GTI GL-20e Transparency ViewerGTI GL 20e Transparency Viewer The GL series of transparency viewers delivers the high quality of Graphiclite products at a cost effective price. Four model sizes to choose from with viewing areas ranging from 10 x 18 to 16 x 42. The LiteGuard II may be added to track lamp hours used and indicate when relamping is required. GL transparency viewers are also available with a rotary dimming feature ( D models) or a digital dimming feature that includes