IX CONGRESO NACIONAL DEL COLOR - Publicaciones de la ...

IX CONGRESO NACIONAL DEL COLOR. ALICANTE 2010
would be used a standardized color chart as the GretagMacbeth ® ColorChecker ® Color Rendition
(CCCR) or the GretagMacbeth ® ColorChecker ® SG (CCSG).
Figure 1. Printed sheet with 33x49 color samples.
Figure 2. Squares with the same color as the patches employed for color measurements.
Two commercial digital photographic cameras were employed in the work, a Canon EOS
350d and a Nikon D80. The Canon camera has resolution of 8 megapixels, CMOS optical sensor
with size 22.2x14.8 mm and the Nikon has resolution of 10.2 megapixels and CCD optical sensor
with size 23.6x15.8 mm. On Canon camera was used a 28-80 mm objective, with maximum
aperture f/3.5 at 28 mm, and f/5.6 at 80 mm. On Nikon camera was used a Nikkor 18-135 mm
objective, with maximum aperture f/3.5 at 18 mm, and f/5.6 at 135 mm. For both cameras, the
aperture was set on the medium value 5.6 and it was not changed throughout the images
acquisition process. However, different ISO values, from 100 to 1600, and exposure times,
between 1/15 and 1/250 s, were employed. The photographs were saved in JPG format with a
resolution of 3456x2304 (8 megapixels). The Adobe ® Photoshop ® CS3 software was used to
obtain colorimetric data, CIELAB coordinates, from the photos, selecting an area of a single
pixel, approximately in the center of the chip image.
Bearing in mind that in case of non-calibrated cameras the transformation used in the
Adobe ® Photoshop ® software is no completely appropriated because this transformation is really
device dependent; the CIELAB coordinates obtained from the images acquired with each camera
have been compared with the ones computes from spectroradiometric measurements, performed
with a spectroradiometer PhotoReseach SpectraScan PR-704, using a aperture of 0.125º. As
perfect diffuser, a surface of PTFE 11 , supplied with the spectroradiometer, were used. The CIE
1931 Standard Observer has been considered in the computation of triestimulus values. Table 1
shows the CIELAB color coordinates of the 8 used samples computed from the
spectroradiometric measurements.
11 PTFE corresponds to polytetrafluoroethylene, which is an organic polymer.
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