Brightness, lamp life and efficiency of LED fixtures

Brightness, lamp life and efficiency of LED fixtures (on photo: Downlight 7 LED Fixture by TheLEDLight.com @ Flickr)

Introduction

A typical LED fixture comprises four major component parts: an LED emitter, the fixture’s heat sink, driver and dimming control, and augmenting optics.

The emitter includes the die, a thermal heat sink, lens, and outer package (Figure 1). The die is the actual LED chip within the emitter.

The color of the light is determined by the energy gap of this semiconductor. The thermal heat sink that is part of the emitter pulls the heat away from the chip and conducts it to the mass of the larger fixture (the fixture’s heat sink).

The power supply electronics are designed to limit the drive current to the LED’s specifications. In this regard LEDs are very sensitive; too much current can shorten their useful life from 50,000 h to nothing in an instant.

This is why manufacturers warn that the light should not be used above a certain temperature. Controlling drive current is critical to the LED’s brightness and useful life.

LED emitter

Figure 1 - LED emitter. At its center is a diode chip, the die, capable of converting electricity to light energy very efficiently. Heat is dissipated into the thermal heat sink. A silicon lens covers this chip.


An LED is a current-driven device, meaning that the intensity of the light generated depends on the amount of electric current flowing through it. Fixture designers try to design their lights with as high a drive current as possible, but there is a three-way relationship between brightness, lamp life and the ability of the fixture to dissipate the heat.

LEDs are brighter at a higher current, but they loose efficiency if their operating temperature is allowed to rise.

The balance a fixture designer can strike will also depend on the limitations posed by the spacing of emitters, the efficiency of the heat management.

This is why almost all LED devices have large heat sinks and fins.

The lethal effect of overheating has prompted some manufacturers to provide safeguards against over-temperature situations by automatically increasing the speed of the cooling fan, and at some point, automatically reducing power or shutting off all together if the light approaches red-line, in order to draw the user’s attention to the heat issue.

Typical 3 W LED bulb Construction

Typical 3 W LED bulb Construction


This can usually be remedied by providing shade or better ventilation.

There are a couple different ways manufacturers can arrange the dimming. One way is to vary the drive current usually using pulse amplitude modulation (PAM). PAM is a method of current control that employs very high speed on/off switching to limit the current. By varying the timing of the switching the current can be lowered to dim the LED.

A fixture dimmed using PAM will cut-out abruptly before it reaches full dim. The other way to dim LEDs is to use pulse width modulation (PWM) downstream of the driver. PWM modulates the intensity by varying the duty cycle at high frequency. This allows smooth dimming to nearly zero.

For LED lights marketed for film and television, the frequency of the electronic power supplies, and PWM dimmer is typically greater than 20,000 Hz, and does not pose a risk of flicker at normal frame rates. Nila has actually done tests up to 7000 fps without capturing any flicker.

However cheaper LEDs designed for the consumer market or club venues may use power supplies that cycle at 1 kHz or even lower, and these pose a definite risk of flicker, especially when the LED itself is being photographed at higher than normal frame rates. Testing is recommended.

The optical components of the LED, lenses and reflectors, extract light from the chip and shape the projection of that light in a focused beam. A total internal reflection (TIR) lens is a small molded lens used to capture light that is emitted in 180 from the die, and form it into a manageable beam of light. Advances in optics accounted for the lion’s share of improvement in LED lumen output in the early years of their development.

More recently improved chip technology and chemistry and better thermal management by the chip itself have contributed greatly to performance improvements.

As mentioned previously, some LED fixtures use interchangeable optics. A thin sheet of glass covers the chip to protect it. The optics should be kept clean, however do not use solvents or window cleaner as these can have adverse reactions with the assembly. Manufacturers recommend using a soft rag with isopropyl alcohol to clean the protective glass. Use water with mild soap for the optics.


Power factor

Power factor may be a concern when using very large numbers of LEDs. Color Kinetics and NILA fixtures are fully power factor corrected. Many of the devices described in this chapter may or may not have power factor correction. If they do not, you might expect to see a power factor of about 0.70.

In a large installation this could create a significant nonlinear load. Check manufacturer’s specifications.


LED useful life

LEDs very rarely just fail or suddenly burn out (unless seriously overheated). Normally they fade slowly over time, at a fairly consistent speed.

Their useful life is defined in terms of lumen maintenance – the number of hours the emitter will operate on average before the lumen output decreases below a given percentage of initial light output.

For example the emitter manufacturer will specify that an LED will produce at least 70% (denoted L70) of its initial output for 50,000 h, when driven at a particular current and a particular junction temperature. This is also sometimes stated as L75 or L50 (75% and 50% respectively).

The L value that the manufacturer uses to produce their advertised lamp life figure makes a big difference. As a practical matter, a light that puts out less than 70% of its initial output would be considered pretty useless in our business. Depending how the light designer configures the electronics and heat dissipation, and exactly which LEDs they choose the estimated lamp life can vary quite a bit.

Manufacturers of lights used in our industry advertise lamp life from 20,000 to 100,000 hours. If you ran a 50,000-hour LED 8 hours a day, every day including weekends and holidays, the light would lose 10% of the initial output in about 6 years. At that rate, it would take a little over 17 years to reach 70% output.

Of course once the LEDs are worn out, you just have to replace the whole fixture. Another factor that is easily overlooked in all this is that in theory the circuit components employed in the drive electronics have a shorter mean time to failure than the LEDs themselves, and may end up being the weakest link.


LED Lighting Technology Overview (VIDEO)

Cant see this video? Click here to watch it on Youtube.

Resource: Lighting Equipment, Practice, and Electrical Distribution – Harry C. Box (get book at Amazon)


About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears. Professional in AutoCAD programming and web-design. Present on



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  1. […] lamp has an initial output (Efficiency) of 85 lumen per […]

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