Measures of Light
For many clinicians, the measurement of light is one of the most confusing topics in the field of clinical optics. This section will attempt to clarify the subject.
As we learned previously, light is a range of electromagnetic radiant energy, comprising the range of wavelengths to which the eye is ordinarily sensitive—about 400–700 nm. The measurement of light thus naturally derives from more general methods for the measurement of electromagnetic radiant energy—specifically, from general methods to measure the transfer of energy by electromagnetic radiation.
Energy is the ability of a system to perform work. Through the insights of physics and thermodynamics, energy is understood to take many forms, including kinetic energy, the energy of objects in motion; potential energy, the energy stored in a system by virtue of its position (such as the height of an object subject to gravity); chemical energy, the potential energy available through chemical reactions; and thermal energy, the energy stored in the temperature of objects. Thus, the units for work and energy are the same—typically joules (J = kg·m2/s2).
Power is the rate at which energy is transformed or transferred from one form to another. It is measured in units of work (or energy) per unit time, typically joules per second (J/s), or watts (W). Historically, this unit and its multiples have served in most contexts for measuring the rate of energy transfer, from the output of a gasoline engine to the energy required to operate a computer microprocessor chip. The watt is used without difficulty whatever the form of the energy transfer, from mechanical devices, to electrical power lines, to heat generation. Derived quantities are described in simple combinations of units—for example, the productivity of solar panels might be described in units of power per unit area—say, W/m2.
Measurements of the power transferred by means of electromagnetic radiation are typically described with derived units in accordance with the above scheme (Table 2-2). In principle, the fundamental measurement tool for this purpose is the blackbody radiometer. This is an object (frequently realized in the form of a hollow cavity), which absorbs radiant electromagnetic radiation at all wavelengths with equal efficiency. The amount of energy absorbed is determined by measuring the increase in the temperature of the blackbody, and converting heat to energy according to the laws of thermodynamics and knowledge of the heat capacity of the detector, and dividing by the length of time the radiometer was exposed to the radiation. (In practice, radiometers are designed to work only over limited ranges of the electromagnetic spectrum, depending on the physics of the detector employed.)
In practice, care must be taken to account for the geometry of the source of the radiation, and for the geometry of the detector. The simplest case is a point source, radiating equally in all directions—the power output of such a device is appropriately measured simply in watts. But it would be difficult to surround such a point source with a detector, which would simultaneously absorb the output in every direction. Typically, the detector will intercept the radiation over only a small area. In this case (assuming the radiation output to be uniform in every direction), it would be appropriate to normalize the measurement by dividing the energy transfer intercepted by the detector by the solid angle it subtends (in essence, the area of the detector divided by the distance from the point source), thus reporting the energy output of the point source in watts per steradian (W/sr), with the understanding that multiplying by 4π (the total solid angle of the space surrounding a point) would yield the energy output of the point source, in watts. This description of the energy output from a point source is referred to as its radiant intensity.
Table 2-2 Comparison of Radiometric and Photometric Units
Similarly, if the radiation source is an extended planar object, the appropriate description of its energy output would be in terms of watts per unit area, say W/m2. This description of the energy output is referred to as the radiant exitance of the extended source. If one prefers to measure the rate at which radiant energy impinges on an extended object, the appropriate units are again, W/m2, but in this context, the energy transfer is referred to as irradiance.
Note that “intensity,” depending on the context or the background of the person using the term, can have alternative meanings. In optics and particular laser physics, it generally refers to power density (ie, radiometric irradiance) and most commonly is expressed in watts per square centimeter (W/cm2).
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.