This instrument incorporates the proven 4-stroke gradient process. The built in timer automatically turns it off and removes the lenses from the tint when the cycle is complete, giving the technician time for other tasks. Produces gradient lenses only. BPI 4-Stroke Gradient III is made of 100% low maintenance stainless steel. The 4-Stroke Gradient III has a one year warranty. In the 4-Stroke gradient process, the lenses are lowered further into the tint bath on each stroke. After the fourth stroke the gradient repeats the cycle. This mechanical process creates a smooth gradient effect.