We all measure L*a*b, but whose numbers are actually correct?
Customers often ask us the question: If I compare L*a*b or other Colour Space Values with another reading from a different instrument and the values differ, whose numbers are correct??
The short answer is that BOTH readings could be “Wrong”.
Quality Assurance people tend to be number orientated as well as precise. The first sign of difference between reported values will cause the shakes and start to question which instrument is correct or reliable to use.
How do we get a correct number?
Beginning with basic Colour Science, unlike Mass or Size, Colour is NOT a property of an object. The appearance of a colour will change if conditions change.
Track the sun during a normal day and as its angle and intensity changes during its passage across the sky, objects in the vicinity will appear to change colour. If you took a reading every hour, of a plant in the garden, the values would change as the conditions change.
The same principal applies when measuring colour with a Spectrophotometer or a Colorimeter. Different “Conditions” or settings will result in different values being reported.
How do we manage this?
Firstly, there is a Global Referee, the CIE or Commission Internationale de l’Eclairage or The Commission on Illumination, whose task is to set International Standards and Norms. If the Instruments you are using are both CIE approved, then you can start to think about comparing values.
The purpose of the CIE is to create standards that emulate Human Perception. To do this, Spectral Data is collected by an Instrument, and depending on its design and settings, will use a Colour Space equation to report a value.
We can list some of the typical settings which must concur with another Instrument to have a chance of returning similar values:
- Illuminant (Typically D65)
- Observer (2◦ or 10◦ )
- Colour Space (L*a*b or L*C*h etc.)
- Colour Space Difference (dE*ab)
- SCI, SCE or SCI+SCE
- Reflectance, Transmittance or other Observation Setting
- Aperture Size (SAV, MAV, LAV)
- Colour Index (ISO Brightness etc.)
- Tolerances
Some “Settings” are cast in stone by the architecture of the Instrument design, for instance:
- Illumination/observation system – (diffused illumination, 8-degree viewing) or 0/45 etc.
- Sphere Size (If d:8)
- Wavelength Pitch ( 5, 10, 20 or other nanometre)
- Light Source (Pulsed Xenon etc.)
- Inter Instrument Agreement (IIA – Critical when comparing colour within groups)
Exterior factors that can change values returned include:
- Ambient Temperature
- Ambient Humidity
- Sample (Object) preparation
- Sample Stability over time or other changes
- Accessories
- Glass Cells or Cuvettes (Colour Neutral?)
If all of these conditions are identical, we can expect to return similar values. However instruments that are different (different model, different brand) will produce different results.
Colour Difference is returned as a single value depending on which values are used. i.e. ∆a*b
Mostly, we do not know what the “Other” settings were, but if both (Or all) parties measure the same Target Colour (Master), then there is the possibility to set Tolerances to that Master when measuring Samples.
Target / Sample comparisons have the benefit that regardless of the original settings, the DISTANCE or Sample to Target ERROR is usually the same. By explanation, if a car travelling at 60km an hour or at 37mph exceed the speed limit, the amount of excess is the same, regardless of the values returned. If the speed limit was 50km an hour, then the km difference was 20%. Likewise, if the speed limit was 30mph, the mph difference is 20%.
Comparing values without CONTEXT, or the CONDITIONS APPLIED are MEANINGLESS.
Comparing a known TARGET to an UNKNOWN sample, which does imply that PHYSICAL SAMPLES are required in BOTH cases, is the only reliable method.
Using Instruments with a HIGH level of Inter Instrument Agreement (IIA) also eliminates or reduces the Instrument Drift factor.
