Ocean Optics Overture - Manual (Page 11)

Chapter 1: Introduction to Spectroscopy
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The CCD array spectrometer
Looking at sunlight through the spectrometer will tell us a lot about how it works. If you have a pocket
spectroscope you can compare the traditional spectroscope view with the Overture software display.
Sunlight enters the spectrometer through a 50 micron-wide slit. That is very narrow; 5/100ths of a
millimeter.
In a conventional spectroscope you will see a spectrum and any absorbance lines will show as dark lines.
These lines correspond to sharp dips in the Overture spectrum graph.
The light passes through an optical geometry of focusing mirrors and a reflection grating. The spectrum
falls on a linear
CCD array with 250 tiny sensors in a row so that each sensor (often called a pixel) in the
array corresponds to one wavelength.
The number of photons hitting each pixel is converted to a voltage which is converted into a y-axis value
on the graph. The x-axis is scaled to the pixel number, which indicates wavelength.
Optical Limitations
The Ocean Optics Spectrometers for Education can display peaks separated by less than 2nm, depending
on the model. This is the limit of its
resolution.
The spectrometer resolution is limited by a number of factors, including:
 Slit width
 Grating specification (lines per mm) and quality
 Number of pixels in the array
 Physical size of the system
This causes an apparent spreading of emission and absorption lines in the Overture display so that they
appear as sharp Gaussian peaks, but this is still remarkably high resolution for a compact instrument.
In chemistry applications this is not a problem as the absorption peaks are usually over a hundred
nanometers wide. In fact you will deliberately “smooth” the spectrum by averaging the array output using
Overture software.
For chemistry applications it is the sensitivity or dynamic range that is more important. This allows the
spectrometer to detect small changes in absorption on the y-axis.