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Calibration Questions

4 Aug, 2003 By: Steve Geishirt imageSource

Calibration Questions

speed of the new HP CLJ4600 and HP CLJ5500 printers is impressive compared to
the older HP CLJ4500 and HP CLJ8500 series-particularly when it comes to
calibration time. I've seen a CLJ4600 get through warm up and calibration in
just 60 seconds because calibration on the CLJ4600 and CLJ5500 has changed in
comparison to the older models. One reason calibration takes less time on these
newer models is the speed of the new single pass, or tandem engines. However,
not all calibrating is performed when the printer is powered on, which increases
the risk of image defects. In this article, we'll examine the four calibrations
of the CLJ4600 and CLJ5500, when they are used and when you should manually
execute a calibration on the printer through the menus.

we get into the types of calibrations, we need to ask the question, why
calibrate? There are a couple different reasons. First, is to keep colors
consistent throughout the life of the consumables, such as the image drum and
toner, while delaying long term degradation of the mechanical and electrical
components of the machines. One of the biggest color issues customers had with
the now ancient, HP CLJ5 was that the colors would change when a new image drum
was installed. Customers would compare color documents from a month ago and
would notice that the colors did not match. Most of these issues surfaced as the
image drum aged, but there was no calibration system to keep the colors
consistent. A second reason for calibration is to align the colors on top of one
another. Without this, trapping issues occur (color is placed a little higher or
lower than the rest of the colors on the page) and results in print quality

of Calibration

There are four calibrations that can be performed on CLJ4600 and CLJ5500
printers. They are DMAX Calibration, DHALF Calibration, CPR or Color Plane
Registration, and Drum Phase Registration. If you're like me, when you first saw
these names in the HP CLJ4600 or CLJ5500, you had little to no idea what each of
these do. So, let's put them into layman's terms.

a density calibration of each toner cartridge color, at 100 percent coverage.
DMAX is included in the initial calibration when the printer is turned on. As
this takes place, a color swatch for each color is written onto the
Electrostatic Transfer/Transport Belt (ETB) which is normally used to transport
paper past the toner cartridges. The transfer belt rotates the color swatches on
the belt up to two density sensors and is located just under the display (See Figure
). These sensors emit a light onto the color swatches, then reads the
amount of returning light. This allows the printer to measure the density of the
toner for each color written onto the ETB. If a color has more or less toner
than it should have, the developing bias for that color is automatically
adjusted to bring the color back into tolerance. The color settings and changes
to the developing bias are stored in the DC controller. This is important as not
all settings for color calibration are set in the DC controller. A picture of a
transfer belt stopped in the middle of the calibration process can be seen in Figure

is also a density calibration for each toner cartridge color, but as a halftone
instead of a full color. Halftone, or dithering, is the process of spreading out
the dots of toner, leaving coverage at less than 100 percent. DHALF also takes
place when the printer is first turned on and each color is written onto the ETB.
The Halftone swatches are also read by the density sensor, but instead of
adjusting the developing bias (already set when DMAX was run), it sends the
density readings to the formatter. The formatter uses this information when
mixing halftones (placing different color dots side by side to create the
appearance of a third color). A good example of both DMAX and DHALF, as used in
calibration, can be found in Figure

the color densities DMAX and DHALF are set, what else is there to calibrate?
While it's great that we now can recreate consistent colors, if we don't lay the
colors one right over the top of the other properly, the image still won't look
right. That's where Color Plane Registration (CPR) comes in. CPR is not run when
the printer is initially started, but automatically runs when a new toner
cartridge, or ETB, is installed. Each image drum or ETB has a slightly different
circumference. The printer must adjust for this or risk color registration
problems. During this registration process, marks are again written onto the ETB,
but a different kind. While the density swatches for DMAX and DHALF were laid
down in blocks, these are laid down in a timing mark fashion (See Figure
). The density sensor is still used, but is now measuring the distance
between the marks. If the distance varies from the last drum unit or ETB, the
motor for that toner cartridge or ETB adjusts its speed to keep its image
rotation in sync with the others.

final registration is called the Drum Phase Registration. This is done only when
"Calibrate Now" is chosen from the Print Quality Menu. This type of
registration aligns all four cartridge motors and their drive gears, setting
them into their home position. It is important to note that when replacing
certain parts in the printer, the "Calibrate Now" function must be
performed. This includes: laser scanner assemblies, drum motors, DC controller
PCB, and formatter PCBs.

replacing or reinstalling a laser scanner assembly, "Calibrate Now"
should be performed. This is used to synchronize the laser beam (to home in the
beam vertically) lest we have a left to right (or vice versa) trapping issue.
This is particularly good to mention as this is not noted in the "Removal
& Replacement" chapter of the service manual for laser scanner
replacement. If this is not done, it will be the cause of many a headache for
the technician. I would suggest writing this into your service manual so you'll
have that information when the day comes to replace a laser scanner.
"Calibrate Now" is mentioned in the Removal & Replacement chapter
for replacement of the DC controller and formatter.


All of these calibrations occur at different times through the life of the
consumables and printer. As found in the HP CLJ4600 service manual, the list
below specifies when registrations and calibrations should be performed.

and DHALF take place when:

  • The
    printer is powered on - 75 seconds

  • A
    new toner cartridge has been installed - 150 seconds

  • 50
    pages have printed after a new toner cartridge has been installed - 75

  • 1000
    pages have printed since the last calibration - 75 seconds

  • Eight
    hours have passed since the last calibration (not while in power save mode)
    - 75 seconds

  • "Calibrate
    Now" has been requested - 280 seconds

Plane Registration (CPR) takes place when:

  • A
    new toner cartridge has been installed - 150 seconds

  • "Calibrate
    Now" has been requested - 280 seconds

Phase Registration takes place when:

  • "Calibrate
    Now" has been requested - 280 seconds

likely note that density and halftone calibration takes place on a regular basis
while CPR and drum phase registration do not. I've seen "Calibrate
Now" fix a number of different color registration problems and it has also
been known to fix some incorrect color issues. Toner cartridges contain smart
chips and will automatically initiate DMAX and DHALF calibration. However, if a
customer or technician substitutes a new or different ETB without resetting the
life of that consumable, the printer will not initiate calibration and image
defects can occur. Since they do not take place as often, CPR and drum phase
registration are regularly the culprits of color issues-specifically when it
comes to CPR. As many of you are aware, knowing CPR (cardiopulmonary
resuscitation) can save a life. In this situation, knowing when to initialize
"Calibrate Now" or to perform CPR can save your own.

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