where h is the Planck constant. Similarly, an electron can “jump” to a higher energy state by absorbing a photon, with the resulting energy equal to the energy difference between states (Fig 8-15). According to EM theory, wave energy depends on amplitude only and frequency does not matter, whereas according to quantum theory, energy depends on frequency only and amplitude does not matter.
Figure 8-15 An electron moves between its lowest energy (ground) state (E0) and an excited state (E1) after absorbing a quantum of light energy (ΔE = E1 – E0 = hν). A, Stimulated absorption. B, Spontaneous emission. C, Stimulated
emission. (Reproduced with permission from Steinert RF, Puliafito CA. The Nd:YAG Laser in Ophthalmology: Principles and Clinical Applications of Photodisruption. Philadelphia: Saunders; 1985. Redrawn b y C. H. Wooley.)
Light Sources
Light sources convert nonlight forms of energy into light and may be categorized as thermal or luminescent.
Thermal Sources
The atoms in material warmer than absolute zero are thermally agitated and radiate EM waves as a result. As temperature increases, the amount of energy radiated increases, and a greater percentage of the total radiated energy occurs at higher frequencies. At body temperature (about 37°C, or 310 K), for instance, most of the radiated energy occurs in the infrared spectrum (eg, the thermal energy detectable by night-vision devices). At body temperature, however, a miniscule amount of visible light is produced well below the visual threshold.
As the temperature of an object increases, it begins to produce EM energy at visible frequencies, first glowing red and, with further increases in temperature, glowing brighter and bluer. The spectrum of all thermal sources of light is continuous; that is, thermal sources produce some EM radiation at
every frequency. Planck showed that the energy radiated at any frequency can change only by discrete amounts (ie, multiples of hν).
Luminescent Sources
Luminescent sources produce light as a result of electron transitions between different energy states— a process different from thermal agitation. In a gas discharge tube, for example, a small amount of gas (eg, hydrogen, helium, or mercury vapor) well below atmospheric pressure is placed in an otherwise evacuated glass tube that is sealed with an electrode at each end. An electrical potential is applied across the electrodes, and the electrical energy raises the electrons of the gas to higher energy states. Almost immediately, the electrons drop to lower levels, emitting light.
The differences (ie, spacing) between energy states in each element are unique. Typically, electron transitions between only a few energy states will produce visible photons. Because every element (or molecule) has a unique set of energy levels, each produces a unique visible spectrum (Fig 8-16), a circumstance that is useful from a practical and scientific standpoint. For example, light produced at the center of the sun is absorbed by atoms at the cooler surface. Only the frequencies corresponding to specific transitions are absorbed, producing discrete dark lines in an otherwise continuous solar spectrum. The pattern of dark spectral lines identifies the elements in the sun. Similarly, each frequency produced by a gas discharge tube can be isolated and used as a light source in its own right. Long before the advent of lasers, gas discharge tubes provided a practical source of monochromatic light.
Figure 8-16 Discrete spectral lines produced by various elements. Every kind of atom has a different distribution of energy states. Each line in an atom’s spectrum corresponds to an energy difference between 2 states.
Fluorescence
When an electron absorbs a photon and jumps to a higher energy level, usually it quickly drops back to the original level and emits a photon identical in frequency to the one it absorbed. However, atoms of some elements have 2 energy levels that are close together (Fig 8-17). When a photon is absorbed, the electron jumps to the highest energy level. Instead of dropping back to the original level, however, the electron transitions to the slightly lower level and emits nonvisible energy. Next, it drops to the initial energy level and emits a photon. The photon emitted is still visible but has less energy than the absorbed photon and, therefore, a lower frequency.
Figure 8-17 Light emission by fluorescence. In this example, an electron jumps from the lowest to highest energy level by absorbing a high-frequency (eg, blue) photon (1). The electron drops to a slightly lower, intermediate energy level through nonradiative processes or by emitting a low-energy (eg, infrared) photon (2). Eventually, the electron drops to its original energy level, emitting a photon of slightly lower frequency (eg, green) than the one it absorbed (3). (Illustration b y Edmond H.
Thall, MD.)
This phenomenon, called fluorescence, occurs only in materials possessing close spacing between energy levels. The essential feature is that the emitted photon has a lower frequency and, therefore, a different color from that of the stimulating photon. The clinical utility of this phenomenon (discussed later) derives from the difference in frequency between the absorbed and emitted photons. Fluorescence is the basis of fluorescein angiography, macular autofluorescence, and the Seidel test. In