Here - PMOD/WRC

Determination of the diffuser reference plane for accurate photometric and
radiometric measurements
J. Hovila 1 , P. Manninen 1 , L. Seppälä 1 , P. Kärhä 1 , L. Ylianttila 2 and E. Ikonen 1,3
1 Metrology Research Institute, Helsinki University of Technology, P.O.Box 3000, FI-02015 TKK, Finland
2 Radiation and Nuclear Safety Authority (STUK), P.O.Box 14, FI-00881 Helsinki, Finland
3 Centre for Metrology and Accreditation (MIKES), P.O.Box 239, FI-00181 Helsinki, Finland
Abstract. A method for determination of the effective
measurement plane of measuring heads with diffusers is
presented. The method was tested with three commercial
photometers and four spectroradiometer diffusers. The
shifts of the diffuser reference planes were determined to
be 5 – 9 mm and 0 – 7 mm, respectively. This effect causes
measurement errors up to 3 % if the detectors are
calibrated and used at different distances from the light
source.
Introduction
Diffusers are often used to improve angular responsivities
of photometers and spectroradiometers. This introduces
additional uncertainty in distance measurements, as the
reference plane for the inverse square law is not always
obvious. In calibrations, distances are typically measured
from the outermost surface of the diffuser. This produces
systematic errors to measurements at any other distances,
if the reference plane is located somewhere else, as it
typically is with dome-shaped diffusers. For accurate
illuminance responsivity and solar UV measurements, the
reference plane positions of the detectors must be known
precisely. 1
We present a simple measurement method for this
effect based on inverse square law and accurate distance
measurements on an optical bench. The method is tested
with three commercial photometers and four diffusers
typically used with spectroradiometers.
Inverse-square law
The method is based on the measurement of
illuminance/irradiance at various distances from the lamp.
The measured values E obey the inverse-square law 2
I
E = (1)
( d + ∆d ) 2
S
+ ∆d D
where I is the luminous/radiant intensity of the point
source, d is the distance between the reference planes of
the source and the detector, ∆d S is the distance offset of the
source and ∆d D is the distance offset of the detector.
The offset ∆d S of the lamp is first determined with a
reference detector having a well-defined aperture plane by
inserting measurement results to Equation (1) and applying
a least-squares fitting. Intensity I and the distance offset
∆d S are used as free parameters (for the reference detector
∆d D = 0). After that, the measurements and the analysis are
repeated with the studied detectors. The known offset ∆d S
is used to obtain desired offset ∆d D for the reference plane
of the detector. 3 The measurement distances are selected in
such a way that the signal varies sufficiently and resolution
of the detector is adequate.
Measurement set-up
Reference detector: In this work, we used a
temperature-controlled standard photometer HUT-2 (of
type PRC TH15) as a reference detector. It has a
well-defined aperture plane with circular aperture of 8 mm.
This plane was used as a reference plane for the distance
measurements.
Studied photometers: The studied photometer heads were
a dome diffuser of Minolta T-1 [Figure 1 (a)] a
dome-shaped diffuser [Figure 1 (b)] and a cylindrical
diffuser [Figure 1 (c)] of Hagner E2.
Figure 1. Images and schematic drawings of the studied
photometer diffusers. Dimensions are given in Table 1.
Studied spectroradiometer diffusers: The tested
measuring heads of the spectroradiometers were three
mesa-shaped diffusers of Bentham and a dome-shaped
type of Schreder diffuser. Images of the diffusers are
presented in Figure 2. The Schreder diffuser has a
removable quartz glass dome for weather protection.
Distances d were measured from the outermost point of the
diffusers for all studied measuring heads.
The incoming light from the measuring heads was
guided with an optical fiber to a trap-detector from which
the photocurrent was measured. Wavelength dependence
of the reference planes was measured by using a filter
between the fiber and the detector. Three filters were used:
a UV-filter of type UG11, a V(λ) filter, and a 700-nm
interference filter. The Si-trap was replaced with a GaAsP
trap when the wide-band UV filter was used.
A 180 W incandescent lamp was used with the
photometers, and a 1 kW halogen lamp was used with the
spectroradiometer heads. The front surface of the
alignment mirror (incandescent lamp) and the front surface
Proceedings NEWRAD, 17-19 October 2005, Davos, Switzerland 201