Instrumentation & Measurement Magazine 25-7 - 21

Reference Radiometer for
Irradiance Meters Calibration:
A Characterization Comparison
Between Two Methods
Samaa M. Faramawy and Sameh M. Reda
T
he continuous need to calibrate radiometers makes
us think and consider which of the utilized calibration
methods provides the desired purpose with a
high accuracy, high degree of stability in repeatability and reproducibility,
and less estimated uncertainty value as possible.
This study compares two common standard calibration methods
that emerged and applied to two radiometers held in the
radiometry lab at the National Institute of Standards (NIS),
Giza, Egypt. The results concluded that the ND-filters method
of calibration achieves the purpose of the study, exhibits more
stability (STDEV 4% and 7%) and less combined uncertainty
2.7% and 9.4% for the two radiometers under test.
Introduction
Calibration of a sensor is the process of characterizing the parameters
required to understand, describe, and quantify its
performance and provide, an evaluation of the sensor performance.
As long as the calibration is mentioned, it is necessary
to mention the importance of traceability, as it refers to the
ability to track a measurement to a known standard unit of
measurement within a given measurement uncertainty [1], [2].
According to metrology science, radiometers are irradiance
output measuring devices for any electromagnetic sources.
These kinds of devices consist of two attached parts, a detector
and a readout unit. A proper detector is selected for a proper
wave band, which can be achieved in case the detector has the
right sensor with an appropriate filter. The incident power
of light on the sensor converts to electrical signal that passes
through an electrical circuit to finally produce the irradiance
unit in W/m2 on the readout display. Radiometry is the measurement
of the electromagnetic radiation power from any
source between the wave band UV to IR, hence, the sources
applied in different fields in medicine, industry and even sterilization.
Therefore, manufacturers build many radiometers to
sense different bands of optical radiation to fulfil the requirement
of the application.
One of the essential techniques of any calibration process
is to verify the stability behavior of the measurement system.
Knowing that a measurement system is stable and repeatable
October 2022
is a comfort to the individuals using it [3]. Calibration studies
provide some information about changes in the measurement
system. Stability is the key to predictability that may be determined
using a control chart. Repeated measurements are
obtained using a measurement device on the same unit. When
recording the measurements, points within limits indicate
that the process has not changed and may not likely change in
the future [4], [5]. In other words, at different times, the same
operator uses the same instrument to perform multiple measurements
on the same properties of the same object and then
calculates the average values for each time period. The measurement
system is stable if the average measured readings are
somehow similar each time [6]. This paper gives a comparison
between two methods for in-house calibration of radiometers
at different wavelengths against a reference detector.
Setup
The system used, shown in Fig. 1, generally consists of a
mercury arc lamp used as a broad radiation source; a test radiometer;
and a standard reference detector. Actually, there were
two radiometers used in this study: The UDT S480 model of
268UVA detector was attached with a read-out unit that delivers
irradiance measurements directly. In this case, the UVA
Band pass filter was used as an optical tool in front of the mercury
lamp to select the UVA band. The other test radiometer
was the UDT S480 model of 268 BLUE-detector that was attached
with a read-out unit, and the source used in this case
was a 450 nm LED source. Finally, the National standard
Fig. 1. Spectral irradiance calibration system.
IEEE Instrumentation & Measurement Magazine
1094-6969/22/$25.00©2022IEEE
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