A P51m has two power 'ranges', 'high' and 'economy'. All operating modes but one use the 'high' range, and different modes have different numbers of power level available. Where modes have fewer than 4 power levels, it is the lower levels which are missing, so that, for example, a 2-power mode has medium and high powers available.
There is one 4-level mode available which operats in the 'economy' range.
Approximate lumen outputs per LED on each range and power setting are:
Within each range, power levels are stepped a factor of
three apart, far apart enough to be usefully distinct,
and covering a good range of brightness settings.
Each beam is controlled by separate electronics, and can be independently set to any one of the available levels for the mode the light is in by manipulating the switch controlling that beam.
Any combination of power levels between beams is available in use apart from having both beams simultaneously on high power.
That single restriction is no real loss, since due to the unique way that the spot beam also gives a good downspill of light, in any situation where the spot beam was on high, whether the flood beam was on high or medium would make little difference to the effective illumination for caving.
Though intended mainly as a super-frugal setting for camp use, sitting around, or for avoiding
lighting up scarily large pitches, even on the economy range, the ultralow power level
is still usable for movement around many kinds of cave passage,
with a runtime that verges on the ridiculous - somewhere around 2-3 weeks
of continuous light on a single 4.5V Duracell alkaline flatpack.
For people buying a bare P51 unit to use with either an Oldham or CEAG
(Speleo Technics) headset, mounting is easy.
All connections are by screw and ring terminal, and no permanent modification to
the headset is necessary.
The P51 works in 'Headlite' systems with all types of battery pack, and also in FX3, and any other commercial or home-made system with regular Oldham or CEAG headsets and the possibility of switching power polarity in the headset.
The unit works from a supply of between 3.6V and 4.5V. In practice,
this means 3x NiMH, NiCd or alkaline cells, or a 3.7V Lithium pack
are suitable power sources.
The P51 is not compatible with Oldham lead-acid lamps, since it requires
opposite power polarity in the headset compared to the Oldham standard for
More information on operation and fitting units into existing headsets is available in the P51 operation manual.