Guidelines / Manual
BTS256-LED Tester / Page 7
About LED Measurement ( Continued )
Version 01.2009-01
Picture 7.1: Auxiliary Lamp Use
sphere through the measurement port and be absorbed
in the dark room. But during actual use, the measure-
ment port of the integrating sphere will be fully or par-
tially covered by the device under test DUT. So light
leaving the sphere through the measurement port will
be reflected back into the sphere adding erroneously to
the DUT light signal. Depending on the spectral reflec-
tivity and color of the DUT the re-reflected light will vary
in intensity and color and effect an unknown measure-
ment error. Auxiliary lamps are used to compensate this
substitution error by measuring the signal of the auxil-
iary lamp with and without the DUT at the measurement
port of the integrating sphere. The difference in intensity
is used as a correction factor for following measure-
ments of the same kind of DUT.
2. The
size of the integrating sphere
should be much
larger than the size of the test sample to keep measure-
ment uncertainty low independent of the spatial light
emission characteristic of the test sample. The smaller
the sphere the greater the interference of the uniform
light distribution due to internal baffles, ports and auxil-
iary lamp. Small size integrating sphere sensors used in
mobile light meters must therefore be calibrated by a
calibration lamp with spatial emission characteristic
close to that of the device under test. Light meters with
small size integrating spheres designed for wide angle
emitting LEDs may have increased measurement uncer-
tainty when used to measure narrow beam emitter type
LEDs. In applications involving large emitting area LED
sources and LED arrays the only choice is large diame-
ter integrating spheres.
Besides light intensity,
spectral intensity distribution
and
color data
are important test properties in LED testing. The
spectrometer is the most commonly used measurement de-
vice for these types of measurements. However the photo-
diode array spectrometer employing low cost CCD sensors
is limited by sensor intensity linearity and stray light charac-
teristics. An alternative method is to mate a photodiode with
a diode array sensor, e.g. Gigahertz-Optik’s
BiTech Sen-
sors BTS
(picture 7.3). It’s photodiode with photometric
sensitivity provides a precise linear ratio between light input
and signal output over a very wide dynamic range for very
accurate luminous flux measurements. Spectral distribution
including the wavelength of peak intensity is provided by it’s
diode array sensor. The spectral data enables the measure-
ment device to calculate color data (xy and u’v’ color coordi-
nates, color temperature, color rendering index and domi-
nant wavelength). A spectral resolution of 5nm is recom-
mended for the color calculation.
The
sensitivity of photodiode
sensor based light meters
can be controlled over different gain ranges due to
switchable feedback resistors in the amplifier circuit. The
response time is short and independent of the gain over a
wide range.
The
sensitivity of photodiode array
is controlled by inte-
gration time so the lower the light level the longer the meas-
urement time. But longer integration times effect increases
Picture 7.3: BITEC Sensor Base Set-up Design
Measurement without DUT
Measurement with DUT
Auxiliary Lamp
Light Detector
Picture 7.2: Calibration Strategy with Hemispherical
Calibration Lamp for Diffuse Emitting LEDs
D
io
de
A
rr
ay
Common Incident Light
Polychromatic or Monochromatic
Photometric
Filter
G
ra
tin
g
P
h
o
to
d
io
d
e
Beam Splitter
Dark Room
