- Luminous flux Φ
- Luminous Intensity I
- Illuminance E
- Luminance L
Basic Lighting Terminology
The most fundamental quantity used to measure light is luminous flux.
Luminous flux is radiant flux multiplied, wavelength by wavelength, by the relative spectral sensitivity of the human visual system, over the wavelength range 380 nm to 780 nm.
The international (SI) unit of luminous flux. This is the amount of light produced from a light source.
This is the final measurement of light emitted from the luminaire. This will take into account the luminous flux from the light source and the LOR of the luminaire.
This describes the quantity of light that is radiated in a particular direction. This is a useful measurement for directive lighting elements such as reflectors. It is represented by the luminous intensity distribution curve and its unit is the Candela (cd).
This is the only lighting unit perceived by the eye. It describes the light source’s impression of surface brightness and is measured in cd/m
This is the quantity of luminous flux (lumens) falling on a surface and its unit is Lux. The simple equation to calculate illuminance is:
Illuminance E (lux) = luminous flux (lumens)
This is the value below which the illuminance level should not fall in the visual task area. The illuminance and its distribution on the task area and on the surrounding area have a great impact on how quickly, safely and comfortably a person perceives and carries out the visual task.
The LOR is the percentage difference between the luminous flux of a light source and the amount emitted from the luminaire it is part of.
LOR = Lumen Output (luminaire)
Lumen Output (light source)
Ingress Protection (IP Rating)
To help identify what luminaire is suitable for which application, they are given an IPXX rating. The IP denotes Ingress or International Protection code and the X denotes first and second characteristic numerals. An IPXX rating categorises the level of protection the luminaire has against dust, solid objects and moisture. These ratings are given in accordance with European standard EN60598-1.
Luminaires installed outdoors will be exposed to rain, dust and solid objects. Luminaires in these environments will typically have an IP rating of IP54 to IP65. Conversely, luminaires installed indoors in an office or corridor would typically have much lower IP rating, i.e. IP20.
LEDs are light emitting diodes. These are electronic components that convert electrical energy directly to light through the movement of electrons within the material of the diode. LEDs are important because due to their efficiency and low energy, they are replacing most conventional light sources.
The term solid state lighting is used because the electronics produce light directly from solid materials in which the electrons are embedded. Other light source technologies, eg. the fluorescent tube requires a gaseous discharge medium to initiate production of light.
The junction temperature is critical in LED lamp or luminaire design. It is the LED’s active region; the point at which the diode connects to the base. This is where the electrons jump between the two semiconductors to produce photons. A low junction temperature helps LEDs to produce more light also reduces lumen depreciation. Junction temperature is affected by the driver current, the thermal path, and the ambient temperature.
LED technology is constantly changing. Rapid innovation continues to improve the performance of LED on an almost daily basis. Future-proofing of LED modules allows luminaire manufacturers to switch from one generation to the next improved generation without major retooling or changes in luminaire design, offering backward compatibility with drivers.
LED chips are mass produced in millions and there are inevitably slight differences in color appearance and light output. Binning is way of sorting the chips so that all the LEDs from one particular bin look the same and have similar light output.
A Light Engine is the LED equivalent of a conventional lamp. It normally consists of a LED chip mounted on a circuit board that has electrical and mechanical fixings, meaning it is ready to be fixed in the luminaire.
The driver is often the critical component in a LED luminaire or lamp, more so than the LED chip itself. The driver has to be matched to the power requirements of the LED chip.
The power factor is the active power divided by the apparent power (i.e. product of the rms input voltage and rms input current of a driver).
The power factor correction is an electronic device, such as an LED lighting fixture, a system of inductors, capacitors, or voltage converters to adjust the power factor of electronic devices toward the ideal power factor of 1.0.
Although the initial cost of conventional light sources is less than LEDs, the operational and maintenance costs of LED are significantly lower. LEDs have a longer life, reduced maintenance and lamp replacement cost. Because LEDs need to be replaced less frequently, the owner spends less on new lamps and the labour needed to change them. LEDs also consume less energy; therefore, the overall cost of a LED system can be significantly lower than that of conventional lighting systems.
The light colour describes the colour appearance of the light. LEDs do not directly produce white light. There are two ways in which white light is produced from LEDs.
Using a blue LED with a phosphor coating to convert blue light to white light by a process called fluorescence. Combining red, blue and green LEDs to produce white light. White light is produced by varying the intensities of the individual red, blue and green chips.
The description used to describe the effect of heating an object until it glows incandescently, the emitted radiation, and apparent colour, changes proportionally to the temperature. This is easily demonstrated when considering metal in an oven that begins to glow red, then orange, and then white as the temperature increases. CCT is defined in degrees Kelvin (K).
Therefore, in addition to the colours of surfaces, it is also the light colour that determines a room’s atmosphere.What defines the colour temperature of a white LED is the thickness of the phosphor layer and the wavelength of the blue chip.
A measure of the degree of colour shift objects undergo when illuminated by the light source as compared with those same objects when illuminated by a reference source of comparable colour temperature. The reference source has a CRI of 100. Natural light (daylight) would also be 100.
Color Rendering Index – CRI indicates the accuracy with which a light source such as an LED can reveal the various colors of an object.
Tm-30-15 is a new method to evaluate colour rendition; it includes several measures and graphics t evaluate the Fidelity (Rf) and Gamut (Rg) of a source when compared to a reference illuminant. TM-30 uses the average value of 99 colour samples; CRI however only utilizes an average score of 8 colour samples. This metric is still yet to become widely adopted in the LED market.
A light source is the visible white light that we see which is made up of a spectrum of various colors of light, ranging from wavelengths of 380nm (violet) to 760nm (red). The SPD is a graph that shows the power (strength) of each wavelength of light produced by a particular light source.
An elliptical region on the CIE chromaticity diagram that contains all the colors that are indistinguishable to the average human eye, from the color at the center of the ellipse. Adjacent ellipses are ‘just distinguishable’ in terms of color. This system is used to refine the binning process of LED colors. Slight color differences in the appearance of LED light are measured in MacAdam ellipses or steps.
Glare is a visual sensation caused by excessive and uncontrolled brightness. It can be disabling or simply uncomfortable. It is subjective, and sensitivity to glare can vary widely. Older people are usually more sensitive to glare due to the aging characteristics of the eye.
Disability glare is the reduction in visibility caused by intense light sources in the field of view, while discomfort glare is the sensation of annoyance or even pain induced by overly bright sources.
When light is scattered it causes a ‘veil’ of high luminance over the retina = reducing contrast.
A great example of this is when you face high-beam headlights when everything else around you is dark.
- Luminance of source (Luminance -the surface brightness of an object measured in candelas/m²)
- Luminance of Background
- Size of Glare Source
- Position of Glare source
‘The rating of discomfort glare shall be determined using the CIE (International Commission on Illumination) Unified Glare Rating (UGR)’. EN 12464-1:2011 Light and Lighting of Work places
(UGR) tabular method. Variations of UGR can be determined using the comprehensive tables for different observer positions, as detailed in report CIE 117-1995
CIE 117-1995 calculates a table of glare indices for a regular grid of luminaires based on a set spacing using the following formula :
The UGR rating helps to determine how likely a luminaire is to cause discomfort to those around it. For example, the discomfort that a LED Panel will cause the workforce within an office
Service Life Data For LEDs
One of the benefits of the LED is its long lifetime. Because they have no movable parts or filaments that may break, LED’s can have long lifetime. However, the light output from all light sources decreases over time.
50,000 hours would imply a service life of 5.7 years if the light is operated for 24 hours in a day, 7.6 years if the lights are on 18 hours per day and 11.4 years for 12 hours a day.
LEDs are measured as lumen depreciation as unlike conventional light sources that reduce in output and eventually fail, it is not normal for LED products to suddenly fail. Instead, the light output reduces over time.
The normal convention is to measure the life from when the output has reduced by 30%, i.e. when there is 70% light output remaining from the initial lumen output. This is often quoted as the L70 life and is measured in hours. The “median useful life” of the LED luminaire is based upon the time elapsed until 50% of the LED luminaires in use reach the stated depreciation.
The thermal management is an important factor that can affect the lifespan of the. Appropriate heat sinks have to be designed to prevent premature failure of LEDs.
It is also important for the correct driver to match the LED chip. Running LEDs at currents higher than specified make the LED run hot. For example, if the driver produces 700mA but the LED needs 350mA, this will put stress on LED and reduce its lifespan.
Higher ambient temperatures than the ones that the LED is rated for will also reduce its expected life.
Unlike many conventional lamps, LEDs are semiconductors and their life span is not affected by the number of times they are turned on and off.