How to Build a Laser Death Ray
Anyone designing a laser death ray runs into a vexing problem: you want the beam of light to be so intense that you can damage your target, but not so intense that your death ray generator melts into slag from its own beam. What can a laser weapons engineer do?
Well, you know how you can use a magnifying glass to get parallel sun rays to converge to a spot? You use the same trick on your laser. You focus the beam through a lens or mirror so that it converges to a very small spot at the target. Now you can keep the beam so diffuse while it is in the death ray device that it never harms the equipment, while allowing the beam to be so intense at the target that the object of your ire is scorched or blasted into oblivion.
This leads to several details that must be solved by the engineer. First, you need to be able to focus the laser at different distances, automatically and rapidly. Second, you need to know the distance at which the beam needs to be focused. If you get either of these wrong, your target will just get heat up a bit, or get a painful but mostly harmless surface burn.
The first issue can be solved by using motors and moveable or deformable lenses or mirrors. This is similar to the auto-focus technology of modern point-and-shoot cameras. However, you will probably want any antipersonnel lasers to focus much faster than a $300.00 camera!
The second issue also uses the same technology that cameras use, either by bouncing a ranging laser pulse off whatever the laser is aimed at and seeing how long it takes to return, or by adjusting the focus until whatever is aimed at displays crisp lines. Both of these can be spoofed - if the target has a very dark surface, very little of the ranging pulse might be bounced back to the ray gun. If the target has very little detail, the sensors might not be able to find anything to focus on. If the target is reflective, either method might cause the laser to lock on to the image being reflected rather than the reflecting surface itself. It is likely that weapons systems will use redundant methods to estimate the range.