I think you mean pyrometric thermometer- the laser is just a pointing device to show you what the pyrometer (Infrared detector) is looking at. How close you have to be depends on the field of view of the IR detector- some project a laser ring to show you what it is measuring. If you are too far away it will average not only your hot CPU (or whatever), but also the cool regions of the motherboard nearby. However, it isn't going to work accurately through plexiglas- the device works on measuring the intensity of IR radiation emitted and some of this is absorbed (blocked) by the plexiglas. (Plexiglas is transparent in the visible but blocks some IR wavelengths like those emitted by warm surfaces.)
