TECHNICAL INSIGHTS

LUMINANCE CONTRAST - IS WHAT YOU SEE, WHAT YOU GET?

Howard Moutrie - ACAA Accredited Access Consultant, ACAA Accredited Fellow, Accessible Building Solutions

The articles featured in Technical Insights are to prompt thought and discussion to assist our members' questions and evaluate their understanding of the technical requirements of Australian Standards and other national/international source material. Technical Insights is intended to provide background information, a different viewpoint, a perspective from an individual with lived experience of disability or to prompt further discussion and/or research by you as an access professional.

Firstly, what is luminance contrast? Materials and surfaces reflect light, if they did not we would not be able to see them. For the purpose of this discussion, the amount of light reflected is expressed as it’s luminance reflectance vale (LRV). Pure Black, which theoretically doesn’t reflect light has an LRV of 0 and Pure White which theoretically reflects all light has an LRV of 100. There are a few ways of determining the luminance contrast of 2 surfaces, which is the difference between the LRV of the 2 surfaces and is normally expressed as a percentage. It is important to understand that luminance contrast and colour contrast are quite different. In fact, the colours red and green have a good colour contrast, they are easily distinguishable except for people with RG colour blindness, but in fact reflect basically the same amount of light and thus have little luminance contrast.

Logic would tell you that to calculate the luminance contrast of the 2 surfaces you would compare one value against the other. A simple formula is C = (Lmax – Lmin) / (Lmax + Lmin). This would be fine for people with good vision, but for those with poor vision, the ability to see the variances diminishes. Some years ago, following some research, the formula was changed to what is called the Bowman Sapolinski Equation C = 125(Lmax-Lmin)/ (Lmax+Lmin+25). This equation forms the basis of the requirements for luminance contrast in Australia. Interestingly, though it is called the Bowman Sapolinski Equation it is in fact a modified version of that equation. The equation was modified by the Standards Committee as the original equation provided a result not between 0-100 which the published equation does. The Australian Standards Committee modified the equation so that a result between 0 to 100 would be achieved.

So how do you determine the LRVs? AS 1428.1-2009 in Appendix B provides two methods, one on site and one in the laboratory. It is important to note that this appendix is “informative” that means it does not form part of the Standard. That is fortunate because the equation given for the on-site testing is incorrect, it has not been updated for the Bowman Sapolinski equation. So in short, ignore this appendix. Appendix E in AS 1428.4.1 is normative and thus is part of the Standard and has the correct equation. These are the methodologies which must be followed. I note that AS1428.1-2021 Appendix B has corrected the formulas but the 2 methodologies are now called contact method and non-contact method, which better describes them (rather than on site and laboratory), but unfortunately has them named the wrong way round. This will be corrected in a future amendment.

The methodologies described, particularly the on-site or non-contact method, are based on testing tactile indicators and are not readily transferrable to the testing of vertical surfaces or more particularly visual marking on glazed doors etc. In fact, it is not clear how the testing of visual indicators on glazing can be achieved and should be specifically addressed in the Standards in the future.

Technically, the use of published LRVs, for example, for paint colours is not addressed in either Standard, so are they valid? I consider that the use of these published figures is a reasonable design tool even though not addressed in the Standards. It could be construed to be covered by the appendix in AS 1428.4.1 on the basis that the formula for the calculation is given separate to the 2 testing methodologies.

But not all LRVs are the same. From my experience, different instruments will provide different LRVs for the same colour. Sometimes this variance will be minimal but can be significant. Thus, I think it is always preferred to use the same instrument to measure all surfaces when calculating a contrast. The chart below shows a comparisons of 3 instruments testing the same standard colours. The differences are obvious and not consistent between the instruments.

The test methodology for given for onsite or non-contact is clearly aimed at tactile ground surface indicators and was originally formulated when there was only the integrated type. For this type of TGSI the method works fine but for discrete and composite tactiles, the field of view of the instrument will detect the surface outside of the tactile and thus give a false reading. On this basis, when using a photometer the test methodology cannot be used to achieve a correct reading.

The original requirement for a 30% contrast was based on an integrated tactile where the whole surface provided the contrast. To compensate for the reduced area of contrast with the discrete and composite tactiles, the contrast was increased to 45% and 60% respectively. The area of a discrete tactile compared to an integrated tactile is 38% and for a composite tactile is 19%. Given that the vision impaired community indicate that the 30% contrast is insufficient and some research at the University of NSW would seem to support this, I am not convinced that the increase in contrast adequately compensates for the reduced area. Thus, given the difficulty to achieve these higher contrasts anyway, I really doubt if either of these types should be used. Unfortunately, they have been introduced, industry has “tooled up” to produce them and they tend to be the type preferred by designers and building owners so I cannot see that will be removed from use.

The use of luminance contrast needs a significant amount of research to resolve these issues. Unfortunately, undertaking research can be costly and research in the field of accessibility seems to be constrained by the ability to obtain participants. Like a number of access related issues, we will have to live with what we have until the research is undertaken to indicate a change is needed.