Welcome to the world of lighting design, where artistry and technology converge to create captivating environments. Lighting is an art form that goes beyond mere illumination and plays a vital role in enhancing the aesthetics, functionality, and ambiance of any space. Whether it's a residential setting, commercial establishment, or public facility, the right lighting design can transform the way we perceive and interact with our surroundings. However, behind the scenes of every stunning lighting arrangement lies a careful consideration of various technical factors that ensure optimal performance and visual impact.

In this article, we will explore the key technical factors that lighting designers must consider achieving outstanding results. From the practical aspects such as lumen output and color temperature to the beam angles and glare control, we explore the fascinating world of lighting technology. These factors, often overlooked by the untrained eye, play a pivotal role in achieving the desired ambiance, visual comfort, and energy efficiency.

Here are the important technical factors in lighting design and technology that lighting designers should consider:

Part 1:

• Lighting measurement

Part 2:

• Color Temperature (CCT)
• Color Rendering Index (CRI)
• Beam Angle and Distribution
• Dimming and Control Options
• Energy Efficiency, luminous Efficacy

Part 3:

• Glare Control
• Light Pollution and Control
• Maintenance and Longevity
• Flicker-free Lighting
• Lighting Uniformity

In this part of the article, we discuss lighting measurements, formulas, their relationships, comparisons, and illustrations.

Part 1: Lighting Measurements

Lighting is a critical component in both residential and commercial settings, affecting everything from mood to productivity. When selecting lighting fixtures, understanding the amount of light is essential for making informed decisions. This is crucial for selecting the right lighting for different applications. It ensures that spaces are adequately lit, enhancing visibility, safety, and ambiance. Whether you are lighting a home, office, or commercial space, knowing the amount of light helps achieve the desired lighting effect efficiently.

The different lighting measurements:

• Luminous Flux
• Luminous Intensity
• Illuminance
• Reflectance
• Luminous Exitance
• Luminance

Luminous Flux (Φ):
Luminous Flux is also referred to as “lumen” with the symbol (lm) which is a measure of the total amount of visible light emitted by a source and indicate the brightness of the light. The higher the lumens, the brighter the light.

Luminous Intensity (I):
Luminous Intensity is also referred to as “candela” with the symbol (cd) which measures the amount of light power (intensity) emitted by a source in a particular direction. It quantifies how bright a light source appears from a specific angle, focusing on the directional distribution of light.

Illuminance (E):
Illuminance is also referred to as “foot-candle” with symbol (fc) or “lux” with the symbol (lx) which quantifies the amount of visible light that hits or passes through a surface area. Unlike lumens, which measure the total light output from a source, Illuminance measures the light intensity over a specific area.

• Foot-candle (fc): A foot-candle is a non-SI unit that measures the amount of light that falls on a surface of one square foot, one foot away (lm/ft²).
• Lux (lx): when the same amount of light falls on a surface of one square meter, one meter away is called lux that is a SI unit of Illuminance (lm/m²).

Reflectance (ρ):
Reflectance is a measure of how much light is reflected by a surface. It is expressed as a ratio or percentage of the light that is reflected by the surface, indicating the surface’s ability to reflect light.
Reflectance can be expressed as a fraction (0 to 1) or as a percentage (0 to 100%).

Luminous Exitance (M):
Luminous Exitance (sometimes called luminous emittance) is a measure of the amount of light that is emitted or exits from a surface per unit area. It quantifies how much light is leaving a surface, regardless of the direction of emission. The unit of luminous exitance is lumens per square meter (lm/m²).

Luminance (L):
Luminance is a measure of the amount of light emitted, reflected, or transmitted from a surface in a particular direction, per unit area per unit solid angle. It quantifies the perceived brightness of a surface from a specific viewpoint and is a crucial concept in fields such as lighting design, photography, and display technology.

The table below (Table 1) compares these different measurements in detail.

Table 1: The different lighting measurements comparison

 

1. Luminous Flux (Φ):

As discussed, Luminous Flux (lumen) is the total amount of visible light emitted by a source which is calculated by this formula (Picture 1):

Consider a room with an area of 20 square meters that requires an illuminance of 300 lux. The total luminous flux needed can be calculated as: 

Φ = E x A = 300 lm/m² x 20 m² = 6000 lm

There is a 15 square meters (m²) room with 12.39 foot-candle (fc) Illuminance. How much intensity with need for this room?

Firstly, we should convert the area from square meters to square foot:

1 m² = 10.764 ft²: (A = 15 m² x 10.764 ft² / m² = 161.46 ft²)
Then:

For residential spaces, comfort and ambiance are key. Here’s a guide to typical lumen requirements:

Commercial spaces often require higher lumen outputs for functionality and safety: 

• Office: 3,000-5,000 lumens per 100 square foot
• Retail Store: 4,000-6,000 lumens per 100 square foot
• Warehouse: 10,000-20,000 lumens per 100 square foot

Understanding lumen output is essential for choosing the right lighting for your needs. Whether you prioritize energy efficiency, brightness, or color temperature, considering the lumen output of different light sources will help you achieve optimal lighting conditions in any setting. As technology advances, newer light sources like LEDs offer higher lumen outputs with lower energy consumption, making them an excellent choice for residential and commercial applications. 

2. Luminous Intensity (I):

Luminous intensity is a measure of the amount of light emitted by a source in a particular direction. It quantifies how bright a light source appears when viewed from a specific angle and is expressed in candelas (cd) (Picture 3).

I = Φ / Ω  where:

• I is the luminous intensity in candela (cd).
• Φ is the luminous flux in lumen (lm).
• Ω is the three-dimensional angle over which light is emitted, measured in steradians (sr).

 

Picture 3: Luminous Intensity (Candela)

Solid Angle

A solid angle (Ω) is a three-dimensional angle that extends out from the source. It can be visualized as a cone with its apex at the light source and its base forming part of the surface of a sphere.

Ω = A / r²  where:

• A is the area of the spherical cap.
• r is the radius of the sphere.

Calculation Example

Let's calculate the luminous intensity of a light source emitting 1500 lumens uniformly over a solid angle of 2 steradians.

Luminous flux (Φ) = 1500 lumens,

Solid angle (Ω) = 2 steradians,

Then:
𝐼 = 1500 lm / 2 sr = 750 cd

So, the luminous intensity of the light source is 750 candelas.

Practical Applications

Luminous intensity is crucial in various fields:

• Spotlights: Design of lighting systems that require focused beams of light.
• Automotive Lighting: Ensuring headlights provide sufficient illumination in specific directions.
• Theatrical Lighting: Directing light precisely to highlight performers or scenes.
• Flashlights: Determining the brightness and directionality of the beam.

Luminous intensity provides a measure of how bright a light source appears in a specific direction. By understanding and calculating luminous intensity, designers and engineers can create effective lighting systems tailored to various applications, ensuring adequate illumination and enhancing visual comfort.

3. Illuminance (E):

Illuminance is a measure of how much luminous flux is spread over a given area. It quantifies the amount of light that hits a surface and is crucial for designing and evaluating lighting systems. Illuminance is expressed in units of lux (lx) or foot-candles (fc) (Picture 4 and Table 2).

Illuminance (E) is defined as the luminous flux (Φ) incident on a surface per unit area (A):

E = Φ / A  where:

• E is the illuminance.
• Φ is the luminous flux in lumen (lm).
• A is the area in square meters (m²) or square foot (ft²).

Units of Illuminance
I. Lux (lx)
Definition: One lux is equal to one lumen per square meter (A: square meter).

Units: Lux (lx) = lumens per square meter (lm/m²).

II. Foot-Candles (fc)
Definition: One foot-candle is equal to one lumen per square foot (A: square foot).

Units: Foot-candles (fc) = lumens per square foot (lm/ft²).

Conversion Between Lux and Foot-Candles

The relationship between lux and foot-candle can be expressed as:

• 1 foot-candle (fc) = 10.764 lux (lx)
• 1 lux (lx) = 0.092903 foot-candles (fc)

 

Picture 4: Illuminance (Lux and Foot-Candle)

 
Examples:

Example 1: Calculating Lux
Suppose a room is illuminated by a light source emitting 2000 lumens, and the room's area is 10 square meters.

E = 2000 lm / 10 m² = 200 lux

So, the illuminance in the room is 200 lux.

Example 2: Calculating Foot-Candles
Suppose a light source emits 2000 lumens, and the area to be illuminated is 100 square foot.

E = 2000 lm / 100 ft² = 20 fc 
So, the illuminance on the surface is 20 foot-candle. 

Practical Applications
Illuminance, measured in lux or foot-candles, is essential for understanding how well a space is lit. By calculating and optimizing illuminance, designers and engineers can create environments that are comfortable, safe, and conducive to various activities. Understanding both lux and foot-candle allows for flexibility in applying international and region-specific standards to achieve optimal lighting conditions.
Illuminance is a critical factor in various applications to ensure adequate lighting levels:

Residential Lighting: Ensuring comfortable and sufficient lighting in living spaces.
Office Lighting: Providing enough light to enhance productivity and reduce eye strain.
Retail Lighting: Creating attractive displays and ensuring products are well-lit.
Industrial Lighting: Ensuring safety and visibility in workplaces.
Photography and Videography: Ensuring appropriate lighting for capturing high-quality images and videos.

The table below also shows the difference among Luminous Flux, Luminous Intensity, and Illuminance.

 

4. Reflectance (ρ):

Reflectance is a measure of how much light is reflected by a surface. It is expressed as a ratio or percentage of the incident light that is reflected by the surface, indicating the surface's ability to reflect light (Picture 6).

Reflectance is defined as the ratio of the reflected radiant flux (Φ_r) to the incident radiant flux (Φ_i): 

ρ = Φ_r / Φ_i  where:

• ρ is the reflectance.
• Φ_r is the reflected radiant flux.
• Φ_i is the incident radiant flux.

Reflectance can be expressed as a fraction (0 to 1) or as a percentage (0% to 100%). 

Types of Reflectance

1. Specular Reflectance: Occurs when light is reflected in a single, specific direction, as with a mirror. This type of reflection preserves the angle of incidence and results in a clear, defined reflection.
2. Diffuse Reflectance: Occurs when light is scattered in many directions after hitting a rough surface. This type of reflection results in a soft, uniform appearance without clear reflections.
3. Mixed Reflectance: Combines both specular and diffuse reflectance, common in many real-world surfaces where light is partially scattered and partially reflected in a specific direction.

Picture 6: How Specular and Diffuse Reflection works

Factors Affecting Reflectance

Several factors influence the reflectance of a surface, including:

• Material Properties: Different materials have different intrinsic reflectance properties. For example, metals typically have high reflectance, while darker, non-metallic surfaces have lower reflectance.
• Surface Texture: Smooth surfaces tend to have higher specular reflectance, while rough surfaces exhibit higher diffuse reflectance.
• Wavelength of Light: Reflectance can vary with the wavelength of the incident light. For example, some materials may reflect visible light well but not infrared light.
• Angle of Incidence: The angle at which light hits a surface affects its reflectance. For example, at grazing angles, more light may be reflected than at perpendicular angles.

Measurement of Reflectance

Reflectance is measured using instruments called reflectometers, which direct a known amount of light onto a surface and measure the amount of light reflected. The reflectance can be determined for specific wavelengths (spectral reflectance) or across a broad range of wavelengths (total reflectance).

Applications of Reflectance

Reflectance is an important parameter in various fields, including:

• Optics and Photonics: Designing mirrors, lenses, and other optical components.
• Astronomy: Studying the reflectance properties of planets, moons, and other celestial bodies to understand their composition.
• Climate Science: Assessing the reflectance of surfaces like snow, ice, and vegetation to understand their impact on Earth's energy balance.
• Material Science: Developing materials with specific reflectance properties for applications such as coatings, paints, and textiles.
• Architecture and Interior Design: Choosing materials and finishes to achieve desired lighting and aesthetic effects.

Example:

If an incident light beam has a radiant flux of 1000 lumens and the reflected flux from the surface is 300 lumens, the reflectance ρ is:

ρ = 300 lm / 1000 lm = 0.3

This means the surface has a reflectance of 0.3, or 30%.

5. Luminous Exitance (M):

Luminous Exitance (sometimes called Luminous emittance) is a measure of the amount of light that is emitted or exits from a surface per unit area. It quantifies how much light is leaving a surface, regardless of the direction of emission.

Luminous Exitance is defined as the total luminous flux (Φ) leaving a surface per unit area (A). The formula is:

M = Φ / A  where:

• M is the luminous exitance [(lm/ft²) or (lm/m²)].
• Φ is the total luminous flux leaving the surface (lumen).
• A is the area of the surface [(square foot) or (square meter)].

Practical Examples and Applications

Luminous exitance is useful in various applications where it is important to quantify the light emitted from surfaces:

• Lighting Design: Helps in designing lighting systems by understanding how much light is being emitted from walls, ceilings, or other surfaces in a room.
• Display Technology: Used to measure the brightness of screens and displays by evaluating the light emitted from the screen surface.
• Building and Architecture: Assists in evaluating the effectiveness of light-reflective surfaces and materials in illuminating spaces.
• Stage and Set Design: Helps in assessing the amount of light emitted from stage surfaces to ensure appropriate lighting for performances.

Example:

Suppose a light-emitting panel has an area of 4 square foot and emits a total luminous flux of 1000 lumens. The luminous exitance can be calculated as:

M = 1000 lm / 4 ft² = 250 lm/ft²

This means the panel has a luminous exitance of 250 lumens per square foot.

Table 4: Comparison between Luminous Exitance and Lux or Foot-candle

6. Luminance (L):