What maintenance factor should a DIALux lighting calculation use?
Choose a defensible DIALux maintenance factor from the luminaire, environment and maintenance plan instead of applying one generic value to every project.
The short answer: use the factor supported by the actual equipment, environment and maintenance plan
There is no responsible universal maintenance factor for every DIALux lighting calculation. Choose and document a value from the selected luminaire and light-source performance, the expected dirt and operating conditions, the room or relevant surrounding surfaces, and a maintenance schedule the owner can realistically carry out. Then calculate maintained performance against the project's governing criteria.
A convenient default can make a result look complete while concealing an unsupported assumption. A professional calculation should identify the method and source data, state the cleaning and replacement basis, show where one factor applies, and explain limitations. If the maintenance plan changes, recalculate rather than editing only the number printed in the report.
1. Understand what the maintenance factor changes
The CIE defines maintenance factor as the ratio of illuminance produced after a stated time interval to the illuminance produced when the installation is new. It accounts for losses such as dirt on luminaires and room surfaces and reduced source output. In practical design, it connects the initial model with the maintained condition expected near the end of the planned maintenance cycle.
This is why maintained illuminance is not the same as day-one illuminance. A lower supported factor generally requires more initial light to meet the same maintained criterion, which can affect luminaire quantity, output, energy and glare. A higher factor may support a leaner scheme, but only when equipment performance and the maintenance programme justify it.
2. Start with a maintenance plan, not an isolated decimal
Ask who will clean the luminaires and room surfaces, how often access is possible, whether cleaning can happen during operating hours, and what triggers replacement or repair. A high-bay fitting above active machinery, a sealed office panel and an outdoor road luminaire face different access, contamination and cleaning realities even when all use LED technology.
Record the calculation period and the intended intervention at its end. If the owner cannot commit to the assumed interval, test a longer realistic cycle or redesign for easier maintenance. A factor tied to an impractical schedule is not a conservative assumption; it is a maintenance promise the project may never keep.
3. Gather manufacturer performance data for the selected variant
Use current data for the exact luminaire, optic, driver, output setting, operating temperature and intended hours. Depending on the selected maintenance method, relevant inputs can include luminous-flux maintenance, source or luminaire survival assumptions and any manufacturer guidance on cleaning, enclosure or environmental suitability. Do not transfer a performance statement from a different wattage, optic or thermal condition without evidence that it applies.
CIE 97 advises obtaining accurate equipment data from manufacturers. Keep the datasheet or declared performance source with its date and product code. If reliable data is unavailable, disclose the conservative assumption and its effect instead of reverse-engineering a factor that happens to make the layout pass.
4. Classify the environment and dirt exposure honestly
Consider airborne dust, oils, fibres, smoke, insects, humidity, traffic residue, washdown, coastal contamination and the way the luminaire is ventilated or sealed. Also consider whether the optical surface faces upward, downward or into a process zone. A clean office and a fabrication bay should not automatically share the same luminaire-dirt assumption.
For interiors, room-surface deterioration can also reduce reflected light. Confirm surface materials, cleaning or repainting expectations and the importance of interreflection to the design. For outdoor work, identify the method appropriate to that application rather than importing an indoor room-surface component. Where conditions are uncertain, model a documented range and show which decision is sensitive to it.
5. Use one detailed method consistently across the calculation scope
DIAL's official version history states that DIALux evo includes simplified or detailed maintenance-factor calculation according to CIE 97:2005, and its current product information lists maintenance-factor and maintenance-planning support. Select the method required by the project and make sure each component comes from a compatible basis rather than mixing values from unrelated guidance.
Large projects may need different factors for genuinely different zones, luminaire types or maintenance cycles. Name those groups clearly and apply them to the correct rooms, exterior areas or light scenes. Avoid using many slightly different values without a maintenance reason; complexity should help the operator understand the plan, not make the report harder to audit.
6. Check the trade-off between maintenance and installed power
A lower maintenance factor can drive additional luminaires or higher initial output. That may improve the end-of-cycle result but can increase initial illuminance, glare risk, installed power and capital cost. A credible option study should therefore compare maintained compliance together with initial conditions, energy, controls, access and the cost of the proposed maintenance cycle.
Do not raise the factor merely to remove a luminaire from the layout, and do not lower it as an automatic safety margin. Consider whether better sealing, more accessible placement, a planned cleaning interval, replaceable components or verified constant-light-output control changes the whole-life solution. Any control strategy still needs documented product behavior and a maintenance basis.
7. Run a sensitivity check when the future condition is uncertain
Calculate the preferred supported case, then test at least one credible alternative when manufacturer data, operating hours, contamination or cleaning frequency remains uncertain. Keep geometry, photometry, output, calculation surfaces and criteria unchanged so the comparison isolates the maintenance assumption.
Review more than average lux. Check minimum values, the required form of uniformity, glare, vertical illuminance, luminance and spill where relevant. A layout with little margin may become sensitive at edges or between fittings even when the average still passes. Use the comparison to decide whether the team needs better data, a revised maintenance plan or a more robust arrangement.
8. Make the report auditable and useful to the operator
State the overall factor or detailed components, calculation period, assumed operating hours, environmental classification, cleaning interval, replacement or survival policy, manufacturer sources and any room-surface assumptions. Identify which areas and luminaire groups use each value. The maintained result pages should be traceable to those inputs.
Also state what the calculation does not verify: actual site cleanliness, future maintenance execution, unexpected thermal conditions, product variation and installation quality. A DIALux calculation can support design evaluation or submission, but acceptance remains with the relevant client, consultant or authority, and ongoing performance still depends on operation and maintenance.
Maintenance-factor decision checklist
Before calculation, confirm: (1) exact luminaire, optic, driver and output; (2) manufacturer maintenance data and source date; (3) operating hours and ambient conditions; (4) dirt and contamination exposure; (5) cleaning access and interval; (6) replacement, repair or survival policy; (7) indoor surface-maintenance assumptions where relevant; and (8) the governing method and project criteria.
Before issue, verify: (1) the correct factor applies to each zone and luminaire group; (2) maintained and initial results are understood; (3) the sensitivity case has been reviewed where inputs remain uncertain; (4) glare, uniformity and other briefed metrics still work; (5) the report records every maintenance assumption; and (6) the owner can realistically implement the maintenance plan.
Authoritative references used for this guide
The CIE International Lighting Vocabulary defines maintenance factor and the losses it represents. CIE 97:2005 explains indoor maintenance planning, equipment, environmental and operating influences, while CIE 154:2003 addresses outdoor systems. ISO/CIE TS 22012:2019 provides a way of working for determining maintenance factors for indoor and outdoor applications using the established CIE methodologies and current equipment considerations.
DIAL's official DIALux version history records simplified and detailed maintenance-factor calculation according to CIE 97:2005, and its current product information includes maintenance-factor and maintenance-planning capabilities. These sources establish the method and software context; the project team must still select verified product data, maintenance conditions and acceptance criteria for the specific installation.
Frequently asked questions
Is 0.8 always the correct maintenance factor in DIALux?
No single value is correct for every installation. The factor should be supported by the selected equipment, operating and environmental conditions, room surfaces where relevant, and a defined maintenance cycle. Use the applicable project method and document the evidence.
Should I change the maintenance factor if the cleaning interval changes?
Yes, when the interval changes the expected depreciation at the end of the cycle may also change. Update the maintenance inputs, recalculate the affected scope and recheck maintained results, initial conditions, glare, uniformity and other required metrics.