 Development of an ultraviolet irradiance metersuitable for measurement of UV-LEDs
K. Miyasaka*, I. Serizawa*, S. Kobayashi*, K. Kinoshita**, K. Godo*** ORC Manufacturing, Co., Ltd., 4896 Tamagawa, Chino, Nagano, 391-0011, Japan** NMIJ, AIST, 1-1-1 Umezono, Tsukuba, Ibaraki, 305-8560, Japan

Abstract-- Many ultraviolet (UV) irradiance meters are designed and calibrated to measure the irradiance of mercury lamps; however,this kind of meters arise a spectral mismatch error of several tens of percent in measuring the irradiance of UV-LEDs. This error wasdifficult to correct in many situations because of the variety of UV-LEDs spectra, resulting in low measurement accuracy. We
developed an irradiance meter with a flat spectral responsivity suitable for UV-LEDs measurement, and improved a calibration method
to minimize spectral mismatch error. Our UV irradiance meter had succeeded in minimize the error from the difference in UV-LEDs
spectra, and thus enable more accurate UV-LEDs irradiance measurements.

I. INTRODUCTION
In irradiance measurements of mercury lamps with little
variation in spectral shape, irradiance meters are designed
and calibrated to measure irradiance of mercury-emission
lines limited by bandpass filters. On the other hand, central
wavelength and spectral shape of UV-LEDs vary amongindividuals and do not match catalog values. To avoid
large spectral mismatch error, it is desirable for UV-LED
irradiance meters to have a flat spectral responsivity withinwavelength range of UV-LEDs. In this study, to minimize
spectral mismatch error in UV-LED measurements, wedeveloped an irradiance meter with flat spectral
responsivity in UVA range and its calibration method.
II. DEVELOPMENT OF UV-LED IRRADIANCE METER
Figure 1 shows a developed UV-LED irradiance meter.
The irradiance meter is constructed from an indicator andan irradiance-sensor head.
Indicator

Irradiance-sensor head
Fig.1. UV-LED irradiance meter

In general, spectral responsivity in an irradiance meter isdetermined by transmittance of optical components, e.g.
optical filters, and spectral responsivty of a photodiode.
To get a flat spectral responsivity in the irradiance meter,
optical filters with relative spectral transmittance inverseto spectral responsivity of the photodiode were integrated
in the sensor head. The black dash line in figure 2 shows
relative spectral responsivity of the sensor head. The

relative spectral responsivity was successfully obtained to
be almost flat characteristic in UVA range.

Fig. 2. Spectral responsivity of the irradiance-sensor head.

III. CALIBRATION AND CORRECTION
To further reduce the spectral mismatch error caused by
the non-flat component of spectral responsivity, amonochromatic light source consisting of a xenon lampand a monochromator was developed as a calibration lightsource. By adjusting slit width and wavelength position of
the monochromator, the light source can generate amonochromatic light with both a full width at halfmaximum (FWHM) of 10 nm, which match to the FWHM
of typical UV-LEDs, and arbitrary center wavelengths.
Using the light source like a UV-LED spectrum, i.e., using
a similar spectral to a test light source in calibration, reduce
the spectral mismatch error in UV-LEDs measurement.
The calibration results for each central wavelength werestored within the indicator, and the measurement results
are corrected for each time measurement. The red line infigure 2 shows relative spectral responsivity of theirradiance-sensor head after calibration and correction.
The spectral mismatch error in each measurement of UVLEDs at 365 nm, 385 nm, and 405 nm was better than
0.5 % after calibration and corecction, that this meter has
sufficient accuracy.
IV. CONCLUSIONOur UV irradiance meter had succeeded in minimize
spectral mismatch error by integrating the special deigned
optical filter and improving calibration method, thus
enable more accurate UV-LEDs irradiance measurements.