Medical Design Briefs - November 2022 - 35

example, a homogeneous energy distribution
in the laser beam is required
for an application in dermatology, the
camera resolution must be correspondingly
high. Otherwise, the power
peaks simply do not show up. On the
other hand, the quality of the software
used determines the accuracy of the
calculated parameters. MKS Instruments,
for instance, uses its own patented
algorithm for Ophir beam profilers,
which enables measurements of
the laser beam parameters according
to the ISO11146-3 standard (see Figures
1 and 2).
n Criteria for Choosing a Device
In order to determine which sensor
is suitable, it is essential to first collect
the specifications of the laser. The following
parameters are crucial:
Which wavelength should be measured?
The wavelengths used in medical
applications range from UV to IR.
As some sensors are considered broadband
and others are highly specific to
just UV or a very narrow band (or even
cover only a specific wavelength), it is
essential to determine the wavelength
before choosing a sensor.
* What is the diameter of the laser
beam? This value is of critical importance,
as it determines the size of the
aperture. Ideally, the aperture size
should be selected such that no additional
beam-reducing or beam-widening
optics is required. The sensor
surface should be at least 10 percent
- but ideally 30 percent - larger
than the beam diameter.
* What power range? If the quantity to
be measured is the average laser power
for a continuous-wave laser, the expected
power must roughly correspond
to the power range of the
sensor. The irradiation duration must
also be known in order to ensure
proper heat dissipation on the sensor.
B
A
Fig. 2 - The beam profile in 2D (b) or in 3D (a) immediately shows whether the beam deviates from the
given definitions. (Source: MKS - Ophir).
* Is it a pulsed laser? In the case of pulsed
lasers, the pulse rate and the pulse width
are also decisive for selecting the right
sensor.
In fact, picking a measuring device for
laser measurement can be very complex.
Ophir, for example, offers various calculators
on its website to help determine a
short list of candidates. Especially in medical
applications, it is always advisable to
contact the manufacturer's experts. In addition
to the standard sensors, many companies
take advantage of the option of havMedical
Design Briefs, November 2022
ing sensors adapted to individual needs as
OEM parts.
n Calibration Provides Surety
Once the decision for the appropriate
measurement technology has finally
been made, there is one more thing to
consider: The measuring devices must
also be checked regularly. In the case of
a power or energy meter, the sensor
should be recalibrated regularly. For this
purpose, the sensor is sent to the manufacturer's
laboratory and compared by
the on-site experts against a silver master
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that can be traced back to PTB or NIST
standards. Since, before each calibration,
the sensor is measured in an input
check, the customer also receives a report
about the device's pre-adjustment
status quo. If the values regularly deviate
by too much, the interval between calibrations
can (or should) be shortened
accordingly. With camera-based measurement
solutions, the camera itself is
tested and recertified. Regular calibration
and/or recertification guarantees
fulfillment of the corresponding documentation
requirements (see Figure 3).
35

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Medical Design Briefs - November 2022

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