LRV stands for Luminance Reflectance Value
TGSI stands for Tactile Ground Surface Indicator

The contrast formula used in the app for Australia is the Bowman-Sapolinski Equation with LRV readings which are to be obtained as noted in Part E3 or E5 of AS/NZ 1428.4.1-2009 and Part B4 of AS1428.1-2009 is

125(Y2-Y1) / (Y1+Y2+25)
where Y2 is the LRV of the lighter area and Y1 is the LRV of the darker area.

Directions for use of calculator:
Input any two fields and the results will provide the response to the missing third field. In some cases the response
can result in 2 LRV readings. This is because a colour can achieve the same luminance contrast with use of a much
darker colour or a much lighter colour.
Where luminance contrast is a minimum of 30%, it is suitable for use for building elements such as doorways, stair nosings,
toilet seats and integrated TGSIs (Tactile Ground Surface Indicators).
Where luminance contrast is a minimum of 45%, it is suitable for use for Discrete TGSIs
Where luminance contrast is a minimum of 60%, it is suitable for use for Composite TGSIs

Please refer to AS/NZ1428 suite of Australian Standards for details.

Note that LRV is on a scale of 0 to 100 (black to white), however both white and black are never absolute and therefore in reality B1/Y2 never equals 100 and B2/Y1 is always greater than 0.

Disclaimer:
Use app at your own risk. The app designer / Vista Access Architects shall not be held liable for use of the app.