Imaging Station Laser scanning
Charging Roller
OPC
Drum Toner
Developer
Cleaning
Blade Y M C K
Paper Paper
Fuser Transfer Belt
Process Direction
Figure 8. In-line color EP print mechanism. There is a separate imaging station for each of the four color separations. Each
imaging station consists of an OPC drum, a charging roller, a laser scanner unit, a toner developer, and a cleaning blade. The
paper is transported past each imaging station by the transfer belt. After all four color separations are transferred to the paper,
it passes between a heated fuser roller and a pressure roller to fuse the toner to the paper.
(a) (b) (c) (d)
0.03 0.03 0.03 0.03
0.025 0.025 0.025 0.025
0.02 0.02 0.02 0.02
Spectrum
Spectrum
Spectrum
Spectrum
0.015 0.015 0.015 0.015
0.01 0.01 0.01 0.01
0.005 0.005 0.005 0.005
0 0 0 0
0 20 40 60 80 0 20 40 60 80 0 20 40 60 80 0 20 40 60 80
Frequency(cycles/inch) Frequency(cycles/inch) Frequency(cycles/inch) Frequency(cycles/inch)
(e) (f) (g) (h)
0.03 0.03 0.03 0.03
0.025 0.025 0.025 0.025
0.02 0.02 0.02 0.02
Spectrum
Spectrum
Spectrum
Spectrum
0.015 0.015 0.015 0.015
0.01 0.01 0.01 0.01
0.005 0.005 0.005 0.005
0 0 0 0
0 20 40 60 80 0 20 40 60 80 0 20 40 60 80 0 20 40 60 80
Frequency(cycles/inch) Frequency(cycles/inch) Frequency(cycles/inch) Frequency(cycles/inch)
(i) (j) (k) (l)
Figure 9. Four halftone patterns (1st row) and the corresponding spectra of projected profiles of constant tone patches. The
velocity modulation of the OPC drum results in the high peaks at 35 cycles/in in the spectra unweighted (2nd row) and weighted
(3rd row) by the contrast sensitivity function of the human viewer at 12 inches viewing distance.
direction is along the short edge of the media; and the exception of the CPR problem, PQ defects are generally
scan direction is along the long edge. most visible in areas of the print that consist of a con-
stant tint fill. Thus we will use a constant tone back-
Test Features in the PQ Test Pages ground, on which other test features will be built. If we
use four different constant tone images, each of which
In this section, we will show how to develop test fea- contains only one of the four primary colors, we can sepa-
tures to make the defects clearly visible. When we de- rately check the condition of each imaging station. For
velop the test feature for a given defect, we should example, if white streaks are found in the magenta con-
consider the condition under which the defect and its stant tone image, the magenta imaging station can be
cause are clearly identified. The basic features that we suspected as being defective.
will discuss are a constant tone background which is Although a constant tone image clearly shows PQ de-
divided into sections, ghosting test blocks, rulers, color fects such as scattered spots or repetitive artifacts, it
plane registration (CPR) test blocks, and page identifi- may also reveal fine pitch banding that might not be
cation numbers. Finally, we will show how to incorpo- visible in the content normally printed by the customer.
rate the test features into a set of PQ test pages for a Thus fine pitch banding is considered to be an artifact
color laser EP printer. caused by imperfection within specification; and it
should be concealed in the test page. In order to find a
Constant Tone Background halftone pattern for the background that can effectively
A PQ test page should be designed to clearly show conceal the fine pitch banding, we next examine the
defects that the manufacturer considers to be out of strength of fine pitch banding for four different half-
specification and which the customer can remedy by re- tone patterns.
placing a consumer-replaceable part, or which should Figure 9 compares the spectra of the projected absorp-
be reported to the customer support center. With the tance6 of four halftone patterns with a 4 4 period for a
Print Quality Test Page Vol. 48, No. 5, September/October 2004 437
