Luminance Contrast Equipment & Equations

  • August 22, 2014
  • Bruce Bromley

The endless complexities of the Standards continue.

I thought I’d post this after having discussions with numerous people regarding equipment and equations. Most people are getting this wrong. Please note that the Standard defines two (2) ways to calculate luminance contrast (Building elements – TGSI’s use the same equation) – most people are just using the Bowman-Sapolinski equation for building elements other than TGSI’s. This equation is ONLY usable for luminous reflectance values and should not be used with luminance values. Luminance values need to use the equation in B5.3 (e) of 1428.1 – 2009. Note that if TGSI’s are being tested then the Bowman-Sapolinski equation is used – another interesting contradiction between these two standards.

An now a little about equipment – if you’re looking at buying some equipment to test contrasts – do yourself a favour and buy a Photometer. In most cases, you will be testing in an on-site setting and this will be fine – note that if your testing TGSI’s then there is a set up that needs to be done. The biggest issue is the laboratory tests.

You cannot use the colorluminator, NCS or similar or any spectrophotometer. The reason for this is geometry. The Standards state for laboratory tests the instrument must be

“A tristimulus colourimeter or spectrophotometer with a diffuse illumination/normal viewing (d/o) geometry is used with CIE Standard Illuminant D65. The instrument has to be capable of measuring absolute CIE for Yxy to be calculated. The measured luminous reflectance is defined by the tristimulus value Y. The chromaticity coordinates x and y provide an indication of the colour.”

The important thing to note is the (d/0) – this means a diffuse illumination (spherical dome illumination) with the receptor (measuring point) at 0degrees.

Most of the equipment that people are using (Tristimulus Colorimeters – including the colorluminator) has a geometry of 45/0. This geometry does not meet the Standards and illuminates from 45degrees and measures from 0degrees. Even more so – there is NO spectrophotometer that has (d/0) – what IS available is diffuse illumination 6-8degree. It is also EXTREMELY important to understand specular component included or excluded. I.E. glossy surfaces will measure differently. Another big question is – Who’s going to tell the CSIRO that their Gardner Gmbh colour Guide doesn’t meet the Standards??? It’s a spectrophotometer with 45/0.

Does the ME64 committee or Standards Australia actually understand what they are including in the standards and the liability & risks these irregularities will cause for specifiers, certifiers etc should an issue in the future go to court?

One more quick equation issue for you to think about. If you have a Dulux LRV value of 50 what is the required LRV’s to achieve the 30% – most people answer by saying 30% difference from the 50. If you review the table B1 you’ll notice that the lighter possible surface is actually 80% difference and the darker 50% difference. We must be careful with these equations.

I’ve spent considerable time working through these issues and welcome any discussions – if you would like further explanation or info please feel free to give me a call or drop me an email.