What do we see when we see L*a*b?

Modern Instruments have made colour measurement easy. Interpreting the data gathered is more problematic and a quick glance at a Wiki Definition is a bit daunting to say the least.

Users and Managers  probably prefer simpler explanation as they report to or have hired nerds to already understand the detail, but usually don’t have time to sit down and have the “talk”.

The image below shows an instrument screen readout of a green shade that has been measured. Lets explain it in small steps: (reflectance colour setup)

Screen

  • L*a*b is a specific colour space developed in 1976 by the CIE to mathematically represent colour. There are a number of different colour spaces available.
  • L = is a number from 0 (Black) to 100 (White) . Therefore without telling us about colour we learn that this is a DARK or nearer to black than white shade.
    L therefore is Lightness to Darkness representation.
  • a* comes in two parts: a*+ is RED and *a- is GREEN. The numbers from 0-60 denote the degree of redness or greenness. This a* – is therefore a type of GREEN
  • b* comes in two parts: b*+ is YELLOW and b*- is BLUE. The numbers 0-60 denote the degree of yellowness or blueness. This b* is therefore a type of YELLOW

Without looking at the colour, we can deduce that its a  green hue with almost equal amounts of green and yellow. The L value (70.96) suggests the depth of shade is between medium and pale, but this perception depends on the application. It may be dark for say an indoor coat of paint, but medium for textile clothing.

This can be seen in a type of 3D chart below (With thanks to Konica Minolta) with the large flat circle plotting the a* and b* values, and the ball at the bottom left the L value

Lab

What can we do with these values so far? BE CAUTIOUS as this is where many colour tasks come unstuck.

These values are only correct under the SET OF CONDITIONS used at the time of measurement and can therefore only be compared to other measurements taken under the SAME EXACT conditions. A Colour Space is designed to encompass ALL the conditions to allow an “Apples to apples” comparison.

What may these conditions be? There are quite a number of criteria that need to be identical to create the same context, as colour is not a property of the object measured, but only of the object under the current context measured. The conditions are: (In random order but all important)

  1. The Instrument, Firmware or Software all adhere to CIE Norms
  2. Instrument is set to measure colour by Reflectance. (Transmission or Emission values will differ)
  3. The Instrument is configured to use the XYZ Tristimulus Equation
  4. The Instrument is configured to measure ADDITIVE Colour
  5. Instrument has a fixed Geometry. (d8)
  6. Instrument has a CIE Norm Illuminant (D65)
  7. Instrument is set for a CIE Norm Observer (10° )
  8. Instrument is using a 30mm Aperture
  9. Instrument is using the CIE Norm Colour Space L*a*b
  10. The Instrument will also return Reflectance Values
  11. Instrument is set to EXCLUDE Gloss (SCE) (Gloss makes up 4% of reflectance values)
  12. Instrument is configured to measure in 10 nm steps
  13. This Instrument is configured to measure from 360 to 740 nm
  14. The Instrument has a high degree of Inter Instrument Agreement (Readings from two identical instruments will be virtually identical under identical conditions)
  15. The object measured is 100% opaque
  16. The object measured is larger than the instrument aperture
  17. The object is measured under a known set of ambient conditions i.e. temperature and humidity.
  18. The Instrument is both USER (Daily) and Certificated (Annually) calibrated.

How important are all of these criteria?
If you want to compare a master (Standard) colour to a sample (Production Batch) colour – important.
If you want to compare a master (Standard) colour to a sample (Production Batch) colour across many different sites – critical.

How does a Master (Target) and a Sample compare? Lets see this image:

Screen1

This particular screen has stored Target Data in L*a*b and in the identical setup is compared to a recently measured SAMPLE.
A new symbol Delta (Δ) is observed in the reading. The SAMPLE data has been subtracted from the TARGET data and the DIFFERENCE (Delta or Δ) returned.
The reading is per equation component with a difference of *ab as well.
Reading differences in the 2nd decimal are MEANINGLESS and can be ignored as no two readings are ever correct to such small values.
In perspective, depending on the object and the importance of colour to the object, a trained eye can see Δ 1.0 and a consumer can even be blind to Δ  3.0 if colour is less important than say value. Automotive and Clothing may work to very close tolerances, while foodstuffs may be sometimes wider, although international food company’s closely monitor colour as part of their Branding.

Future Blog posts will unpack items in more detail, but to end, some history on colour. For a Glossary of Spectral Terms look here.

Colour is a human Sense (A sense is a physiological capacity of organisms that provides data for perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology (or cognitive science), and philosophy of perception. – Wikipedia)

Senses are therefore SUBJECTIVE and colour discussions can lead to conflict.

Prism

Sir Isaac Newton discovered in 1687 that light passing through a Prism separates into discrete colours which represent individual Wave Lengths.
Unlike other senses, these wavelengths could be used for a mathematical calculation because they were reliable and reproducible. So while smell meters are still being developed, light and sound waves allow us to make colour and decibel meters with accuracy.

220px-CIExy1931

This allowed the CIE in 1931 to plot an accurate Colour Chart based on the original X,Y and Z values in the Tristimulus Equation

200px-Cones_SMJ2_E.svg

So colour data is collected (Sensed) by your EYES but seen (Perceived) in the mind. Is it any wonder that our perceptions differ from person to person, and as a last note of caution, do not be surprised that sometimes what you “See”  differs from what your Instrument “Sees”. As per the 18 points of agreement listed above, you and your Instrument are set differently! To overcome this issue, you need to observe the object using a suitable environment, like a Colour Assessment Cabinet.