Luminal Assessment

Luminal Assessment is the collective term for the procedures by which the Librarians determine the illuminative state of a space, an object, or a person, and record that state in a form that may be cited. It is distinguished from mere observation in that an assessment produces a figure, and distinguished from mere measurement in that the figure may be disputed. The Luminal Gradient is the metric on which every assessment terminates; the instruments, oaths, and quarrels by which one arrives at it are the subject of the present article.

Scope and standing

Assessment is not undertaken at will. Third Concordance, Annex 2, fixes the occasions on which a Luminal Assessment may properly be opened: the commissioning of a new floor, the death or transfer of a registered occupant, the discovery of an unlisted aperture, and the recurrence of a Reconciliation of Luminal Gradients sweep. Assessments opened outside these occasions are termed voluntary and are entered in the register in red, a practice retained from an era in which red ink was cheaper than the disciplinary hearing that so often followed.

The Office of Luminal Gradients holds the seal under which completed assessments are enrolled. It does not perform them. That distinction, insisted upon since the Concordance of Seals (Concordance 12, clause 9), has been the source of nearly every subsequent dispute in the field.

Instruments and the question of the Aperture

The standard instrument is the Nocturnal Aperture, operated in accordance with the procedure set out in the Manual of Luminal Gradients. The Aperture does not measure light; it measures the certainty of illumination, a quantity the Manual declines to define further on the grounds that definition would prejudice the reading. Operators are trained to record the certainty before considering its cause, and to record it in pencil where the Aperture is of the pre-Mark VI pattern.

A secondary instrument, the Luminal Diffuser, Mark IX, is held by assessment teams for emergency attenuation only. Its use during a live assessment requires the countersignature of a Clerk of the Beam, and its use during a voluntary assessment is prohibited outright — a prohibition added after the affair of Floor 2,208, where a team attenuated a reading they had been unable to obtain, and were thereafter unable to say which of the two facts had come first.

The Rite and its recensions

Every assessment of a person is preceded by the Rite of Silent Assessment, in which the assessor and the assessed remain together in an unlit room for a period not less than the time required to walk its perimeter three times. The Rite exists in two recensions. The long recension, favoured in the upper galleries, holds that the perimeter is to be walked by the assessed alone. The short recension, current in the lower floors and printed in the Manual since its fourth impression, requires both parties to walk it together, in step.

The disagreement is not ceremonial. Sigma Regulator observed in a marginal note of 4 Ember, 41 that the long recension yields a lower gradient in every case examined, and that the difference is roughly the difference between a room with one occupant and a room with two. The note was not withdrawn, but it was not enrolled either, and it is cited to this day by both parties as supporting their position.

Disagreement: the status of the negative reading

No question in Luminal Assessment is more vexed than what is to be done with a space that returns no certainty at all. Fourth Concordance, Annex 7, directs that such a space be rated at the lowest band, entered as Gradient 1–3, and referred upward for a Hypothetical Durability Assessment. The Guild of Luminal Regulators, which conducts the sweeps in which most such readings arise, has long held that a nil reading is not a low reading but the absence of a reading, and that to rate it is to rate a gap in the record.

"A rating of one is a statement about a room. A rating of nothing is a statement about the assessor, and the assessor is not the subject of the assessment."

— Guild of Luminal Regulators, Memorial to the Standing Committee, 9 Frost, 63

The Office of Volumetric Assessment has declined to resolve the matter, noting only that the question has been referred to it under Vector Beta and remains, in its words, "properly before us." The Circular 45-Sigma of 66 instructs assessors encountering a nil reading to record it twice — once in the band column and once in the margin — a compromise that satisfies neither office and has the practical advantage of doubling the paperwork in precisely the cases where the paperwork is least certain.

Records and the Galleries

Completed assessments are deposited in the Galleries of Attenuated Light, where they are shelved by gradient rather than by floor, so that a reader seeking the record of a particular room must first know how dark it was. Retrieval is undertaken by Lampwrights' Guild porters under standing arrangement; the Guild's rates were last revised in 58 and have not been contested, the porters having declined to contest anything in writing.

An assessment, once enrolled, is held to be true of the moment at which it was taken and of no later moment. This is not a concession to the drift of gradients but a rule of the register, set out in full in the Manual of Luminal Gradients, Appendix C. The effect is that the Galleries contain no current account of any floor, only an unbroken sequence of accurate ones. Visiting assessors are asked not to correct the shelves. Coats are to be left with the porter.