The Maximum Permissible Exposure (MPE) to laser radiation defines a level below
which an acute injury will not be sustained. Values for the MPE are defined by
international standards, based on research into biological laser damage mechanisms and
thresholds. To verify that a given laser installation does not present a hazard it is
necessary to compare accessible levels of laser radiation to the MPE. In most cases
this can only be done by a combination of measurement and calculation.
The standard presents MPE values as tables of formulae. Users find the standards
difficult to interpret and use for practical assessment tasks. It is shown by theoretical
analysis and practical investigation that the measurement process is non-trivial. This
research has identified that general purpose laser radiation measurement equipment is
not capable of undertaking the critical pulsed measurements needed for MPE
assessments. These instruments are usually limited to measurements of average power,
or pulse energy over a limited range of pulse parameters. Analysis of the standards
shows that energy measurement is a critical aspect of the radiation hazard assessment
process. During the practical investigation a radiation hazard was demonstrated to exist in many laser displays used for entertainment purposes.
To meet the measurement criteria laid down in the standard a novel detector strategy is
developed to provide accurate pulse energy measurement. No single detector can
meet these current criteria over the complete laser radiation spectrum. Three types of
detector are used to measure the output of all common lasers. This leads to the
concept of a modular instrument design, incorporating common detector interfaces
with signal conditioning to provide a standard output signal format to an interface unit
which extracts the relevant parameters from the data for processing by a palmtop
computer. The software guides the user through the measurement process, controls
the hardware, determines the measured radiation level from the data, calculates the
appropriate MPE and displays the results. Techniques were developed to minimise the
occurrence of user errors. This required consideration of human-computer interfacing
techniques in the software design and unique coding of each instrument element.
The measurement precision of the instrument was determined using stable laser and
light emitting diode sources. A scanned laser display system was then used to
determine the measurement precision of the combination of a typical source and the
meter. It was found that the instrument precision exceeded that of the source,
essential if the instrument measurement results were to be reliable. For safety critical
instrumentation, calibration is identified as an important issue. Electrical and optical
techniques are discussed.
Alternative applications for the instrument were considered. A technique for high
power laser measurement using a beam sampling technique was demonstrated. This
had advantages when compared to traditional methods of high power laser
measurement. The sampling technique was extended to the construction of a laser
beam delivery monitor capable of monitoring beam power and position and shutting off
the laser in the event of a fault developing.
Since the completion of the research project the instrument has been developed
commercially in collaboration with a UK company. The commercial instrument uses
the same strategy, hardware and software designs as developed during the research
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.