When you measure a colour sample with your spectrophotometer (such as an x-rite i1) it reads the wavelengths of light reflected from an object. The instrument perceives the reflected light wavelengths as numeric values.
These values are recorded as points across the visible spectrum and this is known as spectral, or reflectance data. This reflectance data is represented as a spectral curve, and the curve is the colours fingerprint. In the example below the highlighted curve is the reflectance data of a measured blue colour swatch.
Note: In addition to measured colour samples, ColourSys can synthesise reflectance data for colours made with a colour picker.
As spectral data is not relative to an illuminant or profile, the same data could be used to display on screen and print for different illuminants, where LAB or RGB data for example, will only be correct for the illuminants it was recorded under. Having this data in your colours can help ensure that if you are sending your AVA files to another site to print, your colours will remain more stable.
Also in AVA we can use the reflectance data to increase the accuracy of our overprint simulation, this is essential when doing any Production Colour Management.