PLW7070* Series I2LED High Power LED Colour Over Angle Report

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Introduction White LED light sources usually exhibit a variation in their emitted spectrum as a function of the angle from the perpendicular from the centre point of the light source. This colour variation over angle (COA) can lead to noticeable effects in far-field colour distribution. The cause of the variation is due to; the natural blue emission distribution of the LED chip, the architectural and reflective nature of the package and the behaviour of the phosphor. The cause of COA may be explained as follows. 

The LED chip emits blue light (~455nm) with a spatial distribution from all points at the emitting surface:

Figure 1: Emission from one point on the LED emitting surface 

The composite spatial emission from the total LED emitting surface is generally represented by a Lambertian (Cosine) angular profile:

Figure 2:  

Lambertian (Cosine) angular emission profile

The energy of the emitted photons at ~455nm excites a phosphor layer that covers the LED chip and this re-emits at a range of colours across the spectrum but, with notable red and green components. The emission of the phosphor is carefully designed to achieve the required colour

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

temperature when combined with the quantity of blue light that escapes without conversion. The light is emitted over a wide range of angles and the effective thickness of phosphor varies with angle. Therefore as the level of blue absorption increases with angle the level of red and green increases. This is observed as a change in colour with angle that can take the emitted light away from the desired colour temperature on the black-body curve.

Figure 3: Blue attenuation and red/green enhancement with increasing path length through the phosphor.

Plessey PLW7070* Series The Plessey PLW7070* series of high performance, high power, white LEDs employs a carefully designed surface phosphor layer to produce an excellent representation of a black body emitter over a range of colour temperatures.

Figure 4:

PLW7070GA, 12V, high power LED.

The series are produced using a Plessey custom aluminium nitride ceramic substrate for low thermal resistance and, combined with Plessey’s unique GaN/Si LED technology, provides a long and stable operating life in the most demanding high power lighting applications. In common with other LED light sources the product has a colour variation over angle. This has been measured under careful laboratory conditions and is illustrated here.

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Variation of Spectrum with Angle

Figure 5:

Spectrum at an angle of 0 degrees to the perpendicular.

Figure 6:

Spectrum at an angle of 30 degrees to the perpendicular.

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Figure 6:

Spectrum at an angle of 30 degrees to the perpendicular.

Figure 7:

Spectrum at an angle of 90 degrees to the perpendicular.

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Variation of Colour Coordinates with Angle The effect of change in spectrum over angle is a change in CCT over angle. The observed change in colour coordinates over angle for a nominal CCT of 4000K in shown in the figure below. In this chart the observed net colour coordinates as measured using a sphere is shown for this particular device:

Figure 8:

Change in colour coordinates versus angle for a nominal CCT of 4000K.

Summary The colour over angle behaviour for the PLW7070* series shows competitive performance. In lighting design the behaviour of the products meets demanding applications and can be readily simulated using the ray files that are available.

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Legal Notice Product information provided by Plessey Semiconductors Limited (“Plessey”) in this document is believed to be correct and accurate. Plessey reserves the right to change/correct the specifications and other data or information relating to products without notice but Plessey accepts no liability for errors that may appear in this document, howsoever occurring, or liability arising from the use or application of any information or data provided herein. Neither the supply of such information, nor the purchase or use of products conveys any licence or permission under patent, copyright, trademark or other intellectual property right of Plessey or third parties. Products sold by Plessey are subject to its standard Terms and Conditions of Sale that are available on request. No warranty is given that products do not infringe the intellectual property rights of third parties, and furthermore, the use of products in certain ways or in combination with Plessey, or nonPlessey furnished equipments/components may infringe intellectual property rights of Plessey. The purpose of this document is to provide information only and it may not be used, applied or reproduced (in whole or in part) for any purpose nor be taken as a representation relating to the products in question. No warranty or guarantee express or implied is made concerning the capability, performance or suitability of any product, and information concerning possible applications or methods of use is provided for guidance only and not as a recommendation. The user is solely responsible for determining the performance and suitability of the product in any application and checking that any specification or data it seeks to rely on has not been superseded. Products are intended for normal commercial applications. For applications requiring unusual environmental requirements, extended temperature range, or high reliability capability (e.g. military, or medical applications), special processing/testing/conditions of sale may be available on application to Plessey.

Contact Customer Enquiries/Sales +44 1752 693000 | [email protected] www.plesseysemi.com Plessey Semiconductors Ltd | Plymouth Tamerton Road, Roborough Plymouth, Devon PL6 7BQ United Kingdom P: +44 1752 693000 F: +44 1752 693700