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8.6.Corneal haze secondary to corneal edema is primarily caused by

a.reflection

b.light scattering

c.refraction

d.diffraction

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Appendix 8.1

Radiometric and Photometric Units

Understanding radiometry is a matter of not simply knowing the definitions of various units of measurement but also knowing how to use them appropriately. Every radiometric (or photometric) unit of measurement has a specific purpose.

In studying radiometry, it is helpful to think of light as a collection of photons. Broadband light consists of photons at many frequencies, whereas narrowband light consists of photons at just a few frequencies. For the purposes of this discussion, imagine it is possible to count every photon and identify its frequency.

Radiometry counts every photon—not just visible ones but also those at infrared and UV wavelengths, and all photons at wavelengths between 10 nm and 1 mm. Radiometric units are identified by the subscript (e). Spectral radiometry counts photons within a more limited range— whatever frequencies are of interest in a specific situation. Spectral radiometric units are identified by the subscript (λ).

Radiant energy (Qe)

Radiant energy (Qe) is the total energy produced, typically expressed in joules. Every photon produced by a source is counted, and its energy is calculated using E = hν. For example, an incandescent lightbulb produces not only visible light but also infrared and even some UV light, and photons from each of these spectra need to be included to determine the value for Qe. Also, Qe is a cumulative measurement; that is, it is taken over time. The Qe value for an incandescent bulb operating for 2 hours is twice that for the same bulb operating for 1 hour.

Qe (or Qλ) is the appropriate quantity when cumulative exposure is important. For instance, there is some evidence that cases of post–cataract-extraction cystoid macular edema are related to microscope illumination. Investigators should consider Qe (as well as other parameters) in evaluating light hazards.

Fluence (He)

Fluence (He) is a measure of energy density and is defined as radiant energy (Qe) divided by area.

For example, 6 J of radiant energy applied to an area of 2 cm2 equals a fluence of 3 J/cm2. Fluence measurements are often used clinically in association with laser ablation and photodynamic therapy (PDT).

Photoablation breaks chemical bonds, and in laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK), the number of chemical bonds to be broken by a single laser pulse is proportional to the area of the laser beam. If the pulse energy is low in comparison with the beam area, too few bonds will be broken and the resulting ablation will be uneven. Similarly, in PDT the number of verteporfin molecules to be activated depends on the area of the lesion. Fluence is the appropriate radiometric unit to employ for calibrating lasers used for these procedures.

Radiant flux (Φe)

The word flux means flow, and radiant flux (Φe) is the amount of radiant energy emitted by a source or striking a surface per unit of time. It is a measure of power, typically expressed in joules per second, or watts. A source radiating 60 J for 1 minute produces a radiant flux of 1 W (60 J/60 s).

Irradiance (Ee) or exitance (Me)

Irradiance (Ee) is radiant energy divided by area and time, and it is typically expressed in watts per square meter or watts per square centimeter. For example, a 0.25 cm2 surface illuminated by 2 W has an irradiance of 2 W/0.25 cm2 = 8 W/cm2. Irradiance is used for surfaces illuminated by an external source, but the term radiant exitance (Me) may be used instead when the surface itself is the light source (eg, a light box used to view x-rays). However, irradiance and radiant exitance both have the same units (W/cm2), so some authorities use irradiance in both cases. Clinically, irradiance is the appropriate unit of measurement for photocoagulation equipment.

Radiant intensity (Ie)

Measurements of radiant intensity account for the directional properties of a light source. For example, a flashlight beam diverges somewhat but propagates in the same direction. Some flashlight beams are more narrowly divergent than others, whereas laser beams are extremely directional. It is therefore necessary to incorporate an angular measurement in the description of such light sources, a function served by radiant intensity (Ie), expressed as the radiant flux per unit solid angle or watts per steradian (W/sr).

Radiance (Le)

The measurement of radiance (Le) takes into account irradiance and radiant intensity, and it is the radiometric unit most closely related to brightness. Radiance is radiant flux per unit area per steradian (sr), typically expressed as W/m2 sr.

Photometry

Photometry measures the human visual system’s psychophysical response to light; it is essentially radiometry that takes into account the varying response of the eye to light of different wavelengths.

Based on data from measurements of people, weighting factors (presented as luminous efficacy curves) have been developed for each wavelength of the visible spectrum. There are luminous efficacy curves for photopic and scotopic conditions.

Photometry is a straightforward extension of radiometry—each photon and its frequency are counted. Each photon’s energy is calculated and multiplied by the luminous efficacy for its frequency. (To cite a trivial example of the difference between radiometry and photometry, a source that emits only UV light has radiometric intensity but no luminous intensity.) Photometric units are analogous to radiometric units and are indicated by the same symbols but subscripted with (v) (for visual) instead of (e). Table 8-1 compares radiometric units with analogous photometric units.

Table 8-1

There are many ways to measure photometric quantities. For clinical use, SI units are often not the most convenient. Table 8-2 lists alternative units commonly used to measure illuminance and luminance and their conversion factors. To develop a “feel” for photometric units, Table 8-3 lists photometric values associated with visual functions.

Table 8-2

Table 8-3

The troland (Td) is a unit that estimates retinal illuminance based on source luminance. A source with a luminance of 1 nit produces a retinal illuminance of 1 Td when viewed through a 1 mm2 pupil. If the pupil area is 10 mm2, then the retinal illuminance is 10 Td.