ISO 7668:2010 pdf free download – Anodizing of aluminium and its alloys一 Measurement of specular reflectance and specular gloss of anodic oxidation coatings at angles of 20° 45°, 60° or 85°.
5 Optical standards
5.1 Reference standards
5.1.1 Black glass
The primary reference standard shall be made of highly polished black glass of refractive index 1,567 (or clear glass with the back and edges roughened and coated with black paint), the top surface being plane to within two fringes per centimetre as measured by optical interference methods. The effect of the refractive index on specular reflectance and specular gloss values is described in Annex A.
The glass surface shall be kept in a clean condition and free from surface scratches or damage.
5.1.2 Glass prism (for Method E only)
An alternative reference standard, recommended for Method E and suitable only for 45° reflectometers, is provided by the hypotenuse face of a right-angled glass prism, having dimensions of 25 mm x 25 mm x 35,3 mm and optically worked faces. This uses the principle of total internal reflection for angles greater than the critical angle, but there are losses on entering the prism faces. These can also be calculated from Fresnel’s equation (see A.1) to give the absolute specular reflectance values given in Table 3.
7 Measurement of specular reflectance and specular gloss
7.1 General
Calibrate the instrument as directed in Clause 6 and measure the specular reflectance or specular gloss of the test sample by placing it in firm contact with the instrument so that the plane of incidence and reflection is parallel to the direction of rolling or machining of the test sample. Measurement can be carried out in a plane normal to this direction if the anisotropy of the surface under examination is to be assessed.
7.2 Measurement of specular reflectance
For surfaces of low specular reflectance, set the black-glass primary reference standard (see 5.1.1) to read 10 times the relevant specular reflectance value in Table 4 and multiply each reading by 0,1.
For surfaces with a high specular reflectance, set the apparatus to the relevant specular reflectance value specified in Table 4, using a black-glass primary reference standard. Alternatively, in the case of Method E, the reflectometer may be set to the appropriate specular reflectance value specified in Table 3, using a glass prism (see 5.1.2).
NOTE Specular reflectance values which are encountered in practice can cover a very wide range, from 90 % to less than 0,1 %, and only in the case of bright finishes, or with the 85° method, is it practicable to obtain a direct reading of specular reflectance.
7.3 Measurement of specular gloss
Using a black-glass primary reference standard, set the apparatus to the relevant specular gloss value specified in Table 5.
For surfaces with a high specular gloss, in excess of 100 gloss units, set the black-glass primary reference standard at an arbitary selected value of 0,5 or 0,2 or 0,1 of the specular gloss value in Table 5 and multiply each reading by the corresponding factor (2 or 5 or 10, respectively).
Table 4 gives the calculated specular reflectance values for glass of various refractive indices for each of the measurement angles employed.
The preferred glass has a refractive index of 1,567, but glass with a refractive index of 1,523 has also been used frequently.
A.2 Specular gloss
The specular gloss of the black glass, g’ also depends on its refractive index, ii, and glass of refractive index n = 1,567 shall be assigned a specular gloss value of 100 units. If glass of this refractive index is not available, another material of known refractive index, n, may be used, the specular gloss being raised or lowered according to Equation (A.2).