The four quantities, and which one you are actually designing

Every photometric argument on a project eventually reduces to two people using different quantities and assuming they mean the same thing. The manufacturer talks lumens, the code talks lux, the client talks “brightness”, and the eye — the only judge that matters — responds to none of the above.

There are four quantities. They describe the same light at four successive moments in its life.

1. Luminous flux — light leaving

Φv, in lumens. The total light a source emits in all directions, weighted by the eye’s photopic sensitivity curve V(λ), which peaks at 555 nm.

Lumens are a total. They carry no information about direction. A 3000 lm bare lamp and a 3000 lm narrow-beam projector are the same number and completely different design instruments. This is why “how many lumens do I need?” is not answerable and “what distribution do I need?” is.

2. Luminous intensity — light heading somewhere

Iv, in candelas. Flux per unit solid angle in a specific direction.

Candelas are what a photometric file stores: a table of intensity against vertical and horizontal angle. The polar curve on a datasheet is a slice through that table. When you choose between a 15° and a 40° optic on the same LED module, the lumens barely change and the candelas at nadir change by roughly a factor of six.

3. Illuminance — light arriving

E, in lux (lm/m²). Flux landing on a surface, per unit area.

This is what a light meter reads and what essentially every standard regulates, because it is cheap to measure and easy to verify on site. It is a property of the surface, not of the source — the same luminaire produces different lux on a desk, on the floor and on the wall behind it.

Illuminance is also the quantity most likely to mislead you. A room can meet 500 lx on the working plane and still feel like a car park, because horizontal illuminance says nothing about what the vertical surfaces are doing. EN 12464-1:2021 responded to exactly this by adding requirements for cylindrical, wall and ceiling illuminance alongside the old horizontal number.

4. Luminance — light seen

L, in cd/m² (nits). Intensity per unit projected area of a surface, as seen from the observer’s position.

This is the only one of the four the eye actually responds to. Brightness, contrast, modelling, glare, sparkle — all of it is luminance. For a matte (Lambertian) surface:

L = E × ρ / π

where ρ is the reflectance. A wall at 200 lx with ρ = 0.8 reads at about 51 cd/m². The same 200 lx on a dark timber wall at ρ = 0.15 reads at 9.5 cd/m² — a surface five times darker, from an identical lux calculation that both would pass.

Design in lux because that is what you can verify. Judge in luminance, because that is what your client will see.

The chain in one line

	Quantity	Symbol	Unit	Question it answers
1	Luminous flux	Φv	lm	How much light exists?
2	Luminous intensity	Iv	cd	Which way is it going?
3	Illuminance	E	lx	How much lands here?
4	Luminance	L	cd/m²	How bright does it look?

Two habits worth forming

State the plane. “300 lux” is incomplete. 300 lux horizontal at 0.8 m, 300 lux vertical at 1.6 m, and 300 lux cylindrical are three different specifications and three different schemes. Write the plane and the height into the spec every time.

Sanity-check in luminance before you commit. Take your calculated illuminance on the key surfaces, multiply by the actual finish reflectance, divide by π. If the feature wall lands within 3:1 of the ambient background, the “feature” will not read as one.

Where this goes wrong on real projects

The single most common failure is a scheme calculated entirely in horizontal illuminance and delivered into a room whose finishes were changed after the lighting was signed off. Nothing in the lux calculation moved. Every luminance in the room did. Confirm final reflectances at the end of detailed design and re-run the numbers — it takes an afternoon and it is the difference between the render and the building.