Download Calibrated Specular Reflectance Standards for UV/VIS/NIR

Calibrated Specular Reflectance
Standards for UV/VIS/NIR
Product Note
M
etallic mirrors have long been the medium of choice for calibrated, broad spectrum specular
reflectance standards. Aluminum, silver and gold are the most popular metals for this purpose,
and each has its advantages. Silver and gold display very high reflectivity in the upper visible and NIR
regions, and their spectral profile is relatively flat, but they reflect poorly in the UV. The reflectivity of
aluminum is somewhat lower than the others in the VIS/NIR regions, but this material is much more
suitable for the UV and lower visible wavelengths. The chemical stability of gold against atmospheric
oxidation is a plus, but thin protective layers can mitigate this effect for the other two metals. Regardless
of which material is chosen, another very important factor in evaluating a standard is how accurately the
reflective values are known; therefore, the choice of the best mirror is often determined from the care by
which the mirror is calibrated as well as high reflectivity in the wavelength range of interest.
V
acuum deposited aluminum on glass is
by far the most commonly used reference mirror. Optical Reference Laboratory
offers an aluminum mirror of 1.5 inches in
diameter, mounted in a sturdy screw cap
container, shown in Figure 1. Silver is also
available as the reflective surface, and both
metals are protected by a thin transparent
overcoat. Unmounted mirrors of different
shapes can also be provided.
R
eflectance values are traceable to
primary NIST calibration data. NIST
traceability is available from 250 to 2500 nm,
at incident angles of 8, 30, or 45 degrees. The
mirrors can also be calibrated at other angles,
without the NIST traceability.
Figure 1. Calibrated aluminum reference mirror.
F
igure 2 shows typical reflectance
spectra of aluminum and silver
standard mirrors at 8 degrees incidence,
over their useful calibration ranges. Above
2000 nm the aluminum mirror displays
reflectance values higher than 97%, and the
silver mirror reflects approximately 99% of
the incoming radiation. Spectra of a typical
aluminum mirror at 45 degrees incidence
are given in Figure 3.
Figure 2. Typical spectral profiles of aluminum
and silver standard mirrors, 8 degrees.
C
Figure 3. Typical spectra of an aluminum standard
mirror at 45 degrees incidence, S and P polarization.
T
alibration measurements are performed
with a PerkinElmer Lambda 1050
spectrophotometer, using a relative reflectance
technique. Working replicas of a primary NIST
calibrated mirror are used for baseline scans,
against which the final product mirrors are
calibrated. At incident angles of 30 and 45
degrees, polarization measurements are taken
using a calcite polarizer of the Glan Taylor
design.
o use the standard mirror in relative reflectance, simply perform a baseline using the standard, then
replace it with the sample and scan over the spectral range of interest. Multiply the resulting sample
spectrum point by point with the values of the standard to obtain the true reflectance of the sample.
A
certificate of traceability is
provided with each calibrated
mirror. The certificate contains
information about the original NIST
calibration, as well as the operating
conditions and calibration status of
the spectrophotometer.
For further information contact:
Optical Reference Laboratory LLC
www.OpticalReferenceLab.com
[email protected]
Product Note 1108E
2011, Optical Reference Laboratory
Standard Mirror Specifications
Substrate:
Diameter:
Flatness:
Scratch/Dig
Coating:
Data Interval:
Housing:
Diameter:
Polished clear float glass
1.5 inches
1/4 wave
40/20
Protected aluminum or silver
5 nm unless otherwise requested
Anodized aluminum case
2 inches