Concerns About Heat and Light Discipline
Some excellent questions were raised concerning the light sources heat and whether it would interfere with the wielder's vision. I have brought forward some of my response and elaborated on the virtues of a light source as a tactical weapon. A lantern flame is little hotter than a typical candle, and considerably smaller and less intense than that of an open torch. As these are easily bearable I don't think heat is a concern.
Naturally there is no reason to adhere precisely to historical designs. Which opens up the possibility of using light sources that do not even entail flames or heat. These could be a variety of alchemical, biological, naturally occurring geological artifacts like crystals, arcane or magical effects.
In any case the lantern is well shielded such that light is only allowed to project through the hole in the shield and only when the portal is opened via a lever held in the gauntlet. Therefore the wielder is not blinded or revealed in the dark by his lantern until his gauntleted hand grasps the lever and opens the hasp on the shield face that allows lantern light to pour out into the darkness.
In many survival games darkness is a considerable thing to negotiate. Whether to prevent the spawning of mobs in Minecraft, or simply to allow for navigation and threat detection in most other games, a light in the dark is an appreciable advantage. Tactical use of such a light can benefit from some skillful applications.
Naturally it is doubtful the lantern would be anywhere near this bright but the idea of illumination to allow for navigation and threat detection is illustrated by this image well. Further, disciplined use of the activation of the Lantern could be a concern the devs may want to regulate.
An overly fast alternation between deep darkness and even the warm glow of a lantern could have deleterious effects on players sensitive to strobe effects. Even a slower rate of alternation could allow for use of the Lantern Shield or any covered lantern for use in signaling. Be it "One if by land, two if by sea" or morse code. Alchemical solutions, colored glass lenses or the like could be used to create colored lights.
Could colored lights have in game tactical effects as suggested in the graphic? Could the game develop to the point that infrared might be a spectrum some characters develop a skill to see in, and a lantern casting such light could extend their ability to see where other characters might not? Again, regardless of the form such a lantern takes.
Finally, concerning the tactical use of light in low light conditions as provided by a concealable light source such as the Lantern Shield, disciplined use of light may prevent the wielder of a Lantern Shield from falling prey to ranged weapons with careful use.
By activating the lantern, scanning an area, and then deactivating it; the wielder is then free to move through the darkness that his eyes are more readily able to adjust to for not having been exposed directly to the light source. Then after navigating some distance at an unexpected angle, the wielder can then reactivate the light and scan again. Making it more difficult for an archer or spearman to target the wielder in the dark due to his eyes repeatedly needing to adjust to the light exposure and darkness. Naturally the degree this may be effective is questionable and surely this effect would be less pronounced the further an archer is from the Lantern Shield/covered lantern wielder.
Finally, use of a light tactically in dark areas even today, though admittedly without fancy bucklers full of spikes, is practiced by law enforcement around the world.
I don't think the concept so alien as to lack a place in our fantasy setting, even as low fantasy as Elyria is at its onset.