distribution of a luminaire. Illuminance is the concentration of luminous flux incident on a surface (Figure 1.5). The unit of illuminance is lux (lx), and it indicates the number of lumens per square meter. Lumens per square foot are called footcandles (ftc). Whereas luminous flux relates to the total output of a lamp or lighting product, illuminance relates to the amount of light striking a surface or point. Illuminance depends on the luminous flux of the light sources and their distances from the illuminated surface.
Luminance is a measure used for self-luminous or reflective surfaces (Figure 1.6). It expresses the amount of light, weighted by the sensitivity of the visual system, per unit area of the surface that is travelling in a given direction and is expressed as candelas per square meter (cd/m2). When referring to illuminated surfaces, luminance is determined by the incident light (illuminance) and the reflectance characteristics of the surface. For instance, light- and dark-colored walls will have different luminance values when they have the same illuminance. Luminance is a metric used for internally illuminated variable-sized flat light sources forms, such as sheets or tapes, because the total luminous flux will depend on the surface area of the product.
FIGURE 1.5 Illuminance (lux). The amount of light striking a surface or point, measured in lux (lx).
FIGURE 1.6 Luminance of a luminaire.
The luminous efficacy of a lighting product is the ratio of the luminous flux to the total electrical power consumed and has units of lumens per watt (lm/W). A perfect light source— that is, one that converts all the electricity into visible light— would have an efficacy of 408 lm/W for an assumed color rendering index (CRI; a measure of color quality, discussed below) of 90 (Phillips et al., 2007).5 The luminous efficacy of a typical 60 W incandescent lamp (luminous flux of 850 lumens) is such that only 14.2 lumens are emitted per watt of power drawn by the light bulb. As efficacies increase, more of the power is used to generate visible light, and this leads to a more efficient product. High color quality LEDs currently are being manufactured with efficacies in the range of 60 to 188 lm/W. It should be borne in mind that efficacy is different from efficiency. The efficiency of a lighting system is the ratio between the obtained efficacy and the theoretical maximum efficacy of a light source (408 lm/W for a CRI of 90) and is always expressed as a percentage. Thus, it accounts for the ballast efficiency (if there is one), the light source efficacy, and the luminaire efficiency (see Figure 1.7) in one lumped parameter. Thus, incandescent lamps with system efficacies ranging from 4 to 18 lm/W (depending largely on the wattage of the bulb) will have system efficiencies of only about 0.2 to 2.6 percent. Efficiency does not, however, account for the perceived quality of the light. Using the theoretical maximum of 408 lm/W and the ranges of efficacies for different lighting technologies leads to the ranges of system efficiencies shown in Figures 1.7 and 1.8.
The human eye can generally detect light with wavelengths between 380 nm (corresponding to blue/violet light) and 750 nm (corresponding to red light). The spectral power distribution (SPD) determines several important properties of a light source. The SPD describes the relative amount of light per wavelength per unit time emitted by a light source and is often graphically represented, as shown in Figure 1.9. Figure 1.9 shows the SPDs of a halogen lamp, a red, green, blue (RGB) LED (which produces white light by combining red, green, and blue component LEDs), an OLED, and a combination of four colored lasers.
The color of emitted light as perceived by people, called chromaticity, is regulated by the spectral composition. The human visual system does not process light on a wavelength-by-wavelength basis. Instead, the brain receives signals from only three input channels, the different cone photopigments found in the eye. Because of this, countless different SPDs can produce light identical in chromaticity. To illustrate this,
5 A different choice of color rendering index = 80 would lead to a maximum efficacy of 423 lm/W, and so forth.