Direct Energy, 2018a

7 LAMPS, LEDS, AND LASERS 169
categories of devices while thin borders indicate example types of devices.
Again, this gure does not show a complete list of all possible devices or
device categories, but it does illustrate relationships between some devices
discussed in this chapter.
Devices are usually designed to involve only one of these processes of
absorption, spontaneous emission, or stimulated emission. However, it is
possible for multiple of these processes to occur in a single device depending
on how it is operated. For example, a semiconductor laser converts
electricity to light by stimulated emission when current above the lasing
threshold is supplied. If a weaker current is supplied, the device will act
as an LED which converts electricity to light by spontaneous emission. If
light shines on the device and the voltage across the device is measured,
the same device acts as a photodetector which converts light to electricity
by absorption. Similarly, photomultiplier tubes, gas discharge lamps, and
gas lasers all involve tubes of gas with electrodes to supply or measure
electricity. Like many energy conversion devices, these devices may convert
electricity to light when operated in one direction and convert light to
electricity when operated in reverse.
7.6 Problems
7.1. Identify whether the devices below operate based on spontaneous
emission, stimulated emission, or absorption.
• Light emitting diode
• Gas discharge lamp
• Argon ion laser
• Solar panel
• Semiconductor laser
7.2. Consider a blackbody radiator at a temperature of 6500 K. Use Matlab,
or similar software, to answer this question.
(a) Find the frequency which corresponds to peak spectral energy
density per unit bandwidth.
(b) Find the wavelength which corresponds to peak spectral energy
density per unit bandwidth.