Specular reflection is the mirror-like reflection of light waves from a surface.
In this process, each incident ray reflected at the same angle to the surface normal as the incident ray, but on the opposing side of the surface normal in the plane formed by the incident and reflected rays. The result is that an image reflected by the surface is reproduced in mirror-like (specular) fashion.
The C.I.E accepted Norms when working with Colour Measurement are:
Specular Component Included (SCI)
Specular Component Excluded (SCE)
If one observes natural light striking an object, it is calculated that 4% of the total light is reflected in a mirror-like reflection. Specular Gloss is defined as the ratio of the light reflected from a surface at an equal but opposite angle to that incident on the surface.
If one observes natural light striking water, it is possible to see the mirrored Specular component at any time.
If an object is uneven, then the mirror effect is scattered by the rough surface. 4% of the total light is still reflected but is not perceived, as the light has been scattered by the object’s surface.
So when do you use one setting or the other?
- A choice between SCI and SCE is only applicable to a Spectrophotometer. Colorimeters are by default only SCE.
- If you are measuring an object by direct Reflectance you can select SCI, or SCE, or BOTH. This would allow you to calculate the pure colour values, and the percentage of pure reflection, depending on the scattering properties of the object. The rule of thumb with direct reflectance measurements is to use primarily SCI. This means that you have collected all of the light values that have landed on the object.
- If you are measuring an object in a vessel, cell, cuvette, etc, the default setting is SCE. The 4% Gloss will already have been reflected away from the object IN the cell etc, so it must be assumed that only the SCE component represents the actual cell content values.
To summarise, 4% of all reflected light is Gloss
Seeing the Gloss will depend on the surface of the object. The rougher the surface the more scattered the light the less will be perceived. (SCI or SCE)
If objects are in a glass cell etc, the cell will already have reflected the gloss and does not need to be included in the calculation. (SCE only)
Highly reflected surfaces will allow you to see how much gloss is mirrored so both SCI and SCE can be deduced at the same time. (SCI and SCE)