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Eye strain has nothing to do with the source of light, and has everything to do with the distance between your eye and the screen. Normal reading distance, that is the distance where people with good eyesight would have no eyestrain, is 35 cm (14"). People with myopia (nearsighted) feel better if the distance is shorter, people with hyperopia (farsighted) feel better if the distance is bigger. Young people with good eyesight can adjust to different distances.
If you have no problem reading for hours of your desktop monitor you are probably farsighted, if you can't stand to read of a desktop monitor you might be nearsighted.
With hand held devices, the problem is different. If it is too heavy you will keep it in a position that gives less strain to your hand muscles, since otherwise your hand will start to hurt before you feel eyestrain. And, obviously, you will get eyestrain.
There is a size for the image of letters on your retina that gives best reading speed and comprehension. If the font is too small (even if your eyesight is good and you can see the letters) you will feel the need to bring the device closer to your eyes and you will get eyestrain.
You can't get an accurate impression of a screen if you are looking over somebody's shoulder: the distance is bigger and therefore it is normal to get eye strain.
Any discomfort that you feel from your device in terms of light intensity is not eye strain, that is just your brain telling you to get away from the bright light, and it has to do with the fact that you are going to damage your eyes if you don't. (Long before electronic devices there was always the sun.) This can be avoided by turning the lights on in the room (your pupil will get smaller, as your eyes adjust), turning the brightness down or reading white letters on black background.
There is of course the other side, where there is not enough light intensity, and all I can say to that is: stop it, night vision was not meant for reading.
LCD monitors are not like CRT monitors. The problem with CRTs is that they are emitting small amounts of X-ray radiation, which you wouldn't actually feel, but were damaging your eyes, and the closer you would get to it, the more X-rays you got.
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