How much light do you really need?
The luminaire box lists lumens: the total amount of light the luminaire emits, in every direction. The standard, however, specifies lux, meaning how much light actually reaches the working plane. Between the two lies the whole room: ceiling height, the color of the walls and ceiling, the luminaire's light distribution, the number of fixtures and their placement.
How many lux do you need?
A few guideline values based on EN 12464-1: hallway 100 lux, stairwell 150 lux, dining area 150-200 lux, kitchen countertop 300-500 lux, office desk 500 lux, drafting table or precision assembly work 750-1000 lux. At home these are guidelines, but in the workplace they are mandatory minimums, and an occupational safety inspection may ask for the measurement report.
Besides the lux value, there's another number that gets mentioned less often: uniformity. An office may average 500 lux, but that doesn't help if one desk gets 800 and another 200. That's why the standard also specifies a minimum-to-average ratio in addition to the average, typically 0.6 in work areas.
Why can't you work it out in your head?
The light from a 3000-lumen luminaire isn't distributed evenly. Directly under a narrow 15-degree spot there can be as much as 800 lux, and two meters away just 60. The same luminaire with 60-degree optics spreads the same light out and delivers no more than 300 lux anywhere.
A photometric calculation models this. The software works with the manufacturer's actual light distribution curve, takes the reflectance of the surfaces into account (there can be as much as a 20 percent difference between a white ceiling and a dark wood one), and shows where the dark spots will be.
Start from the function
In practice, then, the starting point isn't lumens but use. First you decide what people do in the space and where; that gives the required lux, and only then comes the luminaire. A kitchen doesn't need a single light level: the countertop needs 500 lux, the walkway 150, and over the dining table it's the ambience that matters more.
That's why two apartments with the same floor area end up with completely different luminaire schedules. The calculation isn't a formality: it's what decides whether the finished space will be pleasant to work in.
