detecting an object hanging under the layer gripped and lifted for the transfer, before said layer is released on the tray, by vertically positioning the object detector at the level of the height of the top layer on the stack before it is lifted. - a support structure comprising at least one vertical guide column, - a tray, mounted on the support structure and defining a tray zone and receiving the pre-formed layer, said tray being movable in the vertical direction, up and down between the top level of the stack and the external conveyor, while a tool mainly moves in the horizontal direction between the stack and the tray, - said tool, defining a pallet zone and mounted on the support structure, for transferring a layer between the tray and the pallet or the stack, said tool being a gripping head movable in the vertical direction, up and down between the top level of the stack and the tray - an object detector with a wide detection angle, for detecting products at the same vertical level in a wide horizontal field of observation, said detector being positioned so as to be able to detect objects in the pallet zone and to be able to detect objects in the tray zone, said detector being movable in the vertical direction along the support structure, said method comprising - transferring a layer to the tray from the stack or pallet and transferring a layer from the tray to an external conveyor, - detecting objects with an object detector, and moving said object detector in a vertical direction, - after the layer has been taken, slightly lifting up the gripping head, then transversally moving it to the tray zone, and lowering it down so that the layer can be directly released on the tray, and - detecting an object hanging under the layer gripped and lifted for the transfer, before said layer is released on the tray, by vertically positioning the object detector at the level of the height of the top layer on the stack before it is lifted.