Who signs the lighting calculation, and what that document actually says

Specifications and supply

The document nearly always comes from whoever is selling, and that is not a flaw in itself. It becomes one when it serves to justify a choice rather than verify it.

The document nobody reads to the end

In a lighting supply the lighting calculation is the bulkiest annex and the one that gets the least attention. It arrives as a PDF, with pages of tables and false-colour images, and almost always whoever decides looks at the last line and moves on.

That is a pity, because it is the only piece of paper that allows two offers to be compared on something other than price. Without it the choice is between two descriptions, and the more reassuring description nearly always wins.

Why the seller produces it, and when that is fine

In most cases the calculation arrives with the quotation, produced by the supplier. That is not a flaw in itself: the supplier is the one holding the photometric files for its own fittings, and it is reasonable that it should run them.

It becomes a flaw when the document serves to justify a decision already made rather than to verify it. The difference is not in the author: it is in what the document declares and what it leaves out.

The sign that shows up at once

There is a signal readable in ten seconds. A calculation that states only average illuminance, without uniformity, without glare and without the breakdown by zone, allows nothing to be contested.

It is not necessarily bad faith: many tools produce that summary as their default output. But a summary is not a verification, and accepting it means giving up the only handhold available if the result does not come out on site.

The quantities that make a calculation useful

A useful document reports few things, but it reports them for each zone of use, not as a building total. These are the ones, and they are the same in a hotel, an office and a shop.

• maintained average illuminance, by zone and not by whole floor
• uniformity, which says whether light is spread or concentrated
• glare, declared with its index and not implied
• colour rendering, with the value that activity actually needs

The total does not say whether people will see well

An average over a large surface can come from a very uneven distribution: two brightly lit zones and one dark one give the same average as three even zones. The average alone does not distinguish those cases, and the eye does.

That is why uniformity has to be asked for alongside the average, always. They are two numbers that say nothing alone and describe the room together.

The assumptions are worth as much as the results

Inside every calculation there are choices that do not appear in the result but determine it: surface reflectances, the maintenance factor, mounting height, and whether furniture is present.

Changing these changes the result without changing a single fitting. That is why a serious document declares them at the top: not out of pedantry, but because without them two calculations are not comparable even when they use the same numbers.

Reflectances, the most optimistic assumption

The commonest and most generous choice concerns the surfaces. A calculation run with pale walls and a white ceiling gives far better results than one run with the real finishes, and hardly anybody notices while reading.

In a contract project with dark panelling and fabrics the difference is large. It is worth asking that the reflectances used correspond to the interior design, and not to the software's default values.

The maintenance factor, which decides the future

This number states how much the installation is assumed to lose over time, through dirt and through source ageing. A high value means assuming the installation will be cleaned often and that the sources will decay little.

It is the assumption that moves the result most, and the one least discussed. Asking for it to be declared and justified is the quickest way to understand how seriously the document was prepared.

A name on it, and what that entails

A design with no name on it binds nobody. If the value is not reached on site, somebody has to be accountable, and the answer cannot be the software that ran the numbers.

That does not mean an external professional always has to sign it: it means somebody takes responsibility in writing. It is a request that changes how the document is prepared, before it changes how it is read.

When an independent eye is needed

Not every project needs an external check, and imposing one everywhere is waste. It is needed where the stakes are high: workplaces with statutory requirements, public spaces, or large supplies where one error is multiplied by hundreds of fittings.

In other cases it is enough to ask for the right quantities and read the assumptions. Most problems do not come from wrong calculations, but from correct calculations of different questions from the ones that mattered.

Comparing two documents, in practice

Comparison is only possible if both reports give the same quantities over the same zones. If they do not, two offers are not being compared: the more reassuring document is being chosen.

When the zones do not match, the quickest route is not to redo the sums: it is to ask both parties to recalculate over the same zones, named by the client. It costs a couple of days and makes the choice defensible.

What to ask before reading the numbers

Before going into the tables at all it is worth checking four formal things. If they are missing, the numbers that follow are worth little, however precise they look.

• the calculation zones, listed and matching real use
• the assumptions declared at the top, not buried in an appendix
• the photometric files for the fittings, cited and obtainable
• the name of whoever answers for the document, with the date

Daylight, which often does not appear

Many calculations are run with no daylight at all, which is correct for a minimum night-time requirement and misleading for a room used by day. The document rarely says so, and the reader assumes it accounts for everything.

If the room has significant windows it is worth asking the calculation to state this explicitly. A second study is not needed: it is enough to know which of the two conditions the document describes.

Acceptance, meaning how it gets checked afterwards

A calculation only becomes verifiable if somebody writes down how and where measurements will be taken. Without that line, on site the measurement happens wherever is convenient, and the results cannot be compared with the prediction.

A few indications are enough: which points, at what height above the floor, with the surfaces in what state, and with the installation switched on for how long. Written beforehand, they discipline whoever produces the document too.

What a missed value actually means

If the result on site falls below the prediction, the possible causes are few and quickly told apart: fittings different from those calculated, surfaces darker than assumed, mounting height changed, or sources already degraded.

Having them as a list saves weeks of argument, because it turns a dispute into a check. And it is also why declared assumptions are worth so much: without them, none of the four causes can be demonstrated.

The quantities, and who answers for each

Lined up, the things that decide whether a lighting calculation is useful do not depend on the same person. Some are fixed by whoever writes the specification, others by whoever runs the numbers, and one is only discovered at measurement.

What changes for a design practice

For an interior designer, having the calculation on their own plans means being able to defend a choice of fittings with a number instead of a preference. It changes the tone of the meeting, and usually shortens it.

It also means noticing in time that a beautiful solution does not meet the requirement, while changing it still costs little. It is the part of the work nobody sees and that avoids the most expensive rework.

False-colour images say less than they seem to

The coloured maps that accompany every report are the most looked at part and the least informative. The colour scale is chosen by the software or by whoever runs it, and changing it can make the same room look well lit or disastrous without a single number changing.

They have to be read together with the legend, and the legend has to be compared between the two documents being weighed against each other. If the scales differ, the two images cannot be placed side by side, however tempting that is in a presentation.

The calculation changes if the fitting changes

It sounds obvious and it is regularly ignored: if a fitting is swapped on site for an equivalent one, the delivered calculation no longer describes the installation that was built. Substitutions during the works are normal, and hardly ever does anybody redo the sums.

The reasonable request is not to forbid them: it is to write that every substitution comes with a check over the affected zones. It is a few hours of work, and it is the only thing that keeps the document and the installation tied to each other.

Asking for the calculation before choosing the fittings

The usual order is to choose the fittings and then have someone verify that they work. Reversing it changes the job: you start from the plans and the requirements, see how much light is needed and where, and only then choose among the pieces that hold that performance.

The advantage is not theoretical. Choosing first, every objection from the calculation becomes an aesthetic sacrifice to negotiate; choosing afterwards, the same information arrives when changing your mind still costs nothing, and the final choice remains with whoever is designing.

The three lines to ask for with the quotation

If only one thing survives from all this, let it be three requests to make alongside the request for an offer. They are short, and they completely change what comes back.

• the quantities by zone, not the building total
• the assumptions declared, reflectances and maintenance factor included
• a name and a date on the document

• A bespoke piece
• The buildings we have lit
• Free lighting calculation
• Lighting knowledge