Page 8 of 14
7. Method of Measurement
The film holder contains an optical transmitter which has a light emission of max. 635 nm (red)
and 565 nm (green). The brightness of the LEDs in the red and green areas can be controlled
using two digital /analog converters. The optical receiver, located opposite the transmitter,
measures the intensity in both the red and the green areas.
During this measurement process, the film thickness is measured in addition to the discoloration
in the red area of the film caused by the ionizing radiation. As a result, the dose readings
determined via discoloration can be adjusted in accordance with the thickness of the film.
Measurement of the dose and film thickness is performed at the same location, thus increasing
the accuracy of the dose reading. The dosimeter displays the same dose if one film is used or
two superimposed films, with the same dose in the measuring head.
After starting, the device tells the user to remove any film remaining in the holder.
First the dosimeter adjusts both colors to maximum brightness and then determines which of
the colors has the highest intensity. The other color is reset until both of the measured
intensities are roughly equivalent. These intensities are stored in the memory as the values
I
g
0
and
I
r
0
.
By comparing the difference between the intensities, scattered light are suppressed.
The measured values are evened out by software components and a mean value calculated
using 8 of the evened values. A measurement period of 3 times 20ms is required for the
evening process (red value, green value and dark value). If the mean value is constant, a colon
is shown before the dose reading. A second mean value is calculated using 32 evened out
measured values. If this mean value is constant, it is output and identified by a preceding
asterisk.
8. Principle of measurement
Light intensity is attenuated exponentially as it passes through an absorbent medium. Red light
is only absorbed in the carbon-black section of the measuring film. The rosalinin which has
been colored red by the dose absorbs only very little of the red light.
The intensity measurement
I
r
can also be used to determine the thickness of the film, if the
other quantities are known.
Green light is also attenuated as it penetrates the film, due to the red discoloration.
However, this red discoloration is proportional to the energy dose (or more specifically the ion
dose) to which the film has been exposed.
