Here - PMOD/WRC

of the lamp housing (halogen lamp) were used as reference
planes of these lamps. Distances were measured on an
optical rail using a magnetic length measuring device with
0.1 mm resolution and accuracy.
Figure 2. Images of the measured spectroradiometer diffusers
(from left to right): Bentham D3 (∅ 25 mm), Bentham D7
(∅ 10 mm), Bentham D5 (∅23.5 mm), and the Schreder
diffuser with the quartz glass dome.
Measurement results
Reference planes of the photometer diffusers: The
reference planes of the tested photometer heads were
determined by illuminance values measured at six
distances between 500 mm and 1500 mm. The results for
the photometer diffusers are presented in Table 1. All
offsets of the reference planes were found out to be several
millimeters behind the outermost surfaces of the diffusers. 3
Table 1. Dimensions and offsets of the photometer diffusers.
Uncertainties are deviations of the mean of four measurements.
Diffuser (a) (b) (c)
D 24.3 16.0 30.0
Dimensions
[mm]
L 8.0 7.1 15.0
W - - 26.1
Offset ∆d D
[mm]
5.0•±•0.5 7.8•±•0.3 8.5•±•0.7
Reference planes of the spectroradiometer diffusers:
Measurements with spectroradiometer heads were carried
out at 10 distances between 300 mm and 2000 mm. This
range includes typical calibration distances of 500 mm and
370 mm used. The results at each wavelength and for each
diffuser are presented in Table 2.
Table 2. Measured reference plane offsets [mm] of the
spectroradiometer diffusers.
Filter /
Diffuser
Bentham
D3
Bentham
D7
Bentham
D5
Schreder
UV 2.3 -0.3 0.4 6.4
V(λ) 2.6 0.1 1.2 7.1
700 nm 3.6 0.3 3.2 7.1
The measurement result, 5.0 mm in the UV region,
indicates that the quartz dome increases the observed shift
of the reference plane.
Conclusions
We have presented a method with which the location of the
reference planes of photometers and radiometers with
diffusers can be determined. Inverse square law works only,
if distance measurements are made with respect to this
reference plane.
The method has been demonstrated with various
photometers and spectroradiometer diffusers. From the
measurement results, we may discover that the distance
reference plane is shifted with measuring heads having
either thick mesa-shaped or dome-shaped diffusers. The
material of the diffuser contributes to the shift. Large
errors appear in the regions where the materials are
transparent.
If these effects are not accounted for, systematic errors
may occur in the calibrations of photometers and
spectroradiometers. For the studied photometers, errors up
to 3 % at a calibration distance of 500 mm would arise.
There would also be inevitable distance dependence on the
correction factors.
With the Schreder diffuser, the observed distance shift
of 6.4 mm in the UV region may cause an error as large as
2 % in the calibration at a distance of 500 mm. This is a
significant contribution in the uncertainty of spectral
global UV irradiance measurements that is nowadays at the
level of a few percent.
The uncertainty of the method may be estimated from
the repeatability noted during the measurements that is of
the order of 1 mm (k=2). It was checked that the
wavelength dependence of the reference plane of the lamp
is of the order of 0.4 mm.
References
1. Y. Ohno, Photometric Calibrations, NIST Special Publication
250-37, U.S. Government Printing Office, Washington DC,
1997.
2. H. J. Kostkowski, Reliable spectroradiometry, La Plata, Md.,
Spectroradiometry Consulting, 1997.
3. J. Hovila, M. Mustonen, P. Kärhä, E. Ikonen, “Determination
of the diffuser reference plane for accurate illuminance
responsivity calibrations,” Appl. Opt. (in press).
It can be seen that the mesa-shaped diffuser D3 has a
significant offset, which is surprising for a planar diffuser.
This is probably due to the poor material used to obtain a
wide wavelength region. The other diffusers are made of
Teflon, which results in increasing offset towards infrared
wavelengths, where Teflon is not a very good diffuser
material. The reference plane of the Schreder diffuser is
surprisingly deep inside the measuring head. The effect is
too large to be explained by the dome-shape only.
The influence of the quartz dome of the Schreder
diffuser on the reference plane position was studied by
repeating the measurement without the quartz glass dome.
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