8
Absorption of Light is a typical phenomenon of interaction between electromagnetic radiation and
matter. When a light beam crosses a substance, some of the radiation may be absorbed by atoms,
molecules or crystal lattices.
If pure absorption occurs, the fraction of light absorbed depends both on the optical path length
through the matter and on the physical‑chemical characteristics of the substance according to the
Lambert‑Beer Law:
‑log
I
/
I
o
=
ε
λ
c d
or
A =
ε
λ
c d
Where:
‑log
I
/
I
o
= Absorbance (A)
I
o
= intensity of incident light beam
I
= intensity of light beam after absorption
ε
λ
= molar extinction coefficient at wavelength
λ
c
= molar concentration of the substance
d
= optical path through the substance
Therefore, the concentration “c” can be calculated from the absorbance of the substance as the
other factors are known.
Photometric chemical analysis is based on the possibility to develop an absorbing compound from a
specific chemical reaction between sample and reagents. Given that the absorption of a compound
strictly depends on the wavelength of the incident light beam, a narrow spectral bandwidth should
be selected as well as a proper central wavelength to optimize measurements.
PRINCIPLE OF OPERA
TION
The optical system of Hanna Instruments’
HI96735
colorimeter is based on special subminiature
LED and narrow‑band interference filters to guarantee both high performance and reliable results.
HI96735
block diagram (optical layout)
A microprocessor controlled special LED emits radiation which is first optically conditioned and
beamed to the sample contained in the cuvette. The optical path is fixed by the diameter of the
cuvette. Then the light is spectrally filtered to a narrow spectral bandwidth, to obtain a light beam
of intensity
I
o
or
I.
The photoelectric cell collects the radiation
I
that is not absorbed by the sample and converts it
into an electric current, producing a potential in the mV range.
