The question arrives in the same form on most projects: how many lux does this room need. It is answerable, but the answer is attached to the task rather than to the room, and the figure that matters is not the one measured on the day the space opens.

AS/NZS 1680 sets maintained illuminance targets for interior and workplace lighting. Maintained is the operative word and it is the source of most of the confusion in practice: the figure is the level held at the end of the maintenance cycle, after lumen depreciation and the accumulation of dirt on the luminaire and room surfaces. A design that hits its target on day one and drifts below it within eighteen months has not met the standard. It has met it once.

Targets by task

The standard is a family rather than a single document. AS/NZS 1680.1 carries the general principles; the AS/NZS 1680.2 series covers specific applications, with separate parts for circulation spaces and general areas, offices and screen-based tasks, educational facilities, industrial tasks and processes, and hospital and medical tasks. AS/NZS 1680.3 deals with measurement and photometric data, and AS/NZS 1680.4 indicates the method to determine luminaire maintenance factor, and the maintenance of the luminaires. Also, AS/NZS 1680.5 serves as the guideline for outdoor workplace lighting.

Typical maintained illuminance targets, as a general orientation only — the applicable figure for any given space comes from the relevant part of the standard for that task:

Two spaces of identical dimensions can carry different targets because the work in them differs, and a single space commonly carries more than one target — the task area at one level, the surrounding circulation at another. Applying one figure uniformly across a floor plate is the most common way to over-light a building and fail its energy calculation at the same time.

What "maintained" costs you in the design

The maintained figure connects to the initial design level through the light loss factor: lamp lumen depreciation, luminaire dirt depreciation, room surface dirt depreciation, and survival factor. The design has to start higher than the target so that it is still at the target at the end of the maintenance interval.

The maintenance interval is itself a design assumption. The same luminaire will depreciate at different rates in a clean, air-conditioned office and in a workshop, while a maintenance schedule that is not followed will invalidate the original assumption. The practical consequence is that the maintenance regime must form part of the lighting design rather than be treated as an afterthought — one of the key principles addressed by AS/NZS 1680.4. In an interior lighting calculation, an appropriate maintenance factor should therefore be assigned to each luminaire based on its characteristics, operating environment and planned maintenance regime.

What the standard controls beyond lux

Illuminance is the number everyone asks for, and it is not sufficient on its own. Three other quantities determine whether a space is comfortable.

Uniformity. A task area averaging its target while varying sharply across the working plane performs worse than a lower, even level. The standard sets uniformity requirements on the task area and its immediate surround and meeting them constrains spacing relative to mounting height.

Discomfort glare. The unified glare rating quantifies the discomfort caused by bright sources in the field of view, and the standard sets limits by application — office and screen-based work being the most demanding case. UGR is calculated from the luminaire's luminous intensity distribution, its position relative to the observer, and the room surfaces. It is not visible in a render, and it does not appear in a lux figure. Normally UGR should be controlled to <19 on task areas in office or schools.

Colour rendering. The colour rendering index sets a floor for how accurately surface colours appear. General interior work sits comfortably at Ra80 and above; spaces where colour judgement is part of the task — retail display of merchandise, medical, printing, anywhere finishes are being assessed — warrant Ra90 and above.

Colour binning. This determines the consistency of colour between different batches of a type of LED lighting product, and it is measured by MacAdam Steps(SDCM). The lower the MacAdam Step value, the higher the consistency of the colour can be maintained between different batch of products. Normally MacAdam 3 step is acceptable for internal lighting, and higher quality luminaire can achieve 2 steps.

Cyanosis observation. In relevant clinical areas, lighting should allow staff to recognise the bluish discoloration of skin associated with cyanosis. Suitability is assessed using the Cyanosis Observation Index (COI), with the applicable requirements determined by the relevant healthcare-lighting provisions.

Hit the lux target while failing any of these and the complaint arrives anyway, expressed as the lighting feels wrong rather than as a number.

Where AS/NZS 1680 stops

The scope is interior and workplace lighting. Several adjacent problems belong to other standards, and using 1680 outside its scope is a recurring source of error:

Exterior roads and public spaces — AS/NZS 1158. Street lighting, pedestrian areas and public domain lighting are categorised and specified under this standard, not under 1680.

Sports lighting — AS/NZS 2560, which sets levels by sport and by level of competition, and deals with the uniformity and glare requirements particular to play and to broadcast.

Emergency and exit lighting — AS/NZS 2293. Emergency provisions are a separate design with separate obligations, and they are not satisfied by the general lighting being adequate.

Obtrusive light from exterior installations — AS/NZS 4282, which controls spill light, luminaire intensity and sky glow at the boundary rather than illuminance inside the site.

Energy — the National Construction Code. Illuminance targets come from 1680; the maximum lighting power density permitted to achieve them comes from Section J7 of NCC Volume One. A design can meet 1680 and fail the NCC, and that combination is common where the target has been applied too generously.

Three misreadings that cost money

Designing to the initial level. The calculation is set up without a realistic light loss factor, the space opens correctly lit, and the complaint arrives two years later. The fix is in the maintenance factor assumption at the start.

One figure applied to a whole floor. Circulation lit to office levels; storage lit to task levels. The energy budget under Section J7 absorbs the excess and the project discovers the problem at documentation stage, when the layout is already coordinated.

Treating 1680 as the only standard in play. A car park designed to interior figures, an outdoor area designed without reference to AS/NZS 1158 or the spill light limits in AS/NZS 4282, emergency provisions assumed to follow from adequate general lighting. Each is a scope error rather than a calculation error, and each is caught by asking which standard governs before opening the software.

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