How to read an LED spec sheet

Monday, April 11, 2016

Many LED manufacturers try to survive on providing a minimum amount of technical information on their products, and even the information that we do get doesn't tell the whole story. This is the information that we really need when we want to specify LED product.

Photometric performance

Rated luminous flux (lumens: lm) 

Traditionally, the luminous flux figure related to the light emitting from the light source, independent of the effect of the luminaire and any optical control. There is no reason why LED manufacturers should report any differently, but this simple figure has been deliberately confused thanks to the ‘embedded' nature of some LEDs within the luminaire body. But this should be dealt with in the Light Output Ratio metric (see below). LED panels have been a particular source of confusion in this area of reporting.  

LED luminaire efficacy (lumens/Watt: lm/W)

This is the figure that every manufacturer has been hunting down for the past decade and more. At first, the practical target was to get above the efficacy threshold required by energy efficiency regulations such as Building Regulations: Part L in the UK. That target was reached some years ago.

There’s a suspicion that the addiction to ever-higher luminous efficacy figures will lead to poorer quality lighting installations, as higher light outputs from luminaires will promote poorer uniformity of illuminance within a space, all in an attempt to hold up the luminous efficacy values .

Luminous efficacy (Light Output Ratio: LOR)

There are two luminous flux values; the source lumen output, which is the light output of the light source itself, and the luminaire lumen output, which takes into account the losses created within the housing and optical system. The Light Output Ratio (LOR) is the difference between the two values.

There is no such thing as a 100% efficient luminaire, despite claims by some LED manufacturers to the contrary. As soon as a light source is fitted into a luminaire the output characteristics of the source changes. Every luminaire must have an LOR, even though many manufacturers may prefer not to tell you what it is.

Intensity Distribution (candelas: cd)

The Polar Curve is the snapshot that tells you what kind of luminaire you’re dealing with. It immediately identifies a downlight, say, as having a narrow beam or a wide beam.

The maximum intensity figure is usually in the centre of the light beam. Beam widths are usually quoted to the half-maximum intensity figure. 

Although things have improved to some extent, it still feels as if the LED manufacturers have decided that the actual luminaire performance didn’t matter, and whether it’s a narrow beam spotlight or wide beam floodlight is irrelevant.

LED Rated Life

Light source engineer James Hooker conducting life tests

Just giving a rated life as a number of hours is pretty much meaningless. The figure needs to come with a qualifying metric, that of the reduction in light output over those hours. This is usually shown as ‘Lx’ where x is the percentage remaining from the initial light output.

50,000 hours at L70 means that the luminaire will have lost 30% of its light at 50,000 hours.
50,000 hours at L90 says that the luminaire has lost only 10% of it light over that same period.

An associated metric is the Fraction Failure. This refers to the percentage of LED modules that may be expected to have failed by the time that the luminaire reaches its rated life. You won't see this figure in many data sheets.

Colour performance

All light sources degrade with use and the LED is no different. As LEDs age, two things happen, the light output drops and the colour quality degrades. Good colour quality reporting will take these age-based changes into account.

 

 

Colour Rendering Index (CRI)

Colour rendering describes how well a white light source performs in accurately displaying the colours that it illuminates. An ideal CRI metric would be 1; an acceptable CRI for most residential and commercial lighting is better than 0.8. A CRI of less than 0.8 should be avoided.

We've talked elsewhere about the proposals to introduce TM30:15 into Europe. Until CIE addresses this issue, we'll continue to refer to the CIE convention.

Correlated Colour Temperature (CCT)

The colour temperature of a white light source refers to how ‘warm’ or cold’ the light appears. Measured in degrees Kelvin (K), the higher the figure, the cooler the light. Domestic interiors usually call for a CT of around 2700K, which imitates the colour of a tungsten lamp (the traditional light bulb); commercial interiors are usually in the 3000K – 4000K range. Recent lighting strategies have engaged with ‘lighting for health’, supporting the body's circadian rhythm. This has seen more use of daylight colour temperatures, above 5000K.

Historically, one of the main problems of many light sources has been the tendency for the lamp to continue working long after its colour qualities have degraded to the point of being useless. The LED is no different and across its rated life, the colour performance can expect to become poorer. The rated life of the LED ought to take this degradation into account as well as the fall-off in light output.  Only a few companies acknowledge the issue.

There are two metrics that would be useful to have available: the colour rendering and colour temperature presenting at the end of rated life (L70, etc)

The MacAdam Ellipse

A simplified version of the MacAdam elipses. Courtesy: Xicato

The LED has also seen the introduction of a new colour metric; the MacAdam Ellipse (McAE). This became necessary because of the inconsistencies in the colour appearance of LEDs as they leave the assembly lines in their mass-produced millions. The McAE is based on the principle of ‘barely perceivable difference’ between LED chips. Where a cluster of chip outputs all appear to be the same, they are considered to be within 1 McAE. The more perceivable difference there is, the higher the number of McAE involved.

A good manufacturer will typically deliver its products within two or three McAE. If the quoted McAE range is greater than six, look for another luminaire.

Again, as the LED ages and the colour output degrades, the colour delivery will deteriorate and spread across more McE, degrading the originally-quoted colour performance. So one further metric would be valuable: the variation in McAE through rated life.

Tunable whites

Something that has recently become relevant is the colour performance of ‘tunable white sources’. These are multi-channel LED sources where the white light can be shifted between warm and cool temperatures. What is rarely reported is the impact on colour rendering as it passes through the tunable range. Ideally, the varying colour temperatures will follow the ‘black body radiator’ curve across the relevant colour temperature range. There will be lighting schemes where this information will be vitally important.

 

Electrical performance

Any LED luminaire is first and foremost an electrical device and there are aspects of LED performance that need to be reported in luminaire data sheets.

Rated input power (in W)

The input power refers to the total power consumed by the luminaire, including any control gear in the circuit.

Drive Current (in mA)

The brightness of an LED is dependent upon the drive current of the driver. The higher the drive current, the brighter the light output, but the life of the LED will be reduced. A typical drive current value is 350mA, but it cannot be assumed.

Good, or unexpectedly high light output figures may be the result of the LED being driven harder, and the rated life figures may be fudged as a consequence.

Power Factor

It is in the nature of an LED circuit to have a poor power factor, less than 0.5.

Although manufacturers are not required to improve the Power Factor of any luminaire rated at less than 26W, it should be a given for any business committed to good technology and good data reporting .

No LED luminaire should have a Power Factor of less than 0.85.