Luminous Intensity is the light emitted in a given direction by a source. It is measured in candela (cd). The candela is an SI base unit from which other lighting-related units are derived. The candela is defined as an emission of monochromatic radiation of frequency 540 x 10^12 Hz and that has a radiant intensity in that direction of 1/683 W/sr.
Luminous flux is the total amount of light emitted from a source in all directions. It can be used to approximate the "brightness" of a source, given that it is an average of the visible portion of the spectra emitted by a light source weighted by a function known as 'v-lambda' that describes the human visual systems sensitivity to light of different wavelengths. The lumen is a derived unit defined as 1 candela emitted in 1 unit solid angle or steradian.
The radiant flux of a light source is a measure of the total power emitted by a source across the entire electromagnetic spectrum, including non-visible portions such as UV and IR. In lighting, radiant flux is used in order to determine the luminous efficacy of a light source.
The luminous efficacy of a light source is determined by dividing the luminous flux by the radiant flux. The resulting fraction or coefficient describes the degree to which a source emits radiation in the visible or ‘useful’ part of the spectrum for lighting purposes. The energy emitted in wavelengths outside the visible portion of the spectrum reduces the overall luminous efficacy of a light source.
Illuminance is the light incident on the surface of a plane. It is a derivative unit where 1 lux = 1 lumen spread over 1 square meter. Illuminance can be further classified as perpendicular or horizontal illuminance, when needing to differentiate in the analysis of a lit environment. The inverse square law can be used to calculate the lux incident on a plane with a known source intensity and distance.
Luminance is the light emitted from or reflected from a surface and approximates the brightness. It is dependant on the luminance of incident light and the reflectance of the surface. It is also commonly used to measure the brightness of a monitor or display.
The colour rendering ability of a light source is the degree to which the source alters the appearance of an illuminate object relative to the appearance of the object under a reference illuminant. The most commonly used system for measuring this is the Colour Rendering Index. A series of coloured patches are evaluated under the source illuminant and an average calculated and indexed to a score (Ra) out of 100. Although in widespread use, there are several issues with the CRI system. One approach is to include a more saturated sample known as R9. An improved alternative to CRI is known as the Colour Quality System (CQS).
The colour temperature of a light source is a measure used to describe the appearance of a white-light source. ‘Cool’ sources are said to have a higher CCT (above 5000K) whilst ‘Warm’ sources have lower temperatures (below 3000K). It it referred to as correlated colour temperature because the appearance of the light source is being compared to that of an ‘ideal’ black-body radiator with a similar surface temperature measured in kelvin (K).
Full Width Half Maximum (FWHM) is an expression often found in the specification of LED optical systems. It refers to the width of the beam where the intensity is 50% of the maximum. This is typically measured by a goniophotometer during standard photometric testing. Some manufactures may use different systems for specifying optical beam performance.
Voltage drop in landscape lighting is the degree to which the starting voltage decreases over a given length of cable as a function of both the current and the resistance of the cable. If the proper cable is not selected, voltage drop can produce faults such as low output or flickering light. Selecting luminaires with the Aqualux MultiVoltage internal driver and using a 24V supply mitigates many of these issues, allowing for cheaper and more flexible installation.