Photometric .docx

Photometric Quantities A description of human visual response is exceptionally complex and involves psychology, physiology, and physics, among other disciplines. The first attempt to quantify human vision was made in 1924 by the newly formed Commission Internationale de l'Éclairage (CIE) and includ...

Photometric Quantities A description of human visual response is exceptionally complex and involves psychology, physiology, and physics, among other disciplines. The first attempt to quantify human vision was made in 1924 by the newly formed Commission Internationale de l'Éclairage (CIE) and included a definition of light intensity, the candle, the footcandle (fc), and candle power. Response of the Eye The CIE recognized the difference between photopic, bright-light vision with cones, and scotopic, dim-light vision with rods. This resulted in the standard CIE photopic and scotopic response curves shown in Fig. 17.1. Bright vision is best at 555 nm, and dim vision is best at 505 nm. Photometric Units Radiologic technologists must have some familiarity with the units used to express photometric quantities. The basic unit of photometry is the lumen. It is scaled to the maximum photopic eye response at 555 nm. he basic photometric unit is the lumen. Luminous flux, the fundamental quantity of photometry, is expressed in lumens (lm). Luminous flux is the total intensity of light from a source. Household lamps are rated by the power they consume in watts. An equally important value found on each lamp package is its luminous flux in lumens. Check it out the next time you go shopping. Illuminance describes the intensity of light incident on a surface. One lumen of luminous flux incident on a single square foot is an fc. This English unit, the fc, is still in wide use. The International System of Units (SI) equivalent is 1 lumen per square meter, which is a lux (lx) (1 fc = 10.8 lx). Luminance intensity is a property of the source of light, such as a digital display device. Luminance intensity is the luminous flux that is emitted into the entire viewing area; it is measured in lumens per steradian or candela. The steradian is the SI solid angle. A solid angle is a three-dimensional version of an angle as shown in Fig. 17.2. Luminance is a quantity that is similar to luminance intensity. Luminance is another measure of the brightness of a source, such as a digital display device expressed in units of candela per square meter or nit, the SI unit. Table 17.1 summarizes these photometric quantities and their associated units. Table 17.2 shows the range of illuminance for several familiar situations. Most indoor work and play areas are illuminated to 100 to 200 fc. Cosine Law Two fundamental laws are associated with photometry. Luminous intensity decreases in proportion to the inverse square of the distance from the source. This is the famous inverse square law (see Chapter 4). The cosine law is important for describing the luminous intensity of a digital display device. When a monitor is viewed straight on, the luminous intensity is maximum. When a monitor is viewed from an angle, the contrast and the luminous intensity, as seen in Fig. 17.3, are reduced. The best viewing of a digital display device is straight on. This reduced projected surface area follows a mathematical function called a cosine. Luminous intensity falls off rapidly as one views a digital display device at larger angles from perpendicular.