4. Optical System
2011-09
Instructions for Use for INFINITE M1000 PRO No. 30064852 Rev. No. 1.0
33
How a Monochromator Works
A monochromator is an optical instrument that enables any wavelength to be
selected from a defined optical spectrum. Its method of operation can be
compared to a tunable optical filter, which allows both the wavelength and
bandwidth to be adjusted.
A monochromator consists of an entrance slit, a dispersive element and an exit
slit. The dispersive element diffracts the light into the optical spectrum and
projects it onto the exit slit. A dispersive element can be realized by using a glass
prism or an optical grating. Modern monochromators such as those used in the
INFINITE M1000 PRO
are designed with optical gratings.
Rotating the optical grating around its vertical axis moves the spectrum across
the exit slit and only a small part of the spectrum (bandpass) passes through the
exit slit. This means that when the monochromator entrance slit is illuminated with
white light, only light with a specific wavelength (monochromatic light) passes
through the exit slit. The wavelength of this light is set by the rotation angle of the
optical grating. The bandwidth is set by the width of the exit slit. The bandwidth is
defined as Full Width at Half Maximum intensity (FWHM).
Monochromators block undesired wavelengths, typically amounting to 10
3
. This
means when the monochromator is set for light with a wavelength of 500 nm and
the detector detects a signal of 10,000 counts, light with different wavelengths
creates a signal of only 10 counts. For applications in the fluorescence range this
blocking is often not sufficient, since the fluorescence light to be detected is
usually much weaker than the excitation light. To achieve a higher level of
blocking, two monochromators are connected in series, i.e. the exit slit of the first
monochromator acts as the entrance slit of the second monochromator
simultaneously. This is known as a double monochromator. In this case, the
blocking count reaches a factor of 10
6
, a value typically achieved by interference
filters.
In the
INFINITE M1000 PRO
, a double monochromator is installed on both the
excitation and detection side. This allows easy selection of arbitrary excitation
and emission wavelengths.
Fiber optic bundle
From the exit slit of the excitation monochromator, the light will be coupled into a
fiber optic bundle guiding the light either to the top measuring optics or the bottom
measuring optics (Figure 12). The lower end of each fiber bundle acts as a color
specific light source. In both cases, a small portion of the light is always guided to
the flash lamp monitor diode.
Flash lamp monitor
The light energy of single flashes may fluctuate slightly. To take these variations
into account, a silicon photodiode monitors the energy of every single flash.
Fluorescence and Absorbance measurement results are compensated
correspondingly.
