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Chapter 5: Read Modes and Read Types
Absorbance Read Mode
The instrument uses the Absorbance (ABS) read mode to measure the Optical Density (OD)
of the sample solutions.
Absorbance is the quantity of light absorbed by a solution. To measure absorbance
accurately, it is necessary to eliminate light scatter. If there is no turbidity, then
absorbance = optical density.
A = log
10
(I
0
/I) = –log
10
(I/I
0
)
where
I
0
is intensity of the incident light before it enters the sample divided by the light after
it passes through the sample, and
A
is the measured absorbance.
The temperature-independent PathCheck® Pathlength Measurement Technology
normalizes your absorbance values to a 1 cm path length based on the near-infrared
absorbance of water.
The instrument enables you to choose whether to display absorbance data as Optical
Density (OD) or %Transmittance (%T) in the Reduction dialog.
Optical Density
Optical density (OD) is the quantity of light passing through a sample to a detector relative to
the total quantity of light available. Optical Density includes absorbance of the sample plus
light scatter from turbidity and background. You can compensate for background using
blanks.
A blank well contains everything used with the sample wells except the chromophore and
sample-specific compounds. Do not use an empty well for a blank.
Some applications are designed for turbid samples, such as algae or other micro-organisms
in suspension. The reported OD values for turbid samples are likely to be different when read
by different instruments.
For optimal results, you should run replicates for all blanks, controls, and samples. In this
case, the blank value that will be subtracted is the average value of all blanks.
% Transmittance
%Transmittance is the ratio of transmitted light to the incident light for absorbance reads.
T = I/I
0
%T = 100T
where
I
is the intensity of light after it passes through the sample and
I
0
is incident light
before it enters the sample.
Optical Density and %Transmittance are related by the following formulas:
%T = 10
2–OD
OD = 2 – log
10
(%T)
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