Why do two instruments measure the same color sample differently?

Main factors:

A. Instrument. The instrument can have different:

  • Aperture
  • Geometry
  • Sensor type
  • various technical issues related to specific technical solutions

B. Source of radiation (UV, visible light, IR)

Fluorescence is an additional factor that may have a huge influence on color depending on the invisible  UV component...
Spectral power distribution is often far from expected and 

 

C. Object

Most instruments are designed to measure flat, mat-finished, uniform objects. In the real-world o lot of samples are very different. Some can be glossy finished some may fave complex non-uniform surfaces. Special effects like metallic, pearl are different when observed from different viewing angles... 

 

Conclusion

There is no -one objective way to specify those parameters. Defending on what we are going to measure some technical solutions may be more sensitive to specific object's properties. In some situations, we have to compare the two different instruments/measurement methodologies to manage properly object appearance properties.

 

Note that some parameters require two different measurement methods compared :

  • OBA Index or Fluorescent Index require two measurements M1 (D50) and M2 (non-UV) 
  • Specular glossiness —  two measurements: SPIN and SPOUT