I pulled together a selection of screen caps from the 2013 BluRay of The Terminator to try and get to the bottom of some of the details of the original design.
Below are a selection of hi-res shots of the eyes featured in the movie, in their various guises. As you'll likely notice, they vary in appearance throughout the movie. This is down to the various roles they were fulfilling and their final scale. Extreme close-ups of the eye (showing the fantastic opening/closing iris) would have likely been accomplished with a larger scale reproduction of the Endo Skull's eye socket, to achieve a greater level detail and fidelity, possibly made at twice the actual size. This shows the lens as having a clear glass finish, revealing the soulless black beyond it when the iris contracts (like another relentless predator of nature, the shark). The red light emanating from within is a flat, even hue.
The super-scale close-up model of the eye suggests the lens is clear
Views of the 1:1 scale animatronic skull seem to show a red-tinted lens, back-lit by a softer red bulb. The image below of Schwarzenegger's puppet with exposed eye suggests the lens is actually slightly pearlescent, although this could just be due to reflections. Subsequent images make it almost look like a large LED has been used (like in my original build of the skull), although it seems likely that all of the 1:1 scale versions used throughout the making of The Terminator would feature the same lens/bulb combo from Stan Winston Studios.
Is it a red lens and bulb, or a clear lens and red bulb?
Does this quick build by Stan Winston for the dying T800 utilise a clear or red lens?
The eyes most commonly found used on shop-bought reproductions of the Endo Skull are those from T2 onwards. They feature a stippled/textured appearance (much like the brake lights on cars), which wasn't an element of the original design from The Terminator.
Sideshow Collectables textured lenses. Note the non-standard darker 'eye ball'
Stan Winston Studios T800 skull (with chromed teeth) from the opening credits of T2. Note the red lens
Oversized skull for close-up shots in Terminator 2 with clear lens.
So what's the conclusion? Well, there is no definitive answer due to different types of lens/bulb being used for different applications during filming. I guess it's up to personal preference.
I personally like the 'clear lens rear-lit with red bulb' combo; I feel the Terminator's eyes should look dead when unlit (ie clear, not red) - this probably stems from my memory of the eyes shown in top image in this post; the soulless black eyes of this relentless killing machine. My preference is reinforced by this last image above (from T2); it looks crisper and 'correct' to me somehow.