Total light, directional intensity and light arriving on a surface answer different specification questions.
A high lumen figure does not, by itself, tell you how an exterior luminaire will illuminate a façade. Lumens describe light output; candela describes intensity in a direction; lux describes illuminance at a surface. A commercial selection needs the quantity that answers the design question.
Lumens: the light output
Luminous flux is expressed in lumens, abbreviated lm. It describes an amount of light weighted for the standard photopic observer. Check whether the quoted figure describes an LED source or the complete luminaire, and which engine, optic, operating level and colour state it represents.
Two fittings with similar lumen output can distribute that light very differently. A total-output comparison does not establish the intensity reaching a distant feature, the uniformity of a wallwash or the appearance of a coloured material.
Candela: intensity in a direction
Luminous intensity is expressed in candela, abbreviated cd. An intensity distribution describes how the light is directed. For a projector, look beyond a peak figure to the distribution around the intended aim: the centre, the edge and the light beyond the target all matter.
A narrow optic can concentrate light into a smaller angular region. That does not mean it produces more total light, or that its beam will suit a broad façade. Use the selected configuration's photometry rather than substituting a family name or nominal beam angle.
Lux: illuminance on the receiving surface
Illuminance is expressed in lux, abbreviated lx; one lux is one lumen per square metre. The installed result depends on geometry and the direction of the incoming light. Illuminance also differs from the luminance of the surface seen by an observer: materials and reflection affect that appearance.
There is no single lumens-to-lux conversion that works without a distribution and geometry. Dividing total lumens by an arbitrary area is not a substitute for calculating the actual installation.
A distance example, not a product prediction
For an ideal point source illuminating a surface perpendicular to the ray, illuminance follows E = I / d². If the intensity in that direction is hypothetically 1,000 cd at 5 m, the example gives 1,000 / 25 = 40 lx. At 10 m, it gives 10 lx.
These are illustrative inputs, not measured Modaluce performance. The example excludes finite-source effects, oblique incidence, obstructions, reflected light and maintenance factors. A real façade calculation needs the relevant angular intensity and surface geometry; this arithmetic establishes neither maintained illuminance nor compliance.
Keep colour and operating state attached to the file
Dedicated white, dedicated colour and multichannel scenes can have different outputs. A white-light file does not describe every RGBW or CHROMA7 mix. Record the tested state and ask for clarification where a file cannot be matched to the intended configuration.
Use the IES and LDT guide to establish file identity, or explore architectural luminaires. Bring the target dimensions, mounting and intended scene to a technical discussion.
Definitions and scope
These references support the terminology. They do not certify a particular fitting or the illustrative calculation as an installation design.
Use the quantity that answers the project question
For the idealised 1,000 cd example, doubling distance from 5 m to 10 m changes the calculated perpendicular illuminance from 40 lx to 10 lx. Those figures are arithmetic using hypothetical intensity, not measured output from the fitting cards below.
Before carrying a number into a project, identify the relevant angular intensity, file orientation, receiving geometry and operating state. A total lumen figure cannot fill those missing fields. Oblique incidence and the actual distribution need their own treatment.
The companion example demonstrates a file/configuration mismatch so the reader can practise retaining an open decision rather than treating a neat calculation as approval.
Use the photometric reading record Excel workbook for the editable record, instructions, field guidance and fictional worked examples. The PDF companion provides a printable guide. All worked examples are fictional and are not project records, product-test results or accepted specifications.
