Grenz rays are part of the electromagnetic spectrum comprising low energy (ultrasoft) X-rays.
These were first investigated by Gustav Bucky in 1923 using a cathode vacuum tube with a lithium borate glass window, which he labeled Grenz rays as he believed that the biological effects resembled those of UV light and traditional X-rays and hence were on the border between the two (Grenz = border in German).
[2] (HVL is half-value layer, and refers to a thickness of a given material that will cut the X-ray flux by 50%).
Grenz rays are absorbed by air and therefore treatments must be given at fixed distances for which the machine has been calibrated.
[3] The exact mechanism of action is unknown but it does have effects on the Langerhans cells in the epidermis.