A is for Amps

LEDs are current driven devices. Typical drive currents are 0.35Amps (350mA) for 1 Watt LEDs and 0.7A (700mA) for 3 Watt LEDs. Combinations of LEDs or arrays of LEDs may need different drive currents ranging from a few hundred milliamps up to several amps.

B is for Beam Spread

The light beam from an LED can be spread by a reflector or lens to give different cones of light. Typical beam spreads are spot, narrow, narrow flood and flood. LEDs are not omni-directional like most light bulbs, so the beam spread is an important consideration for LED light fittings. The definition of the beam spread is the angle that corresponds to half the maximum light intensity. RECOM Lighting TIP: Use the Beam Spread Calculator

C is for Color Rendering Index (CRI)

Daylight has a Color Rendering Index (CRI) of 100 because any colored object illuminated by daylight looks natural to the human eye. If the light source has a different spectrum to sunlight, then some colors can appear unnatural and the CRI is not a perfect 100. For example, any blue object illuminated by a red light will look black, not blue (try this at home to see for yourself!). Most modern white LEDs can reach a CRI of 80 or above, which is considered acceptable by most people.

D is for Driver

As LEDs are current-driven devices, they can’t be connected straight to a voltage supply. A driver is needed to monitor the current flowing through the LED and to constantly adjust the current to keep the LED brightness constant (LED brightness is directly proportional to the current). Some drivers are dimmable so that the LED illumination can be controlled either manually or by a central lighting controller or automatically (e.g. auto-dimming if the LED overheats).

E is for Efficacy

An LED takes electrical energy in and gives light energy out. The energy conversion efficiency, or efficacy, is measured in lumens per watt (lpw). For example, if a 100W light bulb gives out 9000 lumens of light, then its efficacy is 90 lpw. LEDs have efficacies up to 200 lpw.

F is for Fluorescent

Only fluorescent lighting can compete with LEDs in terms of efficacy and running costs. However, LEDs offer a better quality of light (higher CRI), wider dimming range, a longer lifetime and do not contain toxic mercury. Compact fluorescent lamps (CFL) also generate significant mains harmonics; if you have four or more CFLs in your home, then they will generate more electrical interference than all of your other electrical equipment (TVs, fridges, vacuum cleaners, etc.) combined. Mains powered LED drivers with more than 25W output power are required by law to contain power factor correction (PFC) to eliminate such harmonic interference.

G is for Glare

LEDs are essentially point sources of light, so the there is a danger that the very high light intensities can cause discomfort, annoyance or even eye damage if the light fitting is not correctly designed, installed or used. Creating sufficient light without causing glare is one of the main skills of a professional lighting designer.

H is for Heatsink

All high power LEDs need heatsinks to dissipate the heat they generate away to the surroundings. Although LEDs have 3 to 5 times better efficacy than light bulbs, they are still sensitive semi-conductor components and cannot survive very high temperatures. The waste heat from a light bulb is mostly radiated away as infra-red radiation, but the waste heat of an LED must be conducted away with a large metal heatsink.

I is for Illuminance

The illuminance is a measure of the intensity of light falling on to a surface. If the area is measured in square meters, then the units of illuminance is lux (lumens per square meter). If the area is measured in square feet, then the units of illuminance are footcandles (lumens per square foot). 1 footcandle = 10.75 lux. Typical recommended illumination levels are: Public Areas 250 lux Offices 500 lux Supermarkets 750 lux Operating Theaters 1000 lux

J is for Junction Temperature

An LED is a semiconductor device (the “D” is for Diode). Light and heat are created at the junction of two types of semiconductor material (N-Type and P-Type). Only about 20% of the energy put into an LED is converted into light, the rest appears as heat. If the junction temperature rises above 100°C, then the lifetime of the LED will be reduced. A junction temperature of 100°C translates to an LED case temperature of around 85°C.

K is for Kelvin

The color of a light can be expressed as a temperature in degrees Kelvin (°K). The Kelvin temperature scale is the same as Centigrade, but starts at absolute zero (0°K = -273°C). The easiest way to understand the concept of color temperature is to think of metal being slowly heated up from red hot (1000 °K) through to glowing yellow (3000°K) through to white hot (4000°K). Popular LED color temperatures are 3000°K, 3500°K, 4000°K and 5000°K.

L is for LED

A diode allows electricity to flow in one direction only. In the reverse direction (blocking) almost no current can flow through the device, however high the voltage is (up to the limit of the diode). In the forward (conducting) direction, a diode needs about 0.7V to overcome the internal barrier of the p-n junction, and then lets the current flow through unimpeded. A light emitting diode has an artificially high internal junction barrier that requires about 3.3V to force the diode into conduction. Once past the junction barrier, the excess energy appears as light, so the diode becomes a light emitting diode or LED.

M is for MacAdam

Color perception is subjective. In the 1930’s, David MacAdam developed a way of defining color by drawing ellipses on a color chart within which most people could not see any color difference. These ellipses could then be used to define the production quality of a real light source, as any color variation that fell within these ellipses would not be noticeable. LED manufacturers use the same system to measure the color temperature of every LED they make and then “binning” or sorting them into all the same color. Thus when a customer places an order, they will get a delivery where all of the LEDs color-match. The Macadam ellipses can be further defined into “steps”, so a 1-step Macadam ellipse would be a very tight control of the color in which 99% of the population would agree is the same, while a 4-step Macadam ellipse would be judged by only 68% of the population to be all the same color. Most LED manufacturers can achieve 2-step MacAdam color matching.

N is for Norms and Standards

LED Lighting is still a relatively new technology and there are no universally accepted norms or standards for certifying the safety, measuring the LED lifetime or defining the light output, for example. There are some EMC regulations that are required for the sale of LED drivers in the USA and Europe, however. The following list of commonly requested norms is marked -V for a voluntary standard or -R for a required regulation: ANSI/UL8750 Safety of LED equipment used in Lighting - V UL60950 IT equipment Safety (General standard used for almost all power supplies) - V FCC 47 CFR part 15 Radio Frequency devices (USA limits for radiated EMC) - R EN60950 IT equipment Safety (General standard used for almost all power supplies) - R EN55022 Radio Frequency devices (EU limits for radiated and conducted EMC) - R EN61000-3-2 PFC (EU limits for conducted harmonics: LEDs need Class C compliance) – R Energy Star USA EPA Standard for efficacy, lumen maintenance and standby power - V EuP EU Energy Using Product regulation for standby power - R

O is for Optics

Most LED lighting fixtures require optics to shape, limit and steer the light output to create the required illumination pattern. Optical components include reflectors, diffusers, lenses and louvers. Optics play an important role in the overall efficiency of a lighting fixture as up to 20% of the light generated by the LEDs can be lost in the optics if they are not well designed.

P is for PCB

Almost all LEDs are surface mount devices that need to be mounted onto a printed circuit board (PCB) to provide the electrical connections. The choice of PCB material makes a big difference to the thermal transfer characteristics between the LED and the heatsink, as the lower the thermal resistance of the PCB, the better. A conventional FR4 PCB has a thermal resistance of 30 W/mK, adding through hole vias to improve the heat transfer reduces the resistance to 12 W/mK. The best material is a metal core PCB which has a thermal resistance of only 0.25 W/mK.

Q is for Quality

Not all LEDs and not all LED fixtures are of the same quality. Quality starts with the choice of LED manufacturer. A good manufacturer delivers a more consistent product with fewer variations in the die characteristics, a well sealed encapsulant and a long operational lifetime. However the LED is only one component in the system of parts which constitute a LED fixture. If the optical lenses use low grade plastics, they can cloud, craze or yellow with time. If the mechanical construction is poor, then the thermal transfer to the surroundings may be impeded by badly fitting heatsinks, corrosion or by failing fan bearings. However, one of the most important elements to affect the reliability and quality of a LED light fitting is the driver. The driver electronics are statistically the weakest link in the chain and an unreliable driver which uses low quality components, poor internal thermal management or a lowest possible cost circuit design can harm the overall quality significantly. Judging the quality of a product is not easy from a bare datasheet or description in the internet, therefore customers can request a meeting to discuss their needs, see the products first hand and talk to in-house lighting experts to get the best quality solution for them.

R is for Retrofit

Retrofit lamps are designed to fit into standard mains-voltage screw or bayonet-fitting light sockets or low voltage bi-pin MR16 sockets. All of these standard socket systems were designed for incandescent lamps and are not ideal for LEDs – in particular, the space available for heatsinks is extremely limited and proper airflow is impeded by the legacy socket design. The net result is a drastically reduced LED lifetime, and for this reason, RECOM Lighting does not offer retrofit solutions.

S is for Strobe Effect

LEDs can be dimmed it two ways, either by linearly reducing the current flowing through the LED or by repeatedly pulsing the LED quickly on and then off again. The latter method is called Pulse Width Modulation (PWM). If, on average, the LED spends more time on than off, it will appear bright. If it is 50% on and 50% off, it will appear less bright, and if it is more off than on, it will appear dim. The advantage of PWM is that it is easy to implement at the driver IC level and that the color does not vary with dimming. The disadvantage is the stroboscopic effect of the flashing LED. Even if the PWM frequency is too high for the eye to see, any moving objects can appear to move jerkily and interference lines often appear on any video shot under PWM dimmed LED lights. Due to the strobe effect, PWM dimming is not recommended for stage lighting, broadcast studios or public areas like shopping malls, where the cameras in mobile phones can be affected.

T is for Thermal Feedback

LEDs must not be allowed to overheat, or their lifetime will be reduced. Thermal feedback uses a thermal sensor to measure the case temperature of the LED and to automatically dim or reduce the power via a feedback circuit in the driver to avoid over-temperature operation. Thermal feedback can protect a LED fitting from damage due to blocked airflow, accumulated dust in the cooling fins, poor installation or fan failure and is recommended for all high power LED fittings.

U is for Useful Life

Useful life is the length of time an LED fitting can be used without repairing or replacing any of the components. For the LED, the useful life is usually defined as L70, or when the light output falls to 70% of the original light output due to aging effects in the LED. Another definition for lumen maintenance measurements is the LM-80-08 standard.

V is for Vf

Although LEDs are current driven devices, they need a certain threshold voltage before they start conducting. A typical forward voltage (Vf) is 3.3V per LED. If 10 LEDs are connected in series, then the driver must supply 33V to drive the LEDs properly. Some drivers have a limited output voltage range, so the driver needs to be correctly matched to the LED load. For example, if a driver has an output voltage range of 10-34V, it can only be used to drive a minimum of 4 LEDs and a maximum of 10 LEDs. Using too many or too few LEDs will give disappointing results, even if the nominal driver current is correct.

W is for Wallwasher

To avoid glare, LEDS are often pointed along a wall or vertical surface to create a diffuse reflected light. This technique is called “Wallwashing” and typically LED strips are used placed at the bottom or top of the surface so the LED light “washes” across the surface. The same principle is used in cove lighting where LEDs placed behind decorative coving to reflect their light down from the ceiling. Wallwashing is now an over-used technique where hotels, restaurants and bars compete against each other to display ever more garish color changing LED illumination of their facades.

X is for X-ray

LED ballasts (AC powered LED drivers with constant current output) are often used in damp or outdoor conditions. To meet the requirements for IP67 moisture resistance, the drivers are usually “potted” or completely filled with silicone or epoxy resin. If a ballast fails in use, it is not easy to investigate the cause of failure without using an X-ray machine to see through the potting material. RECOM Lighting has access to its own X-ray machine, and can investigate any field failures thoroughly and completely without having to destroy the ballast to analyze for the fault.

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Z is for Zero Defects

The aim of every product and installation is zero defects. Careful production quality control can reduce product failures to a minimum, but if the LED fixture is damaged in transit or incorrectly installed, the end result cannot be zero defects. RECOM Lighting offers not only high quality products, but competent logistics and non-interchangeable connectors that ensure that the LEDs cannot be incorrectly wired up to the drivers and ballasts, thus avoiding accidental damage to the LEDs during installation.