AQ-00275-000, Rev. 3
26
Zeroline Correction
An integrating sphere is sensitive to small-angle scatter from the sample beam coupling optics.
The scattered radiation strikes the wall of the integrating sphere near the reflectance sample port,
creating a "halo" surrounding the port. This halo-effect causes a small error in the measurement of
the reflectance factor that is most significant when measuring samples of very low reflectance.
This error is easy to characterize and correct.
The following procedure may be used to correct reflectance factor measurements of diffuse or
specular samples, in either the 8°/Hemispherical or 8°/Diffuse geometries:
1. Perform a reflectance-factor measurement of a given sample as described in this man-
ual.
2. Replace the sample with a light trap, initiate the scan and record the measurement
data. The light trap reflectance is 0%.
3. Compute the corrected reflectance factor for the sample,
ρ
s
, as follows:
where:
ρ
r
is the reflectance value of the reference standard,
R is the sample data displayed on the instrument and
Z is the data from the light trap measurement.
Note:
This zero-correction procedure can also be used in conjunction with a mask at the
reflectance sample port to accurately measure the reflectance factor of samples smaller than
the accessory reflectance sample port.
Sources of Error
Substitution Error
Substitution error, sometimes called "single-beam sample absorption error," occurs when the refer-
ence and the sample are introduced sequentially into the single beam integrating sphere. Your
spectrophotometer is a double-beam instrument and is not susceptible to substitution error.
Wall Radiance Uniformity Error
Variations in sphere wall radiance can occur due to sphere ports, seams or bright spots reflected
s
ρ =
R
−
Z
(
)
r
ρ
100%
−
Z
(
)
Eq. 15