It’s a far from common occurrence when A. Lange & Söhne cross over to the dark side, but a superb new Datograph Up/Down Lumen possesses a transformational alter ego which is only revealed after daylight has ebbed away and darkness descends.

Typically, if such a word can ever apply to A. Lange & Söhne, the revered German company is a brand for which the highest standards of reserve and sophistication are unfalteringly dialled in codes of conduct for the design team, where nothing, but nothing detracts from the purity of the horological excellence and pristine execution which defines any watch bearing the distinguished crescent of block capitalised font.

Occasionally though, whilst naturally never compromising on any of its core values, Lange has eschewed the classic, traditional aesthetics of its immaculate dials and instead permitted a discreet sight of the inner workings, behind a smoky black sapphire glass which provides a perfect foil for the dramatic light show which comes after dark.

The new Datograph Up/Down Lumen is indeed such a watch. It is only the fourth of a series of Lumen limited editions since it was debuted in 2010 when the Zeitwerk preceded Grand Lange 1 and Grand Lange 1 Moonphase versions, and is produced in a limited edition of two hundred pieces in platinum, It’s a piece which has two distinct personalities, which alternate with each other, transforming between its sober, monochrome daytime appearance, to become a spectacle of vivid green luminescence when the lights are low.

Comprising a glassy central insert surrounded by a matte grey flange, upon which the markings for the tachymetre scale and the minutes are printed, the perlage finishing on the blackened plate below is revealed, glinting under the dark sapphire dial as the reflections move with the wrist. Also visible are the two date rings which carry the tens (1 - 3) and the units (1 - 9), aligning in a prominent display on the upper half of the dial, which The alpha hands in rhodium gold taper to a sharp point, and are inlaid with luminous accents.

The semi transparent sapphire section has been moulded with apertures to accommodate the large double window for the prominent date display and recessed white subdials for the small constant seconds on the left, and the 30-minute chronograph counter on the right. On the surface, the up/down power reserve indicator is at the six, and the hour indices are applied in rhodium gold batons. All of these factors combine to result in a display on which every function is clear and legible at all times

Of course this is a watch that waits impatiently for nighttime, and if the luminous treatment on the hands, subdials and the date display might seem to have been a little enthusiastically applied, as even in daylight there is a hint of green effervescence, there’s little to suggest that as light becomes dark these functions are powerfully accented with a vivid green glow, and the surrounding flange which was grey also transforms to a strong green, and the watch assumes its Mr Hyde persona.

At this point it’s probably worth noting that were this almsot any other brand, the luminescence might be considered a neat trick, and while it undoubtedly is really cool, it’s done in a way that you’d imagine were this glow in the dark technology available two hundred years ago, Ferdinand Lange would have been experimenting with it in much the same way as we see here, so not so much of a gimmick, perhaps more of an alternative to the brand’s more conservative styling language.

While the hands, subdials and the flange area are properly exposed to the UV rays of the sun, and the luminous molecules charge in daylight unfiltered, beneath the glass dial the date requires a little help, and while the smoky sapphire eliminates almost all light getting thorough, it is specially developed so that the UV reaches the luminous compound beneath, meaning that as the date changes on the stroke of midnight, the correct digit snaps into position, already glowing from the off.

It’s interesting to see how the date’s ‘tens’ (1 - 3) are luminous, whereas the units (0-9) are printed in black onto a transparent sapphire ring, and are then illuminated as it stops at its window, backlit by a static luminous background. One assumes that the outgoing unit might have interfered with the current ten as it moved offstage were both done in the same way.

Beating inside, the Datograph Up/Down Lumen boasts the in house A. Lange & Söhne Calibre L951.7 manual winding movement, which incorporates a flyback chronograph whose full suite of stop/start/reset actions can be commanded by the pusher at the 4 o’clock, so one chrono measurement can be stopped for referencing, while another has commenced instantly, which if taken in context with lap times for example, means that while the next one is under way, the previous one can be noted and recorded at leisure.

Underneath the sapphire caseback, the movement comprises some 454 individual components and as is the Lange way, each one is finished to the limits of what can be achieved by hand, using traditional tooling and techniques. Any Lange movement is an extraordinary feast for the eyes and with its lavishly engraved balance cock and bridges with Glashütte ribbing amid a gleaming metal maze of turned levers, springs, blued screws, polished surfaces and pink rubies set into gold chatons, the mechanical heart is a beautiful place in which one could lose oneself for hours.

The numbered platinum case of the A. Lange & Söhne Datograph Up/Down Lumen measures 41mm across and 13.4mm high, and it is presented on a black alligator leather strap with pin buckle,

Datograph Up/Down Lumen Fact file:

BRAND: A. Lange & Söhne
REFERENCE: 405.034
CASE SIZE: 41mm x 13.4mm
CASE MATERIAL: Platinum
DIAL: Semi transparent sapphire
MOVEMENT: A. Lange Söhne Calibre L957.1 manual winding, 18'000 vph
POWER RESERVE: 60 hours
FUNCTIONS: Hours, minutes, small seconds, big date, 30-minute chronograph, power reserve,
BRACELET/STRAP: Alligator
BUCKLE: Pin buckle
WATERPROOF RATING: n/a
PRICE: GB£84,000