Basic principles of optical radiation and some common applications in ...

BASICPRINCIPLESOFOPTICALRADIATIONAND SOMECOMMONAPPLICATIONSIN ANESTHESIA Dietrich Gravenstein, M D , 1 Samsun Lampotang, PhD, Walter Huda, PhD, 2 and Anwer Sultan, M S I

Gravenstein D, Lampotang S, Huda W, Sultan A. Basic principles of optical radiation and some common applicationsin anesthesia. J Clin Monit 1996;12:445-454 The laryngoscope, fiberscope, lighted stylet, and pulse oximeter are among the increasing number o f lightbased devices used by anesthesia personnel for almost every patient. Other medical settings where light-based devices are commonly used include the clinical laboratory, where blood analyses are performed by photometry, and the recovery room, where thermometers measure temperature from the infrared radiation in the internal auditory canal. Light-based devices are generally inexpensive and less dangerous to tissue than ionizing radiation. As a result, interest in the medical applications of technology based on light-tissue interactions is increasing [1-4]. Indeed, an array o f new light-dependent devices are currently under investigation, including devices for the noninvasive measurement o f hemoglobin **a [5,6], serum glucose* [1,2,7,8], and regional perfusion [9] and devices that use light as an antitumor therapy [10-12] or for imaging internal body structures [13]. Even brain oxygenation may eventually be monitored through infrared light [14-17]. This article reviews the basic principles governing the behavior o f light and its characteristics (Table t) and the essentials o f lighttissue interactions and provides examples of clinical applications familiar to anesthesiologists.

THE NATUREOFLIGHT

From the Departments of 1Anesthesiologyand 2Radiology, University of Florida Collegeof Medicine, Gainesville,Florida. Received Nov 20, 1995, and in revised form May 24, 1996. Accepted for PublicationJun 6,1996. Address correspondence to Editorial Office, Department of Anesthesiology, University of Florida College of Medicine, Box 100254, Gainesville,FL 32610-0254, U.S.A.

.Journalof ClinicalMonitoring12: 445-454, 1996. © 1996KluwerAcademicPublishers.Printedin theNetherlands.

Light exhibits a dual nature that has long been a topic o f intensive research [18,19]. Light demonstrates some physical phenomena usually associated only with particles and others usually associated only with waves. Certain physical phenomena of light, such as interaction with the media through which it passes, can only be satisfactorily explained if light is described as consisting of energy-containing particles called photons (quantum theory). Photons exert radiant pressure on surfaces they strike and can be individually counted by means of a Geiger counter or Geiger-Muller detector [19]. On the other hand, phenomena such as diffraction and polarization can only be explained if light is described as an electromagnetic wave (classical wave theory). Electromagnetic waves, o f which visible light is one type, are * Cadell T, Vice President for Corporate Development, CMT Telemetrix, 1993 (personalcommunication). Conis J, Director of Operations, Noninvasive Medical Technology Corp, Ogden, UT, 1992 (personalcommunication). **Gravenstein D, Lampotang S, GravensteinN, Brooks M. Noninvasive hemoglobinometry [abstracted],Anesthesiology81: A576,1994.

446 Journal of Clinical Monitoring Vo112 No 6 November 1996

Table 1. Characteristics of Optical Radiation

Characteristic

Symbol Units

Wavelength

)~

Frequency

v

Velocity

c

Intensity

I

releases energy in the form of photons. The energy o f a photon (E) is inversely proportional to the wavelength and is expressed by the following equation:

Definition

nm

Length of one complete electric field oscillation Hz The number of oscillation cycles per second m/s Distance traveled in one second (3 x 108 m/s in a vacuum) W / m 2 Power (watts) deposited per unit area

C

= hX

where h is Planck's constant. When an atom absorbs a photon of light, an electron moves to a higher energy state, which increases the vibration and rotation of the molecule [21,22]. Traits such as the type of atomic bond(s) within a molecule determine how photons are absorbed and define that molecule's absorption characteristics. The differences in absorption characteristics between molecules are exploited by spectrophotometers to determine the composition of those molecules [21]. Optical radiation travels in a straight line until it enters a new medium where refraction can occur. Refraction causes the light to change direction. The extent to which this occurs is determined by the optical properties of the two media. These properties determine the refractive index of the second medium (n2), which can be calculated using Snell's law (also known in France as Descartes's law):

characterized by their wavelength ()0 and frequency (v) of oscillation (Figure 1). The speed of light (c) in a vacuum is constant for all light at 3.108 m/s. The relationship between wavelength, frequency, and velocity is given by the following equation: c = ,~v = 3.10 8 _m

(1)

S

Electromagnetic wavelengths range from gamma rays, which are shorter than 10-15 m, to radio waves, which are longer than 109 m [20]. Visible electromagnetic waves have wavelengths from 400 to 700 n m (Figure 1) and are an extremely small portion of the electromagnetic spectrum. Frequencies o f visible light are in the order of 1015 Hz. Optical radiation, or light refers to electromagnetic waves in the visible spectrum and waves with both slightly longer (infrared) and slightly shorter (ultraviolet) wavelengths. Electromagnetic radiation is produced when electrons migrate from a higher to a lower energy state, which

n2 nl

iTITllll

ITIT

I I I I O. O

I I I I I l i i l

Wavelength (m)