Page 236 - Photonics Essentials an introduction with experiments

Measurements in Photonics

230 Characterizing Photonic Devices in the Laboratory

Figure 10.3. A lens with a lower f-number (that is, a larger aperture) will focus parallel
light to a point of smaller dimensions than a lens with a larger f-number.

In addition, the diameter of the point of focused light depends on the
f-number. The smaller the f-number, the smaller the diameter. On the
other hand, if you want to make a parallel beam of light out of a
source that is much bigger than a point, you will have better luck with
a larger f-number lens (Figs. 10.3 and 10.4).
You will run into situations where you will be focusing light on the
entrance slit of a spectrometer (focusing down to a point) or taking the
light from the monochromator exit slit and steering it somewhere
(taking light from a point source and turning it into a parallel beam)
Most optical measurements involve this kind of manipulation of light
beams.

10.3 Monochromators and Spectrometers
A monochromator and a spectrometer are the same instrument. The
name depends on whether you are using the instrument to select a
certain wavelength of light from a beam containing many wave-
lengths (such as white light), in which case it is called a monochroma-
tor, or whether you are trying to tell what wavelengths are present in
a beam of light, in which case it is called a spectrometer.

Figure 10.4. A lens with a larger f-number (that is, a smaller aperture) will do a better
job than a smaller f-number lens of producing a parallel beam of light from a point
source with a finite size such as a light bulb filament.

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