Figure 10-27.Control strips read from center of each step with emulsion facing up.
A densitometer is an instrument used for measuring
and reading the density of film and paper directly. Film
is read on a transmission densitometer, and paper is read
on a reflection densitometer. Most densitometers
supplied by the Navy today have both transmission and
reflection reading capabilities on the same meter (fig.
A densitometer uses a photoelectric cell to measure
the light transmitted through film or reflected from
paper electronically. Before a densitometer can be used,
it must be checked and calibrated against a reference
standard. To use the meter, you must place the material
to be measured, emulsion-side up, in the light beam
between the source and the photocell. The density
reading is then read directly from the meter.
There are several sets of filters incorporated in the
head of the densitometer. When you are reading
black-and-white materials, the yellow filter must be in
place. When color materials are read, there are two
different sets of filters that are used. They are Status A
and Status M filters. Status A filters are used to read
color transparencies and prints. Status M is used to read
color negative film that has an orange mask
Only certain steps of the control strip are read. The
steps that you are required to read are established by the
manufacturer of the material or are established by the
quality control technician within your imaging facility.
When you are reading the steps on your control strip, be
sure that the emulsion side is facing up and take the
reading from the center area of each step (fig. 10-27).
PROCESS CONTROL CHARTS
A process control chart provides a visual
representation of a process. Control strips that are
processed and read on a densitometer are then plotted
on a control chart. The points plotted on the graph
indicate what has occurred in the process at the time the
control strip was processed. Through the use of control
charts, a determination can be make whether the process
is operating normally. As stated before, only selected
steps of a control strip are monitored. For black-
and-white film, the minimum steps monitored are as
follows: base plus fog (B + F) or gross fog, high density,
low density, speed point, and contrast.
For each step monitored on a control chart, there are
three lines. The center line represents the mean (X)
(pronounced bar X), or average, a top line that
represents the upper control limit (UCL), and a bottom
line that represents the lower control limit (LCL).