Glossary - R

radiance, L = *d*2φ/[dm (*d*A cos 8)]V =*d*I(*d*A cos 8) (in a direction, at a point on the surface of a source, of a receiver, or of any other real or virtual surface) the quotient of the radiant flux leaving, passing through, or arriving at an element of the surface surrounding the point, and propagated in directions defined by an elementary cone containing the given direction, by the product of the solid angle of the cone, and the area of the orthogonal projection of the element of the surface on a plane perpendicular to the given direction. Note In the defining equation, *8* is the angle between the normal to the element of the source and the given direction.

radiant energy, Q energy traveling in the form of electromagnetic waves. It is measured in units of energy such as joules or kilowatt hours. See *spectral radiant energy*.

radiant energy density, w = *d*Q/*d*V radiant energy per unit volume, for example, joules per cubic meter.

radiant exitance, M† the density of radiant flux leaving a surface. It is expressed in watts per unit area of the surface.

radiant flux (radiant power), φ = *d*Q/dt the time rate of flow of radiant energy. It is expressed preferably in watts. See spectral radiant flux.

radiant flux density at a surface† the quotient of radiant flux of an element of surface to the area of that element, in units such as W/m2. When referring to radiant flux emitted from a surface, this has been called radiant emittance (deprecated); the preferred term is *radiant exitance*: The radiant exitance per unit wavelength interval is called *spectral radiant exitance*. The radiant flux density incident on a surface is called *irradiance* (E).

radiant intensity, I = *d*φ/dm (in a given direction) the radiant flux proceeding from a source per unit solid angle in a given direction, for example, W/sr. See spectral radiant intensity. Note Mathematically, a solid angle must have a point at its apex; the definition of radiant intensity therefore applies strictly only to a point source. In practice, however, radiant energy emanating from a source whose dimensions are negligible in comparison with the distance from which it is observed can be considered as coming from a point. Specifically, this implies that with change of distance (1) the variation in solid angle subtended by the source at the receiving point approaches 1/distance2, and that (2) the average radiance of the projected source area as seen from the receiving point does not vary appreciably.

radiator an emitter of radiant energy.

radiometry the measurement of quantities associated with radiant energy and power.

range lights groups of color-coded boundary lights provided to indicate the direction and limits of a preferred landing path (normally) on an aerodrome without runways, but exceptionally on an aerodrome with runways.

rapid-start fluorescent lamp a fluorescent lamp designed for operation with a ballast that provides a low-voltage winding for preheating the electrodes and initiating the arc without a starting switch or the application of high voltage.

rated lamp life the life value assigned to a particular type lamp. This is commonly a statistically determined estimate of average or of median operational life. For certain lamp types other criteria than failure to light can be used; for example, the life can be based on the average time until the lamp type produces a given fraction of initial luminous flux.

reaction time the interval between the beginning of a stimulus and the beginning of the response of an observer.

recessed luminaire† See *flush-mounted or recessed luminaire*.

reciprocal color temperature color temperature (*T*c) expressed on a reciprocal scale (1/*T*c). An important use stems from the fact that a given small increment in reciprocal color temperature is approximately equally perceptible regardless of color temperature. Also, color temperature conversionfilters for sources approximating graybody sources change the reciprocal color temperature by nearly the same amount anywhere on the color temperature scale. Note The unit is the reciprocal megakelvin (MK−1). The reciprocal color temperature expressed in this unit has the numerical value of 106/*T*c when *T*c is expressed in kelvins. The acronym mirek (for micro- reciprocal-kelvin) occasionally has been used in the literature. The acronym *mired* (for micro-reciprocal- degree) is now considered obsolete as the name for this unit.

recoverable light loss factors factors that give the fractional light loss that can be recovered by cleaning or lamp replacement. Comprised of those components that account for depreciation of luminous quantities in a lighting system. This factor is a function of lighting system age and maintenance processes and is applied to lighting calculations for systems after time zero.

redirecting surfaces and media those that change the direction of the flux without scattering the redirected flux.

reference ballast a ballast that is specially constructed, having certain prescribed characteristics and that is used for testing electric-discharge lamps and other ballasts.

reference line (roadway lighting) either of two radial lines where the surface of the cone of maximum intensity is intersected by a vertical plane parallel to the curb line and passing through the light center of the luminaire.

reference standard† an alternative term for secondary standard.

reflectance of a surface or medium p = φr/φi the ratio of the reflected flux to the incident flux. Reflectance is a function of:

1. Geometry

	1. of the incident flux
	2. of collection for the reflected flux
2. Spectral distribution

	1. characteristic of the incident flux
	2. weighting function for the collected flux
3. Polarization

	1. of the incident flux
	2. component defined for the collected flux.

Note Unless the state of polarization for the incident flux and the polarized component of the reflected flux are stated, it should be considered that the incident flux is unpolarized and that the total reflected flux (including all polarization) is evaluated. Spectral reflectance depends on only the beam geometry and the character of the reflecting surface (and on polarization). Luminous reflectance also is a function of the spectral distribution of the incident flux. If no qualifying geometric adjective is used, the reflectance for hemispherical collection is meant. Certain of the reflectance terms are theoretically imperfect and are recognized only as practical concepts to be used when applicable. Physical measurements of the incident and reflected flux are always biconical in nature. Directional reflectances cannot exist, since one component would be finite while the other was infinitesimal; here the reflectance distribution function is required. However, the concepts of directional and hemispherical reflectance have practical application in instrumentation, measurements, and calculations when including the effect of the nearly zero or nearly 2*π* conical angle would increase complexity without appreciably affecting the immediate results. In each case of conical incidence or collection, the solid angle need not be a right cone but can be of any cross section,including a rectangle, a ring, or a combination of two or more solid angles. For many geometrically specified reflectance properties it is assumed that the radiance (luminance) is isotropic over the specified solid angle of incidence. Otherwise, the property is a function of the directional distribution of the radiance (luminance) as well as the beam geometry and the character of the reflecting surface.

reflectance factor, R the ratio of the flux actually reflected by a sample surface to that which would be reflected into the same reflected-beam geometry by an ideal (glossless), perfectly diffuse (lambertian), completely reflecting standard surface irradiated in exactly the same way as the sample. Note the analogies to reflectance in the fact that nine canonical forms are possible that "spectral" can be applied as a modifier, that it can be luminous or radiant reflectance factor, and so on. Note that reflectance cannot exceed unity, but reflectance factor can have any value from zero to values approaching infinity.

reflected glare glare resulting from reflections of high luminances in polished or glossy surfaces in the field of view. It usually is associated with reflections from within a visual task or areas in close proximity to the region being viewed. See *veiling reflection*.

reflection a general term for the process by which the incident flux leaves a (stationary) surface or medium from the incident side without change in frequency. Note Reflection is usually a combination of regular and diffuse reflection. See *regular (specular) reflection, diffuse reflection,* and *veiling reflection.*

reflectivity† reflectance of a layer of a material of such a thickness that there is no change of reflectance with increase in thickness.

reflectometer a photometer for measuring reflectance. Note Reflectometers can be visual or physical instruments.

reflector a device used to redirect the flux from a source by the process of reflection. See *retro-reflector*.

reflector lamp an incandescent filament or electric-discharge lamp in which the outer blown glass bulb is coated with a reflecting material so as to direct the light (such as R- or ER-type lamps). The light- transmitting region can be clear, frosted, patterned, or phosphor coated.

reflex reflector† See *retro-reflector*.

refraction† the process by which the direction of a ray of light changes as it passes obliquely from one medium to another in which its speed is different.

refractor a device used to redirect the flux from a source, primarily by the process of refraction.

regions of the electromagnetic spectrum for convenience of reference, the electromagnetic spectrum is arbitrarily divided as follows: Note The spectral limits indicated above have been chosen as a matter of practical convenience. There is a gradual transition from region to region without sharp delineation. Also, the division of the spectrum is not unique. In various fields of science the classifications can differ due to the phenomena of interest.

Another division of the UV spectrum often used by photobiologists is given by the Commission Internationale de l'Eclairage (CIE):

regressed luminaire a luminaire mounted above the ceiling with its opening above the ceiling line. See flush-mounted, surface-mounted, suspended, and troffer luminaires.

regular (specular) reflectance the ratio of the flux leaving a surface or medium by regular (specular) reflection to the incident flux. See regular (specular) reflection.

regular (specular) reflection that process by which incident flux is redirected at the specular angle. See bihemispherical reflectance and specular angle.

regular transmission that process by which incident flux passes through a surface or medium without scattering. See *regular transmittance*.

regular transmittance the ratio of the regularly transmitted (straight through) flux leaving a surface or medium to the incident flux.

relative contrast sensitivity (RCS) the relation between the reciprocal of the luminous contrast of a task at visibility threshold and the background luminance expressed as a percentage of the value obtained under a very high level of diffuse task illumination.

relative erythemal factor† See erythemal efficiency of radiant flux.

relative luminosity† an obsolete term for the spectral luminous efficiency of radiant flux.

relative luminosity factor† an obsolete term for the spectral luminous efficiency of radiant flux.

resolving power the ability of the eye to perceive the individual elements of a grating or any other periodic pattern with parallel elements measured by the number of cycles per degree that can be resolved. The resolution threshold is the period of the pattern that can be just resolved. The visual acuity, in such a case, is the reciprocal of one-half the period expressed in minutes. The resolution threshold for a pair of points or lines is the distance between their centers when they can be distinguished as two, not one, expressed in minutes of arc.

resultant color shift the difference between the perceived color of an object illuminated by a test source and of the same object illuminated by the reference source, taking account of the state of chromatic adaptation (see *state of chromatic adaptation*) in each case; that is, the resultant of colorimetric shift andadaptive color shift.

retina a membrane lining the posterior part of the inside of the eye. It comprises photoreceptors (cones and rods) that are sensitive to light and nerve cells that transmit to the optic nerve the responses of the receptor elements.

retro-reflector (reflex reflector) a device designed to reflect light in a direction close to that at which it is incident, whatever the angle of incidence.

rhythmic light a light that when observed from a fixed point has a luminous intensity that changes periodically. See *equal interval (isophase) light, flashing light, group flashing light, interrupted quick- flashing light,* and *occulting light*.

ribbon filament lamp an incandescent lamp in which the luminous element is a tungsten ribbon. Note This type of lamp is often used as a standard in pyrometry and radiometry.

rods retinal receptors that respond at low levels of luminance even below the threshold for cones. At these levels there is no basis for perceiving differences in hue and saturation. No rods are found near the center of the fovea.

room cavity the cavity formed by the plane of the luminaires, the work plane, and the wall surfaces between these two planes.

room cavity ratio (RCR) a number indicating room cavity proportions, calculated from the length, width, and height. See zonal-cavity interreflectance method.

room utilization factor (utilance) the ratio of the luminous flux (lumens) received on the workplane to that emitted by the luminaire. Note This ratio sometimes is called *interflectance*. Room utilization factor is based on the flux emitted by a complete luminaire, whereas *coefficient of utilization* is based on the total flux generated by the lamps in a luminaire.

room surface dirt depreciation (RSDD) the fractional loss of task illuminance due to dirt on the room surfaces.

runway alignment indicator a group of aeronautical ground lights arranged and located to provide early direction and roll guidance on the approach to a runway.

runway centerline lights runway lights installed in the surface of the runway along the centerline indicating the location and direction of the runway centerline; of particular value in conditions of very poor visibility.

runway edge lights lights installed along the edges of a runway marking its lateral limits and indicating its direction.

runway end identification lights a pair of flashing aeronautical ground lights symmetrically disposed on each side of the runway at the threshold to provide additional threshold conspicuity.

runway exit lights lights placed on the surface of a runway to indicate a path of the taxiway centerline.

runway lights aeronautical ground lights arranged along or on a runway. See *runway centerline lights, runway edge lights, runway end identification light,* and *runway exit lights.*

runway threshold† the beginning of the part of the runway usable for landing.

runway visibility the meteorological visibility along an identified runway. Where a transmissometer is used for measurement, the instrument is calibrated in terms of a human observer; for example, the sighting of dark objects against the horizon sky during daylight and the sighting of moderately intense unfocused lights of the order of 25 candelas at night. See *visibility (meteorological).*

runway visual range (RVR) in the United States, an instrumentally derived value based on standard calibrations that represents the horizontal distance a pilot sees down the runway from the approach end; it is based either on the sighting of high-intensity runway lights or on the visual contrast of other targets, whichever yields the greater visual range.