What this calculator works out
This calculator applies the lumen method: it converts a target illuminance across an area into the total light output required, allowing for utilisation and maintenance losses, and divides by the output of one lamp.
It is standard lighting design arithmetic, applied to a temporary scene installation rather than a permanent one. This is a teaching and planning aid, not a casework tool.
Lighting requirement
Before you specify lighting
- Target illuminance depends on the task. General examination needs a few hundred lux; detailed close work needs considerably more.
- The utilisation factor accounts for light that never reaches the working plane — spilled upward, absorbed or lost outside the area. Outdoors with no walls to reflect from, it is lower than indoors.
- The maintenance factor allows for dirt on fittings and lamp output falling over life. For temporary equipment in field conditions, be conservative.
- Lumens per lamp is on the fitting, not the wattage. Wattage tells you consumption, not output.
Lumens measure total light emitted by a source. Lux measures light arriving per square metre — lumens divided by area. Watts measure electrical consumption and say nothing directly about either, which is why comparing a 500 W halogen with a 100 W LED by wattage is meaningless.
How this calculator works
The lumen method, rearranged for the number of lamps:
Required lumens = area × target lux ÷ (utilisation factor × maintenance factor)
Lamps = required lumens ÷ lumens per lamp, rounded upDividing by the two factors increases the requirement, because both represent losses. A utilisation factor of 0.7 and a maintenance factor of 0.8 together mean only 56% of the emitted light does useful work, so the source output must be nearly double the naive figure.
The method gives an average across the area. It says nothing about uniformity, and a scene lit to an adequate average can still have unusable shadow.
Worked example: a 30 m² scene at 500 lux
Using the default figures — 30 m², 500 lux target, utilisation 0.7, maintenance 0.8, 3,000 lumen lamps:
- Naive requirement: 30 × 500 = 15,000 lumens
- After losses: 15,000 ÷ 0.56 = 26,786 lumens
- Lamps: 9 at 3,000 lumens each
The loss factors nearly double the requirement, from five lamps to nine. That gap is the most common error in quick lighting estimates — and outdoors, where there are no walls or ceiling to reflect light back down, the utilisation factor can be lower still.
Common mistakes
- Comparing lamps by wattage. Lumens measure output; watts measure consumption.
- Omitting the loss factors. They can nearly double the requirement.
- Using indoor utilisation factors outdoors. There is nothing to reflect from.
- Designing for average illuminance only. Uniformity and shadow matter as much.
- Forgetting glare. Powerful lamps at low angles blind rather than illuminate.
Frequently asked questions
What illuminance is needed at a scene?
It depends on the task. General movement and orientation needs relatively little; searching and examination need several hundred lux; fine detail work and photography of small items need more again. Published workplace lighting standards give task-based figures that transfer reasonably well.
Why does utilisation differ so much outdoors?
Indoors, light striking walls and ceilings reflects back onto the working plane, so a good proportion of the emitted light is eventually useful. Outdoors it escapes. A utilisation factor that is reasonable in a room may be optimistic by a wide margin in an open field.
Does lighting affect the search itself?
Considerably. Detection rates for small items fall sharply in poor light, and oblique lighting reveals surface detail that flat frontal lighting hides. Some evidence types are searched for with specific light sources and filters rather than general illumination.
What about power supply?
Nine 3,000 lumen LED floodlights might draw 250 to 300 watts in total, which a modest generator handles. The same output in halogen would draw several kilowatts. Cable runs, RCD protection and safe siting of generators are all part of a temporary installation and fall under electrical safety requirements.
Is what I enter stored?
No. Everything you enter is processed in your browser and is never transmitted or retained. Even so, treat any public website as an unsecured environment and do not enter identifiable case information.
Related tools
References
- Health and Safety Executive — lighting at work, temporary electrical installations and generator safety
- GOV.UK — Forensic Science Regulator codes of practice for scene examination
Sources are checked at publication and can change — how I choose and check references.