Caracteristicas De Los Led's
Páginas: 129 (32029 palabras)
Publicado: 1 de octubre de 2012
16
Human eye sensitivity and photometric quantities
The recipient of the light emitted by most visible-spectrum LEDs is the human eye. In this chapter, the characteristics of human vision and of the human eye and are summarized, in particular as these characteristics relate to human eye sensitivity and photometric quantities.
16.1 Light receptors of the human eye Figure 16.1 (a) shows aschematic illustration of the human eye (Encyclopedia Britannica, 1994). The inside of the eyeball is clad by the retina, which is the light-sensitive part of the eye. The illustration also shows the fovea, a cone-rich central region of the retina which affords the high acuteness of central vision. Figure 16.1 (b) shows the cell structure of the retina including the light-sensitive rod cells and conecells. Also shown are the ganglion cells and nerve fibers that transmit the visual information to the brain. Rod cells are more abundant and more light sensitive than cone cells. Rods are sensitive over the entire visible spectrum. There are three types of cone 275
16 Human eye sensitivity and photometric quantities
cells, namely cone cells sensitive in the red, green, and blue spectralrange. The cone cells are therefore denoted as the red-sensitive, green-sensitive, and blue-sensitive cones, or simply as the red, green, and blue cones.
Three different vision regimes are shown in Fig. 16.2 along with the receptors relevant to each of the regimes (Osram Sylvania, 2000). Photopic vision relates to human vision at high ambient light levels (e.g. during daylight conditions) whenvision is mediated by the cones. The photopic vision regime applies to luminance levels > 3 cd/m2. Scotopic vision relates to human vision at low ambient light levels (e.g. at night) when vision is mediated by rods. Rods have a much higher sensitivity than the cones. However, the sense of color is essentially lost in the scotopic vision regime. At low light levels such as in a moonless night, objectslose their colors and only appear to have different gray levels. The scotopic vision regime applies to luminance levels < 0.003 cd/m2. Mesopic vision relates to light levels between the photopic and scotopic vision regime (0.003 cd/m2 < mesopic luminance < 3 cd/m2).
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16.2 Basic radiometric and photometric units
The approximate spectral sensitivity functions of the rods and threetypes or cones are shown in Fig. 16.3 (Dowling, 1987). Inspection of the figure reveals that night-time vision (scotopic vision) is weaker in the red spectral range and thus stronger in the blue spectral range as compared to day-time vision (photopic vision). The following discussion mostly relates to the photopic vision regime.
16.2 Basic radiometric and photometric units The physical propertiesof electromagnetic radiation are characterized by radiometric units. Using radiometric units, we can characterize light in terms of physical quantities; for example, the number of photons, photon energy, and optical power (in the lighting community frequently called the radiant flux). However, the radiometric units are irrelevant when it comes to light perception by a human being. For example,infrared radiation causes no luminous sensation in the eye. To characterize the light and color sensation by the human eye, different types of units are needed. These units are called photometric units. The luminous intensity, which is a photometric quantity, represents the light intensity of an optical source, as perceived by the human eye. The luminous intensity is measured in units of candela (cd),which is a base unit of the International System of Units (SI unit). The present definition of luminous intensity is as follows: a monochromatic light source emitting an optical power of (1/683) watt at 555 nm into the solid angle of 1 steradian (sr) has a luminous intensity of 1 candela (cd). The unit candela has great historical significance. All light intensity measurements can be traced...
Human eye sensitivity and photometric quantities
The recipient of the light emitted by most visible-spectrum LEDs is the human eye. In this chapter, the characteristics of human vision and of the human eye and are summarized, in particular as these characteristics relate to human eye sensitivity and photometric quantities.
16.1 Light receptors of the human eye Figure 16.1 (a) shows aschematic illustration of the human eye (Encyclopedia Britannica, 1994). The inside of the eyeball is clad by the retina, which is the light-sensitive part of the eye. The illustration also shows the fovea, a cone-rich central region of the retina which affords the high acuteness of central vision. Figure 16.1 (b) shows the cell structure of the retina including the light-sensitive rod cells and conecells. Also shown are the ganglion cells and nerve fibers that transmit the visual information to the brain. Rod cells are more abundant and more light sensitive than cone cells. Rods are sensitive over the entire visible spectrum. There are three types of cone 275
16 Human eye sensitivity and photometric quantities
cells, namely cone cells sensitive in the red, green, and blue spectralrange. The cone cells are therefore denoted as the red-sensitive, green-sensitive, and blue-sensitive cones, or simply as the red, green, and blue cones.
Three different vision regimes are shown in Fig. 16.2 along with the receptors relevant to each of the regimes (Osram Sylvania, 2000). Photopic vision relates to human vision at high ambient light levels (e.g. during daylight conditions) whenvision is mediated by the cones. The photopic vision regime applies to luminance levels > 3 cd/m2. Scotopic vision relates to human vision at low ambient light levels (e.g. at night) when vision is mediated by rods. Rods have a much higher sensitivity than the cones. However, the sense of color is essentially lost in the scotopic vision regime. At low light levels such as in a moonless night, objectslose their colors and only appear to have different gray levels. The scotopic vision regime applies to luminance levels < 0.003 cd/m2. Mesopic vision relates to light levels between the photopic and scotopic vision regime (0.003 cd/m2 < mesopic luminance < 3 cd/m2).
276
16.2 Basic radiometric and photometric units
The approximate spectral sensitivity functions of the rods and threetypes or cones are shown in Fig. 16.3 (Dowling, 1987). Inspection of the figure reveals that night-time vision (scotopic vision) is weaker in the red spectral range and thus stronger in the blue spectral range as compared to day-time vision (photopic vision). The following discussion mostly relates to the photopic vision regime.
16.2 Basic radiometric and photometric units The physical propertiesof electromagnetic radiation are characterized by radiometric units. Using radiometric units, we can characterize light in terms of physical quantities; for example, the number of photons, photon energy, and optical power (in the lighting community frequently called the radiant flux). However, the radiometric units are irrelevant when it comes to light perception by a human being. For example,infrared radiation causes no luminous sensation in the eye. To characterize the light and color sensation by the human eye, different types of units are needed. These units are called photometric units. The luminous intensity, which is a photometric quantity, represents the light intensity of an optical source, as perceived by the human eye. The luminous intensity is measured in units of candela (cd),which is a base unit of the International System of Units (SI unit). The present definition of luminous intensity is as follows: a monochromatic light source emitting an optical power of (1/683) watt at 555 nm into the solid angle of 1 steradian (sr) has a luminous intensity of 1 candela (cd). The unit candela has great historical significance. All light intensity measurements can be traced...
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