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AQ-00073-000, Rev. 7 14

where R = reflectance of the sample beam at the sample surface.

During reflectance measurements, the reflected component of the sample beam is collected by

the integrating sphere and detected by the sphere detector. The detector signal represents the part

of the sample beam that is not transmitted and not absorbed by the sample substance.

Double Beam Spectroscopy

In a double-beam, ratio-recording spectrometer, the measurement of reflectance factor and trans-

mittance involves the calculation of a baseline based upon values recorded in a background cor-

rection scan. Background correction is used to compensate for changes in sphere efficiency due

to the introduction of the sample into the system and for any imbalance in the energy of the sam-

ple and reference beams. This correction is performed automatically by the instrument and is the

same routine used during spectroscopy evolutions without the accessory. The theoretical basis

for the background correction, as it applies to simple reflectance and transmittance measure-

ments, is explained briefly in this section.

Radiation from the instrument illumination sources is split into two different beams: the sample

beam and the reference beam. Each beam is interrupted periodically by means of an optical

chopper such that the integrating sphere is illuminated alternately by the two beams. At any

given wavelength, the instrument records the ratio of the signal produced by the detector when

the sphere is illuminated by the sample beam to that when the reference beam is measured.

Therefore, when a background correction scan is performed, for either reflectance or transmit-

tance measurement, the background correction value B recorded by the instrument may be

expressed as:

where:

is the energy of the sample beam,

is the energy of the reference beam,

is the reflectance factor of the sample at the sample reflectance port

is the reflectance factor of the reference port, and

and

are the efficiency with which the energy reflected from the sample and

reference beams is captured by the sphere and converted into a signal by the

detector.

ATR++ 1

----------------=

Eq. 2

Labsphere RSA-PE-20 User Manual

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