None of the 3D game engines currently available can give you 100% accurate photorealism easily. They are all stuck at about <40% of photorealism right now, compared to the visual fidelity that a real physical 4K video camera would record in the same scenario. The light behaviour, material-light interaction, lens behavior, reflection, refraction, shadowing calculations needed for more photorealism are beyond today’s fastest GPUs.
That situation may improve - e.g. to maybe 50% - 55% of photorealism - when new GPUs supporting hardware realtime raytracing come into play in the next year or two.
For example, Nvidia Volta GPUs will soon allow you to realtime raytrace, as will AMD’s new GPUs.
The closest to what you need right now may be CryEngine 5, if you look at this demo:
That’s for open nature. If it is semi-realistic building interiors you are focused on, Unreal Engine 4 seems to do best there:
If you see something that looks more photoreal than the about 40% I mentioned, that is a team of highly trained 3D artists using elaborate artistic tricks to make stuff look better.
There are ways of bumping up the photorealism of realtime 3D with post shader-injection like ReShade and SweetFX here:
There is nothing that renders in realtime on GPU right now that can get anywhere near the photorealism of a real photo/video, or of an offline 3D render engine like Vray, MaxwellRender, Octane that takes several minutes or hours to render one still frame.
You could try contacting GPU maker Imagination about their old realtime raytracing GPU:
It seems to be off the market right now, but they might be able to give you a development board or two that works with Unreal Engine 4.
Also, Intel was working on realtime raytracing using FPGA development boards. Perhaps you can reach out to them for a scientific collaboration.
Basically: Until the next generation of realtime raytracing GPUs arrive, and Unity, Unreal Engine, CryEngine and so forth support it, you are stuck with 40% - 50% accurate realtime 3D graphics.
And you will need to use some basic artistry to get more out of the engines.