Biomedical Engineering Reference
In-Depth Information
2.3 Gloss and optical roughness by DOE
DOE sensors have been proven to be accurate in sensing optical changes in two main surface
properties as permittivity and roughness also in nanometer scale. Specular reflectance is a
function of three variables: complex refractive index/permittivity, angle of incident of the
probe beam and surface topography. Standardized specular gloss (
G
) relates to reflectance
and it is normalized to the reflectance of black glass having gloss value of 100 gloss units
(GU). The optical roughness, which relates to the surface topography, is sensed as optical path
differences which the rough surface generates in reflection. In DOE sensor measurements; (I)
the probe beams angle of incidence remains constant (perpendicular to the surface), (II) the
complex refractive index of the measured surface is known (measured) and (III) the surface
topography acts as a variable. Before DOE measurements, the spectral complex refractive
index (
N
) of each material surface is characterized with variable angle spectro-ellipsometer
(VASE). General principle of DOE is to focus coherent light to a 4
4 dot matrix, which
takes the optimal shape in its focal plane at wanted wavelength. The formed DOE image is
then detected with CCD-camera and saved into personal computers (PC) memory for further
analysis (Fig. 3).
×
CCD
PC
DOE
N
1
N
2
N
3
N
4
Laser
L1
L2
BS
Cuvette
Fig. 3. DOE sensor setup with lenses L1 and L2, beam splitter (BS), sample cuvette, CCD
camera, refractive indexes
N
1
-
N
4
and personal computer (PC) for analyzing. DOE image
with the 4
×
4 dot matrix is shown on the screen of PC.
In figures 4 and 5 are presented the gloss and
R
opt
maps scanned from a polished titanium
(Ti
p
) surface with 20
μ
m probe beam waist diameter.
Scanning was done in air from
2mm
×
2mm surface area.
(a)
(b)
Fig. 4. Gloss from Ti
p
surface measured in air. The gloss from the surface is presented (a) as
colormap and (b) as meshmap.
