Glossary - R

R 

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. 

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

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. 

reflectance of a surface or medium, ρ = Φr/Φi the ratio of the reflected flux to the incident flux. Reflectance is a function of: 

1. Geometry 

a. of the incident flux  b. of collection for the reflected flux 

2. Spectral distribution

a. characteristic of the incident flux  b. weighting function for the collected flux 

3. Polarization 

a. of the incident flux  b. component defined for the collected flux. 

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. 

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. 

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. 

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 quickflashing light, and occulting light. 

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.