I was involved in a project that tries to determine the quality of wines and in doing literature survey I found out that there are various color parameters that need to be determined for assessing the quality. Along with many others like color intensity, Tint, percentage redness, percentage blueness I also found that there is one parameter called CIELab space. It is used to calculate hue value and chroma but in method it is written that we should use D65 illuminant at first as a reference. So what is this D65 illuminant and how to make measurements of CIELab?
D65 is a standardized illuminant defined by CIE which simulated the normal daylight (inlcuding UV part). As it is mathemtical defined by CIE it can be easily integrated into calculation using white point transform with the D65 data.
You can do this by choosing the D65 illuminant during measurement, if there is no option for that take the X, Y and Z values and multiple it by the D65 values.
Light sources are physical emitters of electromagnetic radiation such as a light bulb or the sun. Unfortunately there is usually some variation between light sources (whether between phases of daylight or e.g. different fluorescent tubes). An illuminant is a table of values used to describe either a phase of daylight or a particular light source. A blueprint of a light source.The CIE use illuminants as part of their XYZ system of colour specification to minimise variability due to the light source. They do something similar for the observer.
It turns out that colour and temperature are linked and this relationship was modeled by a German physicist called Max Planck. Using Planck's Blackbody Radiator equations (by substituting a value for temperature in Kelvins) we can produce a range of spectral power distributions that correlate well to the different phases of daylight (from dawn to dusk). These D class illuminants include one that approximates to northern daylight called D65 where the 65 means 6500 kelvin (although the standard uses 6504 kelvins). This has been adopted by many of the coloration industries such as textiles.
A colour space relates numbers to colours. CIELAB is a colour space and was introduced in 1976 to be perceptually uniform. This means that the distances between two points could be used to calculate a perceptual or colour difference. CIELAB is a transformation of the CIE XYZ values. Designed to mimic simple opponent models of human colour vision the system can represent the three values of lightness (L*), red-greenness (a*) and yellow-blueness (b*) or lightness, chroma (C*) and hue (h degrees or metric hue H*).
You will need a colour measurement device to capture colour information. This would typically be either a colorimeter to capture CIE XYZ values or a spectrophotometer to measure reflectance values. There are then a set of equations you will need to apply to calculate the CIELAB values (this may be done for you depending on the combination of hardware and software you use).
I wrote a blog post on the subject...
http://johnthemathguy.blogspot.com/2014/11/how-many-d65s-are-there-in-2-degree.html
The short answer is that : 1) D65 refers to the color temperature of daylight; and 2) you need a colorimeter or (preferably) a spectroradiometer to make CIELab or similar color space measurements. As Peter Kapusta noted in an unacceptably rude manner, information on these topics is available through Wikipedia.
More usefully, I recommend doing a Web search for "Sudraud method." This will present you with a number of authoritative discussions on .judging the quality of wine using spectroscopic methods, including practical white papers by the instrument manufacturers. (HunterLab offers a particularly useful discussion, with literature references.) It is a complex topic, with much more information than we can provide in a ResearchGate discussion exchange.
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