This is a reply from the MOD experts after I was lased? about a month ago.
I have shown the clip to laser experts at DSTL and we feel that the device in question is a laser. The beam is very narrow and has no side lobes that would be expected if a powerful hand held lamp was used.
The pure green colour strongly suggests that it is doubled neodymium at a wavelength of around 530nm. Hand held lasers of this frequency (colour) are readily available.
These hand held lasers have a beam divergence of about 1mrad (milli-radian) and this is typical of lasers readily available over the internet. At a distance of 1.4 miles (say 2.5kms) these produce a laser ‘splash’ of about 2.5m diameter, enough to fill the cockpit of a medium size helicopter.
The laser is certainly more than a typical presentation pointer. A CCD camera at a range of 2.5kms would be dazzled by a 50 to100mW laser at night. As the helicopter was operating in an urban environment the effect of the urban lighting would reduce the camera aperture which would indicate that the laser was probably nearer to the 100mW. Our assessment is that this likely to be a 100mwish laser operating at 532nm. Such a laser would have an NOHD (Nominal Ocular Hazard Distance – the distance at which you could view the beam without risk of eye damage. It would be very uncomfortable. Any closer to the laser increases the risk of eye damage) 70m and would definitely cause eye damage at about 25m. This is all based on experience and It would be very difficult to prove any of this from the video evidence alone. We would need to know more about the ambient illumination, particular content of the atmosphere and the camera being used.
Shaun