PerkinElmer EnVision, Instrument Manual

Page: 33 / 111

Functional description
23
Figure 26. Excitation from above/Dual reading from above (using the S and
P polarizors)
Absorbance mode
For absorbance measurements the same excitation light source is used as for
FI, TRF and FP measurements. The wavelength of the light is selected by an
optical filter placed in the excitation filter slide and it can be in the range
230-950 nm. A 405 nm absorbance filter is supplied with the instrument.
In absorbance measurements, light is directed through a light guide to the
bottom measuring head where the intensity of the light is measured using a
reference photodiode. The light is then directed through the bottom of the
plate and focused into the sample. The focal plane is the same as for FI,
TRF and FP measurements.
The intensity of the light is first measured without any sample and then the
samples in one plate are measured.
The light intensity is measured by a photodiode placed at an optimal
position directly above the plate. The light path for absorbance
measurements is thus different than for FI, TRF, FP and luminescence
measurements.
The absorbance value is calculated by the equation
A = -log (I/I
0
)
where I
0
is the light intensity without any sample and I is the intensity after
an absorbing or reflecting medium.