Illuminance surveys: what a lux level survey is, and when you need one

What is a lux survey?

A lux survey is a set of light readings, in lux, taken at evenly spaced points across an area with a calibrated lux meter, together with a record of how and where each one was taken. "Illuminance survey" is the same thing under its technical name.

Luxwork is a lighting survey app for iPhone and iPad, in beta: readings from your calibrated meter go on the plan where you took them.

Illuminance survey, lux testing or light level survey: one job

"Illuminance" is the technical term for light falling on a surface, and lux is the unit it's measured in, which is why the same work goes by both names. Some firms call it a lighting assessment.

A few things share the name but are different jobs:

• An LED or energy lighting survey. Lighting suppliers often offer a free survey so they can quote for new fittings. It may include some lux readings, but its purpose is a quotation, not a record of light levels.
• An emergency lighting test. Checking that emergency fittings switch on and last for their rated time is routine maintenance. Measuring the light they give is a separate check, done in the dark with the normal supply off. How to check emergency lighting lux levels.
• A luminance survey. Luminance is how bright a surface looks, measured in candela per square metre with a different instrument. A lux survey measures the light arriving, not the light coming back off a surface.

When do you need a lux survey?

No law in Great Britain names a lux survey as such. What the law asks for is lighting that's good enough, and a survey is how you find out whether it is.

• The duty. Regulation 8 of the Workplace (Health, Safety and Welfare) Regulations 1992 requires suitable and sufficient lighting in every workplace, and emergency lighting where people would be in particular danger if the normal lighting failed. It gives no figures.
• The figures. HSE's guidance, HSG38 Lighting at work, gives an average and a minimum for five kinds of work. It's guidance, not law. A reading at or above HSG38's figure doesn't make a workplace lawful, and one below it doesn't make it unlawful. HSG38's table, and what the two columns mean.
• Risk assessment. The Management of Health and Safety at Work Regulations 1999 require employers to assess risks, and to review the assessment when there's reason to think it's out of date or something significant has changed. Poor lighting is one of those risks.
• Screen work. The Health and Safety (Display Screen Equipment) Regulations 1992 ask for lighting that gives satisfactory conditions and suitable contrast, without distracting glare or reflections. They give no lux figure either.

In practice, these are the usual reasons someone commissions or carries out a survey:

• people complain about the light, eye strain or headaches, or an accident raises the question
• the risk assessment is being reviewed, or the use or layout of a space has changed, for example new racking or new tasks
• new lighting has gone in and someone wants to know what it actually delivers
• a client, landlord, insurer or contract asks for evidence of light levels
• the lighting has aged, because lamps dim and fittings get dirty over time

Figures for offices, warehouses, corridors, kitchens and other common areas are on Lux levels by area.

How a lux survey is done

The method in brief. The full, step-by-step version, with the plane, the grid and what to record, is in How to do a lighting survey.

1. Decide what it's measured against. HSG38, a client's figure, or none. This sets the plane and the figures the results are compared with.
2. Check the meter. Note its make, model, serial number and calibration certificate.
3. Set the conditions. Lamps run up to full output, daylight kept out or recorded, every fitting working, or the ones that aren't counted.
4. Lay out the grid. Evenly spaced points across the area, set back a little from the walls. Best practice sets the number of points from the size and shape of the area and the height of the fittings.
5. Take the readings. Sensor flat on the plane at every point, with the surveyor standing clear so their body doesn't shade it.
6. Work out the results. The average, the lowest reading and the uniformity, which is the lowest divided by the average. Compare each with the figure it's judged against.

The average alone can hide a dark patch, which is why HSG38 gives a minimum as well, and why a survey should report both.

What an illuminance survey report should contain

A report is usually read by someone who wasn't there. They should be able to check it from the report alone. At the least, it should state:

• the site and area, and what the survey was measured against
• the meter, its serial number and its calibration certificate
• the measurement plane and how the grid was set
• the conditions confirmed before the first reading
• every reading, with where it was taken
• the average, the minimum and the uniformity, each with the figure it was compared with
• anything not measured, and why
• who did the survey, and when

What a lighting survey report should contain goes through each item on a specimen report, with questions to ask of any survey you receive.

What affects how long it takes and what it costs

• Size and number of areas. More floor area means more points, and each separate area is its own grid with its own result.
• When it can be done. Electric lighting is usually measured after dark or with the daylight kept out, which can mean a visit outside working hours.
• Access. High racking, live operations, permits or areas open only at certain times all add time on site.
• Emergency lighting. Measuring it means switching off the normal supply and working in the dark, so it's often a separate visit.
• What the report has to show. A plan with every reading and results area by area take more work than a list of numbers.

Who can carry out a lux survey?

The regulations don't name a qualification for taking lux readings. The person needs to be competent: they know the method, use a calibrated meter, and record enough for a reader to check the work.

Many workplaces use an electrical contractor, a lighting engineer, an occupational hygienist or a health and safety consultancy. Some facilities teams do their own, with a calibrated meter and a written method.

If you're hiring someone

Ask these before you book:

• What will the survey be measured against, and will the report say where that figure came from?
• Which meter will you use, and is its calibration in date?
• Will you survey after dark or with the daylight kept out, or record that it was present?
• Will the report show where each reading was taken, or only an average?
• Will it give the minimum and the uniformity as well as the average?
• Will it say which points couldn't be measured, and why?

Questions people ask

Is a lux survey a legal requirement?

Not by name. The law requires suitable and sufficient lighting and an assessment of risks. A survey is the usual way to show what the lighting actually delivers, and a client or contract may require one.

How often should a lux survey be done?

No fixed interval is set in law. Survey when something changes: new lighting, a new layout or use, complaints, or a risk assessment review. Light output also falls as lamps age and fittings get dirty, so periodic checks make sense. If a client or contract sets an interval, follow it.

What is a lux level survey?

Another name for a lux survey: readings of the light level, in lux, at points across an area, recorded with how they were taken.

What lux level does an office need?

HSG38 lists offices under work requiring perception of detail: an average of 200 lux, and at least 100 lux at any point. A design brief often asks for more. Lux levels by area covers other spaces.

Can I do a lux survey with my phone?

Not one that anyone else can rely on. A phone's light sensor is not a calibrated instrument, and published tests have found phone readings well away from a reference meter. Are lux meter apps accurate enough?

Does a lux survey prove the lighting is acceptable?

A survey records what was measured, and where each figure came in: at or above the figure it was compared with, or below it. Whether the lighting is acceptable is a judgement for the engineer or the person responsible for the workplace.

Is lux testing the same as an illuminance survey?

Usually, yes. When a firm offers lux testing of a workplace, it normally means taking lux readings across each area with a calibrated meter and reporting them, which is an illuminance survey. The phrase is also used for laboratory testing of lighting products and for checking lux meters themselves, which are different jobs.

Is an emergency lighting lux survey different?

Yes. It's done in the dark with the normal supply switched off, against the figures in BS 5266-1, the emergency lighting code of practice, rather than HSG38's table. We don't reproduce those figures. Emergency lighting lux levels sets out the method and what to record.