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507 555 1800 1800 1600 ☼ 1400 scotopic Luminous Efficacy (lm/W) 1200 1000 800 600 400 photopic 683 200 0 400 500 600 700 Figure 1: The response of two types of vision can be described by photopic (red) and scotopic (blue) Luminous Efficacy curves. Wavelength (nm) Photometry is distinct from radiometry, which is the science of measurement of radiant energy (including light) in terms of absolute power. There are two parallel systems of measurement known as photometric and radiometric quantities. Every quantity in one system has an analogous quantity in the other system. In photometric quantities every wavelength is weighted according to how sensitive the human eye is to it, while radiometric quantities use unweighted absolute power. Photometric and Radiometric Units Radiometric Term Unit Photometric Term SI Unit English Unit Radiant flux Watt Luminous flux Lumen Lumen Radiant intensity Watt/steradian Luminous intensity Candela Candela Radiance Watt/steradian/m 2 Lumiance cd/m 2 Footlambert Irradiance Watt/m 2 Illuminance Lux Footcandle TM www.edwardssignaling.com 21
TM Science of Light Luminous flux Typically, manufacturers measure the total lumen output by placing a photometer at the exit port of an integrating sphere. The light source is placed inside the integrating sphere – a highly reflective sphere with a white, spectrally nonselective coating. These devices also are used to measure directional sources such as laser beams and light-emitting diodes (LEDs). Figure 2. The steradian is the solid angle at the center of a sphere that subtends a surface area of r 2 . The fundamental quantity used in photometry is luminous flux, which is the visible light energy per second radiated by a source or amount of photometric energy traveling through space in a given time interval. The lumen is the unit of luminous flux, and 1 W of radiant flux at the peak photopic wavelength of 555 nm is equivalent to a luminous flux of 683 lumens. If the radiant flux (power) of a polychromatic light source at every wavelength of the visible spectrum from the blue to the red end is known, the luminous flux can be calculated by mathematically integrating the power values with the CIE-defined photopic value for each wavelength. For historical reasons, the candela – the unit of luminous intensity – takes precedence over the lumen. The candela originally was derived from the light emitted by a candle; its current definition is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10 12 Hz and that has a radiant intensity in that direction of 1/683 W per steradian (540 × 10 12 Hz is equivalent to 555 nm). The lumen is the luminous flux, emitted per unit solid angle from a point source whose luminous intensity is one candela. The unit of solid angle is the steradian (Figure 2), and because the surface area subtended by a steradian is r 2 , a sphere with a surface area of 4πr must have 4π steradians. Therefore, the total luminous flux emitted from a one-candela point source is 4π lumens. Luminous intensity Luminous intensity, or candlepower, is the luminous flux per unit solid angle emitted by a uniform point source of light. This is the quantity used to measure the output of point (small) sources such as an LED. The unit of luminous intensity is the candela, and it is given in lumens per steradian. It is important to note that in many practical applications, point sources are directional and do not radiate uniformly. Therefore, candlepower is measured in a specific direction over a small collection angle along the axis of radiation. LED receptors, which use the technique to measure LEDs in millicandelas, consist of tubes with baffles that have a specific collections geometry (2° to 15° are common). The CIE has recommended in CIE 127 that LEDs be measured using 2° and 6.5° geometry, and that these two readings are averaged to yield "Averaged LED Intensity." A typical LED receptor is a tube in which one end is attached to the light measuring device and the other end receives the LED. The collection geometry is defined by a field stop in the tube. The receptor is then calibrated using a source of known luminous intensity. Flashing lights are also commonly used in many signaling applications. The visibility or conspicuity of flashing lights varies depending on the duration and waveform of flashes for the same physical energy and spectrum of the flashes. To take into account such visual effects, the term, effective intensity, is used to specify the intensity of flashing lights for signaling applications. Effective intensity is defined as the luminous intensity (cd) of a steady light source that would have the same luminous range (or visual range) as the flashing lights in question. An additional source of potential confusion regarding intensity measurements involves the difference between Mean Spherical Candela and Beam Candela, both of which use the candela unit (lumens per steradian). Mean spherical measurements are made in an integrating sphere, and represent the total output in lumens divided by 4πsr in a sphere. Thus, a one candela isotropic lamp produces one lumen per steradian. Beam candela, on the other hand, samples a very narrow angle and is only representative of the lumens per steradian at 22 www.edwardssignaling.com
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