Historically, lithophanes are designed to reveal an image in thin ceramic when backlit. These prints in glass are different. They are designed to create an image using front lighting. By taking advantage of transparent colored glass ability to transition from clear at the surface to black at a given depth. Matching (kinda) the relief in the underlying print to the transparent glass' optical properties yields continuous tone.
If you took your run of the mill 3D printed lithophane and printed a positive & a reverse x negative - one print being opaque & the other a colored transparent... Put the two together. Then look through the transparent print to the opaque print to reveal an image... different.
Closest is a cameo engraving with a couple differences. Cameo's are typically engraved in layered glass so that the geometry is fairly close to the subject. For the glass prints, the geometry in the background is mapped to grayscale - which doesn't always match the geometry in the subject. For example, dark areas in the image are deeper (or farther away from your eye) than dark objects in the foreground... in most photographs, generally, usually, most often, lighter areas in the image are physically closer to you... there are always exceptions.
Because there is physical relief to these prints, it messes with Weber-Fechner a tad too... ( a proposed relationship between the magnitude of a physical stimulus and the intensity or strength that people feel.) When I place one of these prints in a persons hand, the emotional effect is rather dramatic - or amplified a bit. (I've had to dry Max off a couple times.)
I also create something of a hybrid type lithophane in glass that is designed to reveal an a positive image when front lit & a negative image when backlit.
These are not Lithophanes
They are Lithovitrum Prints. Refractory mold is the stone (lithos), and the glass is the ink (vitrum). No oil is used tho...