FIGURE 2.45 The narrow range of visual sensitivity. Vision is elicited by electromagnetic radiation. However, humans are blind to all radiation outside the narrow range of wavelengths from 400 to 700 nm.

in Figure 2.45, people are sensitive only to wavelengths between approximately 400 and 700 nm (1 nm = 10-9 m). Regardless of the intensity of illumination, they see no light below 400 nm or above 700 nm. Therefore, a set of photometric units has been created to supplement the radiometric units discussed earlier.194,196 The photometric units "correct" the radiometric units according to the human eye. Radiometric and photometric names and units of measurement are shown in Table 2.3. At peak sensitivity (555 nm), an irradiance of 1 W • m-2 corresponds to an illuminance of 683 lux (sometimes abbreviated as lx). At shorter and longer wavelengths, 1 W • m-2 corresponds to

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