You mean frequency, not amplitude--- you mean chasing the light until it is too weak to burn through the iron. But then the iron is rushing toward the light, and the relative motion of the iron and the light is what determines the impact energy, and this is unchanged in any frame.

### EDIT: Perpendicular motion

Now that you said what you meant--- you meant perpendicular motion. Then the beam is slanting down, and like a flashlight shining at an angle, you assume that it covers more area and is reduced in intensity. This is just not true. The reason a flashlight gets dimmer at an angle is that the same number of photons are hitting more area, because a given angle-spread at the emitter gets turned into more area at a further distance.

The laserbeam is just tilted by your motion, not spread in angle, so it hits the exact same area when you are moving, in fact, a smaller area because of the Lorentz contraction. The intensity goes _up_ not down, but the atoms Lorentz contract just the same, so that the number of photon collisions per atom stays the same. Each collision is physical, so there is no mystery why it should be invariant.

The frequency of the light is also increaseded by your motion. But the relative motion of the light and the atom is unchanged, as I said above.

### EDIT: Amplitude decreasing

The amplitude of light is not a length, and does not extend in physical space. It's an internal thing. A high amplitude wave can extend over a big, or small, area, which is independent of the amplitude. There is no relation between space and "amplitude space".