Luminance Contrast

The Importance of Luminance Contrast

Tactile Ground Surface Indicators (TGSI’s) are designed to assist pedestrians with vision impairment to navigate the urban environment safely through visual and sensory stimulus. Generally, people with low vision are able to see colour and differentiate between different colours, however, their ability to discriminate colours may also be impaired. A small percentage of vision impaired persons are completely blind and so, the luminance contrast of walkways and tactile indicators allow for people with low vision to distinguish between surfaces and gather information to detect hazards and safe paths of travel. It is essential that the luminance requirements are met so that the contrast in luminance of tactile indicators relative to that of the surrounding surface are distinguishable to allow for safe navigation.

The Building Code of Australia (BCA), which forms Volumes 1 and 2 of the National Construction Code, and the Disability (Access to Premises – Buildings) Standards 2010 require minimum ‘deemed to satisfy’ requirements for levels of luminance contrast that must be provided to parts of a building and on accessible path of travels. Some of the materials that are used in building that must meet minimum luminance requirements are Tactile Ground Surface Indicators (TGSIs), stair nosings, raised tactile and braille signage, doorways, door glazing, lift call buttons and toilet seats. The appropriate selection of internal and external finishes with sufficient luminance contrast prove to assist people with low vision to identify features pertaining to the built environment for safe access and travel.

What is Vision Impairment?

Vision impairment or low vision is defined as a permanent decreased ability to see that can’t be treated by usual means such as glasses or contact lenses. The most common causes of visual impairment are cataracts (31%), and age-related degenerative eye diseases such as macular degeneration (3.1%), diabetic retinopathy (2.8%) and glaucoma (2.3%). Uncorrected refractive errors such as near sightedness, far sightedness, presbyopia and astigmatism are a form of treatable vision impairment that affects far more people than those with permanent vision impairment. The diseases that cause vision impairment, including uncorrected refractive errors, contributes to more than 90% of vision impairment for Australians aged 55 or more. Low vision affects many aspects of a person’s daily life and can make tasks such as crossing the road, identifying signs and public navigation very difficult.

Estimates provided by Vision 2020 Australia indicates that there are over 575,000 people who experience blindness or vision impairment in 2019. About 85% of all vision impairment affect those aged 50 and over, with a dramatic increase expected due to diabetic retinopathy arising from the diabetes epidemic in Australia. The prevalence and causes of eye disease in Australia is currently being undertaken by the National Eye health survey to determine ways in which we can prevent, detect and intervene in order to support and minimise people subjected to vision impairment in Australia. Currently, there is a need for national services for eye health and the implementation of funding programs and policy changes that aim to eliminate avoidable blindness and vision impairment.

What is Luminance Contrast?

Australian Standard 1428.1-2009 defines luminance contrast as ‘the light reflected from one surface or component, compared to the light reflected from another surface or component’.  The light reflective properties of a material are influenced by the degree of light absorption of different colours. This means that luminance contrast is not the difference in colour, but the difference of the light reflective properties of materials.

Fig. 1 and 2 highlight the the difference between properly defined stair treads and nosings that have a luminance contrast strip providing more than 30% contrast and that of the same staircase with no visual or tactile cues. These images highlight the importance of clearly defined treads and edges to allowing people with low vision to more easily differentiate the start and end of each step, reducing the risk of falls and providing a safer environment to for people to navigate.

LRV ranges from 0% to 100%, where  absolute black has a value of 0% as it completely absorbs all light and pure white has a value of 100%, reflecting all visible light. These properties influence the heat capacity of building materials which generally explains why darker materials can get hotter at a faster rate than that of materials that of light materials that reflect more light energy.

In practice, we do not encounter materials that truly absorb or reflect all light. Realistically, all LRV’s of everyday materials will lie approximately between 5% and 95%, with all colours fitting within these two extremes.

Luminance Spectrum Gradient
Luminance Spectrum Gradient LRV's

The minimum luminance contrast values in Section 2.2 of AS/NZS 1428.4.1: 2009 specifies the minimum luminance contrast values and provides testing methods that are suitable for the measurement of the Light Reflectance Value (LRV) of a material that is used to determine the luminance contrast with respect to another material.

The LRV of a material is measured with a tristimulus colorimeter or spectrophotometer and can be performed in a laboratory or on-site. The LRV of each material are used in the Bowman-Sapolinski equation C = 125 (Y2 − Y1)/(Y1 + Y2 + 25)) to calculate their luminance contrast of the materials. An online Luminance Colour Contrast Calculator has been developed by Luminous Consulting which only requires the two LRVs to determine the luminance contrast, saving you the hassle of calculating it yourself. The contrast calculator is particularly useful if you have known LRV’s for materials that can be entered directely into the calculator. The luminance contrast provided will only be accurate if the materials have been tested on-site or sent to a laboratory for analysis and tested using the correct testing metholodogies specified in Australian Standard AS 1428.1-2009

* The Luminance Colour Contrast Calculator is to be used as a design tool and should not replace  independent testing from a third party to determine the luminance contrast of building materials.

Stair Nosing and TGSI luminance contrast
Fig. 1: Tactile Indicators and stair nosings with sufficient luminance contrast.
A view of a flight of stairs for vision impaired persons
Fig. 2: A view of a flight of stairs for vision impaired persons.
Tristimulus Colorimeter
Tristimulus Colorimeter
Portable-Spectrophotometer
Spectrophotometer

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