Terms

LED diode (Light Emitting Diode) – an optoelectronic component that emits radiation the source of which is a recombination of carriers (electrons and electron holes) in a semiconductor under the influence of external electric field (electroluminescence).

Monochromatic radiation – radiation that carries energy using one wave length, e.g. visible radiation of the low pressure sodium lamps that carries its almost full power on one wave length of 590 nm.

Complex radiation - radiation that carries energy using many wave lengths of the visible spectrum. A particular case of this radiation is the continuous radiation, the power of which is distributed on all waves from the range of 380–780 nm.

Wave length – the distance between two wave points being in the same phase of vibration. Usually measured as a distance between crests of a wave. Different wave lengths in the visible spectrum reflect in different colors of light.

Visible spectrum
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Diode lumen depreciation (L70) – the period of time after which the diode light output drops by 30%, so it reaches only 70% its initial output. The parameter is defined in this way because this change in light output can be noticed by a human eye. Diodes used in the light industry have the life span from 30 000 to 50 000 hours, but the best diodes with appropriate power supply and cooling can function for up to 70 000 or even 100 000 hours.

Luminous flux – a quantity which defines the power output of a light source in relation to the light impact on a human eye (the light color — or the wave length that the eye is the most sensitive to, differs depending on the day time). The quantity unit of the luminous flux is the φ [lm].

Luminous intensity – the basic physical quantity in photometry. Defines the light power being emitted by a light source in the given solid angle. The quantity unit is the candela I [cd].

Luminance – defines the quantity of light that crosses or is emitted by the given surface and is contained in the given solid angle. Its quantity unit is the candela per square meter Lv [cd/m²].

Illuminance – defines the level of light intensity of a surface. Its quantity unit is the lux E [lx].

Color temperature – refers to the tone of the emitted white light. The tone is defined as the comparison with a perfectly black body heated up to the particular temperature, so its quantity unit is kelvin [K]. It is a commonly used standard in industry that the most popular tones are called: warm (2600–3700 K), neutral (3700–5000 K) and cool (5000–8300 K). The symbol of this parameter: CCT [K].

Range of color temperatures for various light sourcesimage003

Colour Rendering Index – defines how faithfully the colors of items are reproduced in the given light source. It is represented by a number from 0 (for monochromatic light) to 100 (for white light); the higher the number, the better the color reproduction is. The most commonly used symbols of this parameter: CRI or Ra.

Light efficiency – it is a relation of the light emitted by a light source and the consumed power. Its quantity unit is the lumen/Watt [lm/W].

Rank – a batch of diodes that has been tested and sorted (usually already during the production process) in respect of:
- emitted light flux
- color temperature (refers to the white light diodes)
- wave length (refers to the color diodes)
- voltage
- color rendering index

Photopic vision (daytime vision) – this term defines the work of a human eye (seeing) in normal conditions, which means conditions in which the light intensity allows for maximum usage of the eye, e.g. during the day. Only cone cells are used for recognizing the light impulses.

Mesopic vision (twilight vision) – this term defines the work of a human eye (seeing) in transitional conditions, which means conditions in which both cone cells and rod cells are used for recognizing the light impulses, e.g. during twilight.

Scotopic vision (night-time vision) – this term defines the work of a human eye (seeing) in extremely adverse conditions, which means conditions in which only rod cells are used for recognizing the light impulses, e.g. in the night. The rod cells allow to see only in the monochromatic scale (gray scale).

Relative sensitivity curve of a human eye for photopic V(λ)
and scotopic V' (λ) vision according to CIE
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