nt into photometric units both the spectral response of the eye and the spectral output of the light source must be taken into account. The conversion is then achieved by multiplying the energy radiated at each wavelength by the relative lumen/watt factor at that wave- Visible length and summing the results. Note that in the ultraviolet and infrared portions of the optical spectrum although one may have high output in terms of watts the photometric value in lumens is zero Wavelength (nm) due to lack of eye response in those ranges. However, it should be said that many observers can see the 900nm radiation from a GaAs laser or the 1.06um radiation from a Nd: YAG laser since in this instance the intensity can be sufficiently high to elicit a visual response. Viewing of these sources in practice is not to be recom- mended for safety reasons and the moderately high energy densities Figure 7.1 The visible spectrum at the eye which are involved. 7/4 Optics and vision Radiance Power/solid angle/projected area arriving at a surface Radiance 507 nm 555 nm Intensity/projected area leaving a surface Scotopic vision (for low light levels) Photopic vision (for normal Radiant intensity Power/solid light levels) angle Surface acting as a secondary light source Irradiance Power/area incident on a surface Point source of radiant energy Radiant power Radiant energy/time Wavelength, a (nm) (flux) Radiant exitance Power/area Figure 7.2 The photopic and scotopic eye responses leaving a surface Figure 7.3 A visualisation of radiometric terms (from Zaha, 1972) Units of measurement 5. Radiance, Lc. The radiant intensity per unit area, leaving, pas- sing through, or arriving at a surface in a given direction. The There are many possible measurements for characterising the output surface area is the projected area as seen from the specified of a light source. The principles employed in defining radiometric direction. Expressed in W cm -2 and photopic measurement terms are very similar. The terms em- ployed have the adjective radiant for a