22P095-P
Intensity Calibration of the Liquid-Crystal-Based
Tunable Lyot Filter Spectra Camera Using Large White LED Panel
大型白色LEDパネルを用いた
液晶リオフィルタスペクトラカメラの感度較正
Shota Abe, Shinicihro Kado, Satoru Tanaka
阿部 翔太, 門 信一郎，田中 知
School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan東京大学大学院工学系研究科 〒113-8656
In order to apply simple and portable calibration system to the imaging spectroscopy using a tunable
Liquid-Crystal-Based Lyot filter, a light-emitting diode (LED) display panel was examined if it can beused as a standard light source having continuum spectrum in visible wavelength region. By comparing
with the standard light of the tungsten halogen lamp, the radiance of the LED panel was measured. Thenthe calibration factor for the intensity and for the intensity ratio was examined.

1. Introduction
In nuclear fusion study, so-called “line intensity
ratio method” based on the optical emission
spectroscopy (OES) has been applied quite
frequently in these one or two decades, in order tomeasure the plasma parameter. Recently,
importance of the imaging diagnostics has been
particularly emphasized because the plasmaparameter has spatial distribution both along andacross the magnetic field. Therefore, in order toapply the intensity ratio method to an imaging
spectroscopy, we have developed the tunable Lyot
filter spectra camera that can be used with acollisional-radiative model (CR model) [1].
Conventionally, in the calibration of this kind ofimaging spectroscopy system, a tungsten halogen
lamp and a perfect diffuse reflector plate are used.
However, the size and the uniformity of theirradiated surface area of the diffuse reflector can bea problem. This is because the surface area forwhich the calibration data of the halogen lamp isattached usually limited in a small region. In
contrast, the white LED panel has a large surface,which emits continuum spectrum in the visible
region for a long time, and is easy in use and lessexpensive. However, the commercially available
LED panels are not intended to be used for optical
diagnostics, so that their optical properties have not
yet well evaluated. Therefore, in this research, theseproperties of the LED panel were examined interms of the applicability as the standard lightsource for the intensity or intensity ratio
measurement in a visible region.
2. White LED flat panel
Required properties for a standard light sourceare (i) uniformity of irradiance distribution for eachwavelength and (ii) small angular dependence of its
irradiance. The A3-size LED panel (412 × 228 mm2
in light-emission surface area), examined in thisresearch, is equipped with the white LEDs at bothedges of the acrylic plate. Emitted light from theLED is reflected by dot-shaped white inks, on theback surface of the acrylic plate, which makes themarble pattern. The emission from the surface isnon-uniform, due to this marble pattern, so that amilky-white acrylic plate 4 mm thick was used as adiffuse reflector.
The A6-size LED panel (140 × 91 mm2 inlight-emission surface area) was also used for theangular distribution measurement. This panel isequipped with LEDs on one edge and the reflection
mirror on the other edge of the acrylic plate.
3. Examination of the LED panel(A) a low dispersion spectrometer (StellarNet inc.
EPP2000C) and (B) a digital camera (Nikon: D90
CMOS detector 4288 × 2848 pixels) attached with
the Liquid-Crystal-Based Tunable Lyot Filter (CRi:VariSpec VIS-7-20) were used for the evaluation.
The filter has 7 nm FWHM pass band and 12 %transmittance at 550 nm. The wavelength range of
the A3 LED panel is 420-750 nm as shown in Fig. 1.
While the tunable range of the Lyot filter we use is400-720 nm, the leak-band, which appears
presumably due to the misalignment of the internal
optical components, were observed at longer than548 nm at a specific wavelength setting below 455nm.
The angular dependence of the irradiance of theA6 LED panel was measured using the system (A).
Sweeping the angle between the viewing chord andFig. 1. Wavelength distribution at the center of theA3 LED panel. The tunable range of the Lyot filter isindicated.

Fig. 2. The angular dependence of the irradiance at
the center of the A6 LED panel, normalized to theirradiance for 0 degree. The widths, labeled F/1.4
and F/1.7, are the corresponding to spread angles
measured from the optical axis.

Fig. 3. Spatial distribution of the irradiance for theA3 LED panel surface, normalized to the value at thecentral region. 1D profiles of the relative irradiancealong the horizontal and vertical lines on the image
are also appended to the image. The uniformity
within ±5 % is observed in the squared area.

the normal direction of the panel surface, theirradiance and the wavelength distribution of thecenter point of the panel were obtained. Thisobservation shows that, at the angle within ±30Fig. 4. The ratio of the image of the irradiance tuned
to 667nm and that for 471 nm. 1D profiles of the ratio
along the horizontal and vertical lines on the image
are also appended to the image. The uniformity within±3 % is observed in the squared area.

degrees, the irradiance is not smaller than 95 % to
the irradiance at the 0 degree. It indicates that on
the light axis, the bright optical system such as
F/1.4-1.7 can be used (Fig.2).
Spatial distribution of the irradiance for the A3
LED panel was examined using the system (B).
Considering the spectrum from the helium atom,
the pass band of the Lyot filter was set to 471 nm,
501 nm, 587 nm, 667 nm and 691 nm respectively.
Figure 3 shows the image for 471 nm obtainedusing the system (B). The wavelength tuned to 691
nm for the image of the LED panel is slightly
shifted from the helium line of 706 nm because
sensitivity of the digital camera degrades above 695
nm. The spatial distribution of intensity normalized
to that for 471 nm is shown in Fig. 4. The area ofapproximately 160 × 175 mm2 exhibits theuniformity within ±3 %. If one applies to the line
intensity ratio method, one needs not consider thespatial distribution of the calibration factor over thisarea.
4. Application to the sensitivity calibration
It has been turned out that the variation of thecalibration factor, namely the calibration function,needs to be introduced outside the uniform-ratio
area. The calibration function is determined by
combining the absolute value of the irradiance at
the center of the LED panel, obtained by the system(A), and the images of the irradiance at tuned
wavelength.
References[1] S. Kado, H. Suzuki, Y. Kuwahara, et al.: PlasmaFusion Res., 2 (2007) S1125.