The P51m is a twin-LED unit, with one LED providing a great spot
beam, and the other a smooth flood, each LED running from its own
independent circuitry for maximum reliability.
The highest-efficiency Cree XP-G LEDs are used to maximise output.
Compared to most LED lights, which generally use acrylic optics
for their spot beam (or their only beam), the cunning Bisun 'halflector' arrangement
not only gives a tight spot beam from the lower LED, but also gives a bright down-spill
of light from the same LED, just where it is most useful for walking, climbing, etc.
The optical design also means there is minimal spill
above the spot beam. This mean that even running at full power and close up,
the spot beam is easy on other cavers' vision as long as it is aimed
even slightly below their eyes.
The wide (or 'flood') beam uses a specially made conical micro-reflector round the LED,
which gives a smooth spread of light, usefully biased towards the centre.
This gives a particularly good flood beam for use while caving,
being significantly better at lighting
things in the near-distance than the pure flood obtained from a
'naked' LED at the same power level, without feeling
artificially concentrated, or sacrificing illumination in useful directions.
Given the nature of the two beams, and the numerous possibilities
for mixing the beams at different power levels, all kinds
of lighting demands can be accomodated by a P51m, in the way that
few other lights could equal, let alone beat, even running at rather higher power levels.
Each beam can be run at up to four brightness levels, but the P51m is configurable to operate with anything between one and four brightness levels, with multiple 3-power and 4-power modes for extra choice, including the option of an 'economy' 4-power mode with lower power consumption.