Glossary - P

P 

panel (open) face exit sign† a transilluminated sign where both the exit legend and background are translucent. 

parking lamp a lighting device placed on a vehicle to indicate its presence when parked. 

partial diffusion† See incomplete diffusion. 

partly cloudy sky a sky that has 30 to 70% cloud cover. 

passing beams† See lower (passing) beams. 

pendant luminaire† See suspended (pendant) luminaire. 

perceived light source color the color perceived to belong to a light source. 

perceived object color† the color perceived to belong to an object resulting from characteristics of the object, of the incident light, and of the surround, the viewing direction, and observer adaptation. See object color. 

percent flicker a relative measure of the cyclic variation in output of a light source (percent modulation). It is given by the expression where A is the maximum and B is the minimum output during a single cycle. See Chapter 6, Light Sources. 

perfect diffusion that in which flux is uniformly scattered in accord with Lambert's cosine law. 

peripheral vision the seeing of objects displaced from the primary line of sight and outside the central visual field. 

peripheral visual field that portion of the visual field that falls outside the region corresponding to the foveal portion of the retina. 

phosphor mercury lamp† see mercury-fluorescent lamp. 

phosphorescence the emission of light as the result of the absorption of radiation, and continuing for a noticeable length of time after excitation (longer than approximately 10−8 s). 

phot, ph a unit of illuminance equal to one lumen per square centimeter. The use of this unit is deprecated. 

photobiology a branch of biology that deals with the effects of optical radiation on living systems. 

photometer an instrument for measuring photometric quantities such as luminance, luminous intensity, luminous flux, or illuminance. See densitomer, goniophotometer, illuminance (lux or footcandle) meter, integrating photometer, reflectometer, spectrophotometer, and transmissometer. 

photometry the measurement of quantities associated with light.  Note Photometry can be either visual, in which the eye is used to make a comparison, or physical, in which measurements are made by means of physical receptors. 

phototherapy the treatment of disease involving the use of optical radiation. 

physical photometer an instrument containing a physical receptor and associated filters that is calibrated so as to read photometric quantities directly. See visual photometer. 

pilot house control a mechanical means for controlling the elevation and train of a searchlight from a position on the other side of the bulkhead or deck on which it is mounted. 

point-by-point method† a method of lighting calculation, now called the point method. 

point method a lighting design procedure for predetermining the illuminance at various locations in

point source a source of radiation whose dimensions are sufficiently small, compared with the distance between the source and the irradiated surface, that these dimensions can be neglected in calculations and measurements. 

point vision the mode of vision of a small source of light such that the sensation is determined by its intensity rather than by its size. Point vision occurs with sources so small that their form or shape is not perceived and that they appear as points of light; this generally means less than 1-minute angular subtense.

pole (roadway lighting) a standard support generally used where overhead lighting distribution circuits are employed. 

portable lighting lighting involving equipment designed for manual portability. 

portable luminaire a lighting unit that is not permanently fixed in place. See table lamp and floor lamp. 

preheat (switch start) fluorescent lamp a fluorescent lamp designed for operation in a circuit requiring a manual or automatic starting switch to preheat the electrodes in order to start the arc. 

projector a lighting unit that, by means of mirrors and lenses, concentrates the light to a limited solid angle so as to obtain a high value of luminous intensity. See floodlight, searchlight, and signaling light.