A
absorption coefficient
The fraction of light absorbed per unit distance in a participating medium. In SI units, this is specified as a fraction per metre. In Radiance the absorption is given separately for red, green and blue.
adaptation
The process which takes place as the visual system adjusts itself to the brightness of the visual field.
adaptive sampling
The automatic sending of additional samples in cases where the default number of samples is deemed inadequate to achieve the desired accuracy.
albedo
The unitless ratio between the scattering coefficient and the extinction coefficient for a participating medium.
alias
In a Radiance scene description, an alias associates a new identifier with a previously defined primitive.
altitude
The angle between a vector and the surface horizon.
ambient
An ambient value approximates the global average of radiance over all directions and all points of interest in a scene.
ambient file
A binary file in which indirect irradiance values and their gradients are stored for reuse in other calculations.
angular fish-eye perspective
A perspective view projection in which the angle from the central view direction is proportional to the distance from the centre of the image.
animation
A sequence of images rendered from the same scene description and lighting but from a changing view point and/or view direction. Some animations may fix the viewing parameters and change the lighting conditions, e.g. change the time of day.
anisotropic
A reflection or transmission distribution function that varies with rotation about the surface normal.
aspect ratio
The ratio of the width of an image to its height.
azimuth
The angle about the surface normal relative to some standard direction, such as south.
B
BRDF
Bi-directional reflectance distribution function is a mathematical function that describes the way light is reflected from a point on a locally planar surface.
BRTDF
Bi-directional reflectance-transmittance distribution function. (see BRDF)
BSDF
Bi-directional scattering distribution function. (see BRDF)
C
CAD
Notation used for Computer Aided Design or Computer Aided Drafting.
candela
The SI unit for the luminous intensity (total visible light leaving a light source in a certain direction), one candela equals one lumen per steradian.
candlepower distribution
The light output distribution for a source, in candelas (in SI units), given as a function of direction (two angles in general).
Cartesian co-ordinate system
A system for locating points based on an origin and two or more perpendicular axes passing through it. In Radiance, 3D co-ordinates are specified via a right –hand Cartesian co-ordinate system, where the z-axis is the direction of the thumb of the right hand.
chromaticity
The exact colour (but not the luminance) of a light source or a reflecting surface.
CIE
The Commission International de l'Eclairage, which sets international standards for lighting, daylighting, colour measurement and estimation.
CIE colour
A system for defining colour as proposed by the CIE.
CIE standard sky distribution
Sky light is a continually varying and unpredictable quantity, so for analysis and comparison purposes, the CIE recommends a specific set of standard sky distributions that approximate average skies. The most commonly applied are the CIE overcast sky and the CIE clear sky.
colour rendering
A general expression for the appearance of surface colours when illuminated by light from a given source, compared with their appearance under light from some reference source.
contrast
In lighting, contrast is defined as the difference between the foreground luminance and the background luminance, divided by the background luminance. Contrast is what the human eye relies on to see object detail, and is the most important quantity to reproduce faithfully in any rendering algorithm.
D
daylight factor (DF)
The ratio of the illuminance at a point on a given plane within an interior due to the light received directly and indirectly from a sky of assumed or known luminance distribution, to that on a horizontal plane due to an unobstructed hemisphere of this sky. Direct sunlight is excluded from both values of illuminance i.e. using a CIE overcast sky.
deterministic
Any algorithm that consistently produces exactly the same results for exactly the same input. (see stochastic and Monte Carlo)
dielectric
A participating medium that refracts, and may absorb, but does not scatter radiation. Examples include water and glass.
diffraction
The deviation from linear propagation that occurs when light passes a small object or opening. This phenomenon is significant only when the object or opening is on the order of the wavelength of light.
diffuse reflection
Lighting in which the luminous flux comes from many directions, none of which predominates.
direct component
The illumination arriving at a surface point directly from a light source.
dispersion
The tendency of light to scatter spatially when refracted through a dielectric medium whose index of refraction varies with wavelength.
E
emittance
The visible light emitted (but not reflected) by a surface, expressed in the SI units of lumens per square metre.
exitance
The total visible light leaving a surface, including reflections.
exposure
A scaling factor used to map the computed luminances to the appropriate range for display.
extinction coefficient
The fraction of light lost to scattering and absorption per unit distance in a participating medium.
F
fovea
The high-resolution central region of the retina, spanning about 1 degree of the visual field in humans.
G
gamma correction
Computer monitors exhibit a response function that approximates a simple power law, and the exponent is called the monitor’s gamma. Gamma correction is to make allowances for this exponent for a particular monitor.
glare
The discomfort or impairment of vision experienced when parts of the visual field are excessively bright in relation to the general surroundings.
global illumination
The general problem of light propagation in simulated environments, accounting for the many interreflections between scene surfaces.
H
Hartmann constant
A parameter that approximates the change in the index of refraction of a particular dielectric material as a function of wavelength.
I
IES luminaire data
Candlepower distribution data together with other miscellaneous information in a standard format proposed by the IESNA.
IESNA
The Illuminating Engineering Society of North America, a professional organisation that sets standard practices for lighting design in US and Canada.
Illuminance
The integrated visible light arriving at a surface, expressed in SI units of lumens per square metre, or Lux.
Index of refraction
The ratio of the speed of light in a vacuum to the speed of light in a particular participating medium. This value determines how light is refracted (bent) as it passes from one medium to another.
indirect component
The component of light arriving at a surface indirectly via bounces off other, non-emitting surfaces. This may include specular and diffuse reflections.
indirect irradiance
A quantity equal to the integrated radiation arriving at a surface point excluding light sources that are part of the direct component.
irradiance
The integrated radiation arriving at a surface. (see illuminance)
isolux contour
A line corresponding to equal illuminance values on a surface.
isotropic
Equal in all directions. Isotropic scattering occurs when light is scattered equally in all directions.
J
JPEG
A compressed image format as defined by the Joint Photographic Experts Group. Usually the quality of the compressed image can be defined by the user (0 = bad – 100 = good).
K
L
Lambertian
Ideal diffuse reflection, in which the luminance is radiated identically in all directions.
lumen
The SI unit of luminous flux, used to describe a quantity of light emitted by a source or received by a surface.
luminaire
An apparatus which controls the distribution of light given by a lamp or lamps and which includes all the components necessary for fixing and protecting the lamps and for connecting them to the supply circuit. Luminaire has officially superseded the term “lighting fitting” which is still used colloquially.
luminance
The visible light passing through a point in a certain direction, expressed in SI units of lumens per steradian, or nits.
luminous flux
The light emitted by a source, or received by a surface.
M
material description
A Radiance scene description item that defines the properties assigned to a particular material, i.e. type, parameters, modifiers.
Monte Carlo
Random sampling in the context of an integration or averaging problem. In the absence of better information or techniques, this results in the most efficient sampling pattern.
N
nits
Unit of luminance, shorthand for candelas per square metre.
O
octree
A data structure that subdivides a volume of space into eight cubes, each of which may be subdivided into eight sub-cubes, and so on recursively. This data structure is created by the program oconv and is used to speed up ray intersection tests on the scene. This data structure is stored in the octree file.
P
participating medium
A solid, liquid, gas or colloid that affects the transport of light through its volume. Examples include water, glass, fog and smoke. Clear air is also a participating medium, but one that has an effect only over great distances.
pattern
A variation in surface colour, which can be described in Radiance as a procedure. In the IES Radiance interface we differentiate between the application of a procedural definition (a pattern) and a picture, which we refer to as an image. These definitions are found in the map file along with the material definitions.
penumbra
The partial shadow between the umbra and the unshaded region.
photometric
A quantity related to the human photopic response to visible radiation.
photometry
The measurement of visible radiation.
photopic response
The average spectral sensitivity of the human cone system to radiation.
pixel
Short for picture element. The smallest measured, calculated or displayed point in an image.
Q
R
radiance
The radiation passing through a point in a certain direction. (see luminance)
radiometric
A quantity related to radiation, usually measured in terms of power. Examples of radiometric units are watts and watts/steradian. Examples of radiometric quantities are radiance and irradiance.
radiosity
A physical quantity equal to the radiant exitance of a Lambertian surface.
radiosity method
A formulation of the global illumination problem based on the solution of radiosity for a finite number of points in a 3D scene geometry.
recursion
A function that calls itself to arrive at a solution.
reflectance
The ratio of the light leaving a surface to the light striking a surface.
refraction
The bending of light as it enters a dielectric medium at a nonzero polar angle. (see index of refraction)
refractive index
See index of refraction.
retina
Rear surface of the eye
RGB color
Red, Green and Blue representation of colour.
S
sampling
The process of determining an average or integral value by evaluating a function at one or more positions.
scattering coefficient
The fraction of light scattered per unit distance in a participating medium, given in SI units as a fraction per metre.
scotopic response
The average spectral sensitivity of the human rod system, which dominates vision in dark environments. See photopic response.
secondary light sources
A surface through which large quantities of light are directed e.g. a window.
shadow ray
A ray sent towards a light source to determine if the point is in shadow.
SI units
Standard International units. A specific recommended set of metric units for physical quantities such as length, volume, temperature, mass, etc.
sky component
The ratio of the illuminance at a point on a given plane within an interior due to the light received directly from a sky of assumed or known luminance distribution, to that on a horizontal plane due to an unobstructed hemisphere of this sky. Direct sunlight is excluded from both values of illuminance i.e. using a CIE overcast sky. Note that this is the same as the Daylight Factor except the indirect component has been removed. The vertical sky component is where the measured illuminance occurs on a vertical surface.
sky distribution
The luminance or radiance arriving at a specific point on the earth’s surface from the sky. Determined by the date and time. See CIE standard sky distribution.
solid angle
A solid slice of space starting from a point and extending to infinity in a region of directions that can be indicated by an area on the unit sphere.
spectral power distribution
The density of radiative energy as a function of wavelength, usually over the visible spectrum.
spectrum
A continuum or sequence, such as the spectrum of visible light.
specular
Pertaining to highly directional reflection or transmission. Specular reflection is usually centred about the mirror direction.
specularity
The amount of light reflected (or transmitted) by specular mechanisms.
steradian
The unit of solid angle. The maximum solid angle equals 4 p (area of a unit sphere).
stochastic
A stochastic method observes random occurrences (samples) generated by some probability distribution function. In practice, digital computers usually do not model ideal stochastic processes, since most random-number generators actually generate a fixed sequence of values (hence the term pseudo-random numbers). See Monte Carlo.
surface normal
A vector that is perpendicular to the tangent plane of a surface.
T
texture
An effect applied to a flat surface to yield the appearance of a macro-scopically rough surface e.g. fabric, wood, etc. Usually applied as a pattern.
transmission coefficient
The fraction of light that is passed per unit length. Equals 1 minus the absorption coeffiecient.
transmissivity
The fraction of light that passes through the interior of a glass pane at normal incidence. This does not consider light lost to refection by the front or back surface, or multiple internal reflections. From the transmissivity and the index of refraction, total transmittance and reflectance can be computed for any incident angle.
transmittance
The total light transmitted by a system, usually given for normal incidence. This is the quantity most easily measured, although it is not the most convenient to work with mathematically.
U
umbra
The region in complete shadow, where no portion of the light source is visible.
V
veiling luminance
Light scattered in the lens of the eye that makes it difficult to see areas immediately surrounding bright light sources.
virtual light source
An imaginary light source corresponding to a reflection or a redirection of a real light source (e.g. in a mirror).
void
This is a special identifier used to indicate the absence of a modifier in a Radiance scene description.
W
watt
The power equal to one joule per second. One horsepower = 746 watts.
wavelength
The longitudinal distance from one peak to another on a wave form. For light, this corresponds to the speed of light divided by the radiation frequency. The visible spectrum begins in the infrared at 780 nanometres and continues into the ultraviolet at 350 namometres.
X
Y
Z
