|
Camera objective characterization methodologies are widely used in the digital camera industry. Most objective
characterization systems rely on a chart with specific patterns, a software algorithm measures a degradation or difference
between the captured image and the chart itself.
The Spatial Frequency Response (SFR) method, which is part of the ISO 122331 standard, is now very commonly used
in the imaging industry, it is a very convenient way to measure a camera Modulation transfer function (MTF). The SFR
algorithm can measure frequencies beyond the Nyquist frequency thanks to super-resolution, so it does provide useful
information on aliasing and can provide modulation for frequencies between half Nyquist and Nyquist on all color
channels of a color sensor with a Bayer pattern. The measurement process relies on a chart that is simple to manufacture:
a straight transition from a bright reflectance to a dark one (black and white for instance), while a sine chart requires
handling precisely shades of gray which can also create all sort of issues with printers that rely on half-toning. However,
no technology can create a perfect edge, so it is important to assess the quality of the chart and understand how it affects
the accuracy of the measurement.
In this article, I describe a protocol to characterize the MTF of a slanted edge chart, using a high-resolution flatbed
scanner. The main idea is to use the RAW output of the scanner as a high-resolution micro-densitometer, since the signal
is linear it is suitable to measure the chart MTF using the SFR algorithm. The scanner needs to be calibrated in
sharpness: the scanner MTF is measured with a calibrated sine chart and inverted to compensate for the modulation loss
from the scanner. Then the true chart MTF is computed. This article compares measured MTF from commercial charts
and charts printed on printers, and also compares how of the contrast of the edge (using different shades of gray) can
affect the chart MTF, then concludes on what distance range and camera resolution the chart can reliably measure the
camera MTF.
|
