There are many different terms used in the world of lighting, some of them quite similar but with very distinct meanings. Can you tell the difference between luminance and illuminance?
In professional lighting design, these two concepts — luminance and illuminance — are essential to creating well-balanced spaces. Understanding them will help you evaluate how much light reaches a surface and how that light is perceived by the human eye.
Whether you're new to lighting or need to clarify these concepts for a technical project, Faro Barcelona is here to help. But first, let's go over a few key terms:
Key concepts to understand luminance and illuminance
- Luminous flux: this is the portion of radiant energy emitted by a light source — natural or artificial — in the form of electromagnetic waves that create the sensation of light (400–700 nm). Measured in lumens.
- What is a lumen (lm)? A lumen is the unit of visible light output we can measure. It refers to the luminous flux of monochromatic radiation with a frequency of 540 × 1012 Hz and a radiant power of 1/683 watt.
- Luminous intensity (I): measured in candelas, this is the magnitude that describes the distribution of luminous flux in space. It helps evaluate the amount of light emitted in specific directions.
- Candela (cd): the basic unit of luminous intensity. One candela equals 1 lumen per steradian (lm/sr). It refers to the intensity of light emitted in a specific direction, depending on the beam angle.
What is luminance?
Luminance, or photometric brightness, is the amount of luminous flux that hits, emerges from, or passes through the apparent surface of a primary or secondary light source from a specific solid angle. The “apparent surface” refers to the projection of the actual surface onto a plane perpendicular to the line of sight.
If two light sources have the same luminous intensity per unit of apparent surface, but one has a smaller area, it will appear brighter to the human eye.
How is luminance measured?
Luminance is expressed in candelas per square meter (cd/m²). It's calculated using the following formula:
Where:
- Lv = luminance in candelas per square meter
- F = luminous flux in lumens (lm)
- dS = surface area (m²)
- d = solid angle (steradians)
- s = angle between the surface normal and the observation direction
Tools for measuring luminance
Luminance can be measured using tools such as a spectroradiometer, a luminance meter, or a thermal sensor. Depending on the application, different beam angles are used: narrow angles for precision tasks, and wider angles for measuring average luminance across an area.
What is illuminance?
Illuminance (E) is the amount of light falling onto a surface. It's also defined as the number of lumens spread over 1 square meter.
The standard unit is the lux (lx), which equals 1 lumen per square meter.
A common alternative term is “incident light”. Keep in mind: while the luminous flux of a bulb stays constant, the illuminance increases as the light source gets closer to the surface.
Types of illuminance: horizontal and vertical
When light falls on a horizontal surface, it’s called horizontal illuminance — such as on floors or desks. When it falls on a vertical surface, like walls or windows, it’s called vertical illuminance.
How to calculate and measure illuminance
Illuminance is measured in lux (lx) and can be calculated using this formula:
To measure it, use a lux meter, which converts the light signal into an electrical signal calibrated in lux. Always calibrate the device beforehand to ensure accuracy.
The difference between luminance and illuminance
In short, illuminance measures the light falling onto a surface, while luminance measures the light that is reflected or perceived by the eye. This distinction is critical in any professional lighting project.
Visual comparison and summary
Illuminance tells you how much light hits a surface. Luminance tells you how much light comes from that surface to your eyes. Here's a quick breakdown:
Illuminance
- Independent of the surface characteristics
- Not affected by:
- Number of light sources
- Direction of the light
- Position of the light sources
- Type of surface being illuminated
- Not directly visible to the human eye
- Measures light falling on a surface
- Varies based on distance from the light source
Luminance
- Depends on the surface
- Affected by:
- Direction of light incidence
- Viewing angle
- Reflective properties of the material
- Visible to the human eye
- Measures light reflected toward the observer
- Varies depending on surface reflectance
Applying luminance and illuminance in your projects
Once you understand the difference, the next step is applying these concepts effectively in your lighting designs.
Luminance and illuminance are both crucial for architectural, retail, and residential projects. Getting them right means creating efficient, comfortable spaces with lighting levels tailored to each task.
Looking to improve the technical lighting in your projects?
At Faro Barcelona, we help you calculate the optimal levels of luminance and illuminance and select the right luminaires for each application. Feel free to contact one of our lighting specialists — we're here to help!
