inversely to the fourth power of the wavelength; thus, blue light (at 400 nm) is scattered about 9.4 times as much as red light (at 700 nm). The effect of Rayleigh scattering and the amount of melanin in the iris stroma determine iris color.
Mie Scattering
Mie scattering is caused by particles in a medium that are one or more wavelengths larger than the incident light. The effect varies with wavelength in a complex way, but the variation with wavelength is not very strong. In Mie scattering, forward scattering tends to be stronger than back scattering. Mie scattering accounts for the white color of the sclera and of clouds.
The Tyndall Effect
Light scattering from the Tyndall effect is due to light reflected by fairly large particles in a suspension. Anterior segment cells are visible during slit-lamp examination because of Tyndall-effect light scattering by cells (or proteins) in the aqueous humor.
Radiometry and Photometry
It is necessary at times to measure the amount of light produced by a source or illuminating a surface. For example, the bowl of a perimeter must be accurately illuminated in order for any comparisons of visual field measurements performed on different instruments or on the same instrument at different times to be meaningful.
Radiometry is the measurement of EM radiation occurring between 3 × 1011 Hz and 3 × 1016 Hz, a range corresponding to wavelengths between 10 nm and 1 mm. Therefore, radiometry spans the microwave, infrared, visible, ultraviolet, and soft x-ray regions of the EM spectrum. Often, such broad measurements are not necessary, and spectral radiometry is used instead to measure a narrower portion of the EM spectrum.
Photometry measures the human visual system’s psychophysical response to light. Basically, photometry is a type of radiometry that takes into account the varying sensitivity of the eye to different wavelengths. Based on data from measurements of numerous people, weighting factors (presented graphically as luminous efficacy curves) have been developed for each wavelength across the visible spectrum. Different luminous efficacy curves are used to assess vision under photopic and scotopic lighting conditions.
In photometry, the spectral characteristics of a light source are determined by spectral radiometry. Then, each wavelength measured is multiplied by the appropriate luminous efficacy factor, and the responses are summed across the visible spectrum to estimate the response of the visual system. To give a simple example, a light source that produces only UV light has radiometric intensity but no luminous intensity.
To many clinicians, radiometric and photometric units of measurement are unfamiliar and confusing. For additional detail, a discussion of such units is provided in Appendix 8.1, at the end of this chapter.