KNOWLEDGE

MINTO GARDENS, NSW

LUMINANCE

Luminance refers to the intensity of light emitted or reflected from a surface per unit area in a given direction. It is a measure of the brightness perceived by the human eye and is often expressed in units such as candelas per square meter (cd/m²). In lighting we come across luminance in two key areas; luminance emitted from a luminaire itself can often cause glare when emitted at particular angles, and luminance reflected off objects is how we perceive the world around us.

ILLUMINANCE

Illuminance is a measure of the amount of light falling on a surface per unit area. It quantifies the light level that the surface receives from a light source. Illuminance is typically measured in lux (lx), which equals one lumen per square meter (lm/m²).

In practical terms, illuminance indicates how well an area is illuminated by a light source. It is an important consideration in lighting design to ensure that spaces are adequately lit for their intended purposes, whether for visibility, safety, or ambiance.

In exterior lighting we typically refer to illuminance on a "horizontal" plane and a "vertical" plane. Horizontal planes are often used to predict the amount of light hitting a ground surface, whereas vertical planes are often used to predict the amount of light hitting a person's face.

It's important to remember that whilst illuminance quantifies the amount of light hitting a surface, the brightness (luminance) of that surface will be affected by its reflectance.

GLARE

Glare refers to the discomfort or impairment of vision caused by excessive brightness or contrast in the field of view. It occurs when there is a significant luminance contrast between objects or areas within the visual scene, leading to a sensation of discomfort, reduced visibility, or even temporary vision impairment.

Glare can be categorised into two main types:

Discomfort glare: This type of glare occurs when bright light sources or reflections within the field of view cause discomfort, but do not necessarily impair vision. It can be distracting and make it difficult to see details or focus on tasks.

Disability glare: Disability glare occurs when the contrast between bright and dark areas in the visual field is so extreme that it impairs the ability to see clearly. This type of glare can reduce visual acuity and make it challenging to perceive details, especially in low-light conditions.

Glare is influenced by factors such as the intensity and direction of light sources, the design of luminaires or light fixtures, the reflective surfaces in the environment, and the sensitivity of the observer's eyes. In exterior lighting we consider glare through a number of metrics - AS 1158.3.1 references Luminous Intensity at specific angles and the Discomfort Glare Index.

DISCOMFORT GLARE INDEX (DGI)

The Discomfort Glare Index (DGI) is a metric used to quantitatively assess the potential for discomfort glare caused by a luminaire. It provides a numerical value that indicates the likelihood and severity of discomfort glare experienced by an observer.

The DGI takes into account several factors that contribute to discomfort glare, such as the luminance of the glare source, the angle of view, and the size of the glare source.

Higher values of the Discomfort Glare Index indicate a greater potential for discomfort glare, while lower values suggest that glare is less likely to occur or is less severe. Lighting designers use the DGI to evaluate and optimise lighting designs to minimise discomfort glare and ensure visual comfort for occupants in various outdoor environments.

Because many Australian Standards for exterior lighting require vertical illuminance on a person's face, DGI and distance between poles (pole spacings) becomes a balance. Greater pole spacings typically result in increased DGI values.

COLOUR RENDERING INDEX

CRI stands for Colour Rendering Index. It is a quantitative measure that indicates how accurately a light source illuminates the true colours of objects compared to a natural light source (such as sunlight or incandescent).

The CRI is calculated based on a scale from 0 to 100, where higher values indicate better colour rendering ability. A CRI of 100 would mean that the light source renders colours similarly to the reference light source. A lower CRI indicates that the light source may distort certain colours, making them appear different from how they would appear under natural or reference lighting conditions.

Colour rendering is particularly important in applications where accurate colour perception is crucial, such as in retail environments (to showcase products accurately), art galleries (to display artwork with fidelity), and medical settings (to examine skin tones or evaluate diagnostic images). Different light sources, such as LEDs, fluorescent lamps, and incandescent bulbs, can vary significantly in their CRI values.

It should be noted that the colour of an object can also be affected by the Colour Temperature of a light source, since CRI is determined against a natural light source with an equivalent colour temperature.

COLOUR TEMPERATURE

Colour temperature refers to a characteristic of visible light that describes its colour appearance and is measured in units of Kelvin (K). It indicates whether a light source emits a warm, neutral, or cool colour tone.

In practical terms, colour temperature describes the colour of light emitted by a lamp or light fixture, ranging from warm tones (reddish-yellow hues) to cool tones (bluish-white hues). The terminology originates from the concept of heating a "black body" radiator (a theoretical object that absorbs all incident radiation) to a certain temperature, where the colour of the emitted light changes predictably.

UNIFORMITY

Uniformity is a term used broadly in lighting practice in reference to how uniform lighting levels are on a certain surface or imaginary plane. In exterior lighting we usually refer to uniformity of illuminance on a horizontal plane. Because the plane surface is assumed to have consistent reflectance, the uniformity of that surface (represented as a ratio) then is said to represent the perceived brightness of that surface. For this reason, uniformity shouldn't be calculated over a plane (either physical or imaginary) which has multiple reflectances.

Under Australian Standards, when we talk about uniformity we are referencing the ratio of the maximum illuminance to the average illuminance within a certain area. Higher uniformity (represented by a lower number) is typically desirable for exterior lighting applications, but the appropriate contrast and uniformity should be assessed for each space carefully.

MAINTENANCE FACTOR

A Maintenance factor refers to a numerical factor used to account for the gradual decrease in lighting performance over time due to factors such as lamp lumen depreciation, dirt accumulation on luminaires, and aging of electrical components. It quantifies the reduction in the initial light output of a lighting installation after a certain period of operation (typically the end of useable life).

Maintenance Factors relate to lighting designs rather than luminaires. This is because the depreciating light levels of a lighting design are affected by many aspects including build up of dirt on the luminaire, degredation of luminaire materials, depreciation of surface finishes. A key consideration for Maintenance Factor is the reduction of output in the light source itself. With LED technology this occurs from a gradual breakdown in the diode junction, and this process is accelerated by increased drive current and heat.

LUMINOUS INTENSITY

Luminous intensity is a measure of the amount of light emitted or transmitted from a source in a particular direction. It quantifies the intensity of a light source as perceived by the human eye in a specific direction, taking into account the sensitivity of the human eye to different wavelengths of light. Luminous intensity refers only to the quantity of light leaving a surface in a certain direction, but doesn't accurately describe the brightness of a surface as it doesn't consider the area of the surface emitting the light. The luminous intensity at a specific angle divided by the size of the area creating that intensity is the surface or source luminance.

BEAM ANGLE

The beam angle indicates the width of the light beam. A larger beam angle means the light will spread out more quickly as it travels, covering a wider area. For floodlights with rotationally symmetrical beams, the beam angle is determined where the light's intensity (measured in candelas) drops to 50% of its peak.
Another commonly used measurement in lighting is the Field Angle, which is the angle at which the light's intensity decreases to 10% of its peak. Whilst these measurements help with the initial selection process of a luminaire, more detailed calculations are often necessary to fully anticipate the illumination outcome.

LUMINAIRE CLASSIFICATION SYSTEM (LCS)

The IES Luminaire Classification System is a method developed by the Illuminating Engineering Society (IES) to categorise outdoor light fixtures based on their light distribution. This classification helps in designing lighting systems that minimise glare, and reduce light pollution and spill light. The system is particularly important in outdoor lighting, such as streetlights and parking lots, where controlling light spill and uplight is crucial.

The LCS looks at 10 light zones:

Forward Zones (in front of the fixture):
- High-Forward (FH): 60°–80° from nadir.
- Low-Forward (FL): 30°–60° from nadir.
- Very-Forward (FVH): 80°–90° from nadir.
Back Zones (behind the fixture):
- High-Back (BH): 60°–80° from nadir.
- Low-Back (BL): 30°–60° from nadir.
- Very-Back (BVH): 80°–90° from nadir.
Uplight Zones (above the luminaire’s horizontal plane):
- Low-Uplight (UL): 90°–100°.
- High-Uplight (UH): 100°–180°.

BUG RATING SYSTEM

The IES LCS is often used with the BUG (Backlight, Uplight, and Glare) rating system. This rating assesses luminaires based on their performance in reducing:

Backlight (B): Light spilling backward.
Uplight (U): Light contributing to skyglow.
Glare (G): Light causing visual discomfort.

A luminaire's BUG rating (e.g., B2-U0-G1) indicates its control in each of these areas, helping designers to quickly understand how the luminaire’s distribution will perform with respect to their project requirements. For B, U and G, a value of 0 indicates the most control and 5 indicates the least control.

In Australia, we already have strict requirements for Upward Waste Light Ratios (UWLR), so the Uplight (U) value is of lesser interest. For Backlight (B), a high value can be helpful for achieving the AS1158.3.1 requirement for Horizontal Illuminance to the verge 5m behind a pathway, and a low value will be better where a property boundary and/or AS4282 compliance is in close proximity behind a path. The Glare (G) value proves to be particularly useful in Australia, with a low value quickly confirming low Lumens emitted within specific light zones.