Three terms, three measurements
The lighting vocabulary that buyers meet most often contains three words that are routinely used as though they were the same thing. Lumens describe the total light leaving the source. Beam angle describes how that light is distributed. Illuminance describes how much light lands on a surface. They are related, and none of them can replace another, because each answers a different question and each is measured under different conditions.
This glossary defines each term, states what test condition it carries, and sets out the boundaries between them.
Lumens: total light leaving the source
Luminous flux, usually reported as lumens, is the total quantity of light emitted by the source in all directions within the measured cone. It is a property of the measurement rather than of the product, and it moves with the machine, the mode, the detector distance, the drive current, the ambient temperature and the warm-up. A lumen figure without those conditions is a result without a method.
The condition to request is the machine type, whether an integrating sphere or a goniophotometer, and the mode reported, whether total flux or a centre-beam value. The two are not interchangeable: a total flux figure describes all the light leaving the source, while a centre-beam figure describes the light in a narrow direction. Both can be expressed in lumens, and neither can be compared with the other without knowing which was reported. Lumens without test conditions sets out the full condition table.
Beam angle: how the light is distributed
Beam angle describes the spread of the light. A narrow beam puts the available flux into a small patch and reads bright at the centre. A wide beam puts the same flux into a larger patch and reads less bright at the centre while covering more area. The angle is a distribution property, and it is measured with a goniophotometer or a related instrument.
The condition to request is how the angle was defined: the angle at which the intensity falls to a stated percentage of the centre value. A beam angle quoted without that definition is not comparable, because two suppliers can use different cut-off percentages and report different angles for the same light.
Illuminance: the light that lands on a surface
Illuminance is the light arriving at a surface per unit area, usually reported in lux. It is the quantity the user actually feels, because it decides how bright the table, the tent wall or the page appears. It falls with the square of the distance from the source, so it depends on the mounting height as much as on the source itself.
The condition to request is the distance at which the reading was taken and the area the measurement covers. A centre illuminance reading taken close to the source reads high and describes a small patch; a reading taken farther away reads lower and describes a larger patch.
The three terms side by side
| Term | What it measures | What carries its condition | What it cannot replace |
|---|---|---|---|
| Lumens | Total light leaving the source | Machine, mode, distance, current, ambient, warm-up | Beam angle and illuminance |
| Beam angle | How the light is distributed | The cut-off definition used | Lumens and illuminance |
| Illuminance | Light landing on a surface per area | Distance and the measured area | Lumens and beam angle |
| Centre beam value | Light in the central direction | Machine, distance, mode | Total flux and illuminance |
| Beam pattern | The shape of the distribution | The instrument and the sample orientation | The headline angle |
A lumen figure ≠ an illuminance figure
A lumen figure is not an illuminance figure. Lumens describe what leaves the source; illuminance describes what arrives at a surface. The two are related by distance and distribution, so a high lumen figure can produce a low illuminance on a distant surface.
A beam angle is not a brightness figure. A narrow beam makes the centre read bright while the surrounding area reads darker, and a wide beam does the opposite. The angle describes the distribution, not the brightness.
A third boundary: an illuminance figure is not a coverage figure. Illuminance describes a point or a small area; coverage describes how far the light reaches across a surface. A reading at one point does not describe the patch.
Five questions for the supplier
- Which machine produced the lumen figure, was it an integrating sphere or a goniophotometer, and what mode was reported? Total flux and centre-beam values are not interchangeable.
- How was the beam angle defined, and what cut-off percentage was used? The angle is not comparable without the definition.
- At what distance was the illuminance measured, and over what area? Illuminance falls with the square of the distance.
- Which brightness mode does each figure belong to, and was the sample stabilised at a stated ambient temperature? Conditions define the result.
- Can you send the test-condition sheet together with the quotation rather than after the order? The comparison should be made on the conditions.
Next step
Use the three terms as three separate lines in the request, and keep the conditions beside each. When the figures arrive in that shape, the comparison becomes a comparison of products rather than of measurement practices. Related reading: beam angle and illuminance for the detailed relationship, lumens without test conditions for the light-side conditions, and IP-rated and ingress terms for the protection vocabulary. To compare two models on these terms, start at contact.
Part of the Lumizuno outdoor-lighting notes. Illustrative values only; lumens and ingress ratings must be read with their test conditions.