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Gloss standard calibration by spectrophotometer reflectance
measurement-suggested method
Anne Andersson
SP Swedish National Testing and Research Institute, Borås, Sweden
Abstract. By definition, the ultimate reference or
primary standard for gloss measurements is a glass surface
having a refractive index of 1,567 for the wavelength
589.26 nm for all angles, (Budde, ISO2813, Andor). This
paper suggests a method to calibrate gloss by near normal
reflection measurements in a spectrophotometer. The near
normal reflectance of a high gloss standard is measured at
589.3 nm in a spectrophotometer with reference to a
standard Aluminium mirror with traceability. From the
reflection value the refractive index is calculated using
Fresnel equations. The monochromatic gloss value, G, can
be obtained for all different angles. Correction factors
between the monochromatic definition and the
polychromatic luminous use in the gloss meter is added as
suggested by Budde, 1980. The total uncertainty of gloss
reference standard value G lum is calculated for all angles
20º, 60º and 85º to be < 1 gloss unit.
Introduction
Specular gloss is widely used in the paint, paper, plastic
and textile industries for the characterisation of mirror-like
appearance of a surface by specific reflectance
measurements. The measurement geometry and conditions
are well defined in international and national standards, f
ex ISO 2813. By definition [1,2], the primary standard for
gloss measurements is a piece of highly plane black glass
having a refractive index of 1.567 at the wavelength of the
sodium line D (589.3nm). For all angles of incidence this
surface by definition has an assigned gloss value of 100
GLU (Gloss Units). For calculation of the corresponding
specular reflectance, r(angleº, n=1,567), values the Fresnel
equations are applied, giving:
r (20º, n=1.567) = 0.049078
r (60º, n=1.567) = 0.100056 (1)
r (85º, n=1.567) = 0.619148
Primary standards of different refractive index than n =
1.567 results in different, G(angleº, n), gloss values in
GLU (gloss units). From the reflection, r(angleº,n) values
the G(angleº, n) can be calculated for all angles with
formula (2):
G(angleº, n) = 100* r(angleº,n)/ r (angleº, n=1.567) (2)
The r(angleº,n) values are in this work calculated from
the refractive index assuming an ideal surface having the
property of all light that are not reflected are absorbed.
However the gloss meter specified with the standard uses a
daylight source (CIE C) and a photometer with a CIE
V(lambda) spectral response function covering a rather
broad spectral range. The inconsistency between the
monochromatic specification of the reference and the
polychromatic instrument leads to ambiguities in the
calibration of the used reference standards and may cause
differences in the practical gloss measurements of up to
0,5 %, according to Budde. Correction factors between the
monochromatic definition and the polychromatic use in the
gloss meter is recalculated, from values presented by
Budde. The ratios of luminous data to spectral data, at the
wavelength 589,26 nm, are presented in Table 1.
Table 1: Conversion from reflection at 589,26 nm to luminous
gloss values G lum for angles 20º, 60º, and 85º,according to Budde
1980.
Angles 20º 60º 80º
r(lum)/r(589,26) 1.005 1.003 1.000
Method
The reference gloss value are obtained by calculating
the refractive index from the near normal
spectrophotometer measurement and deduction of the G
for each angle followed by multiplication of the
conversion factor to get the luminous G lum values that the
instruments are working with.
1 measure the reflectance r at near normal
incidence (8º)
2 calculate the refractive index from the
Fresnel equations (in this work assuming
equal polarisation of p and s components)
3 reflection at other angles was calculated
with the same assumption as in 2.
4 the monochromatic gloss value G was
calculated for angles 20º, 60º and 85º from
(2)
5 apply conversion to luminous G lum for all
angles
Reference and sample preparation The quality and long
time stability of the reference standard is of crucial
importance. Cleaning of the surface of the gloss standard
used in this work is done with alcohol with a developed
technique until the gloss meter presents same values as
calibrated values within ± 0.3 GLU for all angles. If any
G(angle) deviates more it is an implication of having
scattering effects and the surface is not cleaned properly.
Proceedings NEWRAD, 17-19 October 2005, Davos, Switzerland 321