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Talking Photometry: LED Colour Zoning

The most common technique for generating white light from an LED is to take a blue LED and shine the beam through a material that exhibits phosphorescence, i.e. a phosphor. In a phosphor, light of shorter wavelengths is absorbed and re-emitted at longer wavelengths. The combination of blue light from the LED combined with the green to red light emitted by the phosphor creates white light. The trouble with simple "phosphor-white" LEDs is that differences in the optical path length through the phosphor can lead to significant differences in the correlated colour temperature (CCT) at different angles from the LED. This is known as colour zoning and is a very undesirable feature.

Colour Zoning

The picture above illustrates the problem of colour zoning from LED spotlights. This is a true colour image (no Photoshop shenanigans!) in which there is a high colour temperature (blue white) central zone surrounded by an area of low colour temperature (yellow white). The problem occurs when the blue light from the LED passes through the phosphor coating. In the forwards direction, a minimum of blue light is converted to longer wavelength light by the phosphor, hence the light in that direction appears more blue (high CCT). However, blue light passing though the phosphor at higher angles is more absorbed due to the longer optical path length and hence more blue light is converted to longer wavelengths. Consequently, light at higher angles appears more yellow (low CCT).

LED CCT Variation

We recently had a sample spotlight in our lab which exhibited a marked colour zoning problem. To quantify the performance, we carried out a measurement using our Radiant Zemax NFMS imaging goniophotometer. Using our ProMetric™ imaging colorimeter, we set up a goniometer scan to record the chromaticity and CCT as a function of angle. The results are shown in the chart below.

 In the centre of the beam (at 0° inclination), the colour temperature (CCT) reaches a maximum of 7,500 Kelvin (giving rise to a distinctly blue hue). At about ± 30°, the CCT has dropped to a minimum of about 4,500 Kelvin (leading to the more yellow-white hue shown in the photo to the left). Thus, from 0-30°, the CCT has dropped by about 3,000 Kelvin. When you consider that the minimum detectable difference in colour temperature for the human eye is about 50 Kelvin, one can readily understand that this level of colour variation is a real problem. As an aside, although it is hard to spot in the photo due to the lower levels of light, the CCT again raises as we approach ± 90°. This was one very colourful spotlight!

Fortunately, not all LED-based solid state lighting (SSL) suffers from this level of colour zoning. Photometric Testing is one of the few UK lighting test laboratories able to provide measurements of this type. Whether you wish to determine the level of colour shift which your product exhibits as part of a devlopment study, or perhaps you want to give yourself a competitive edge and demonstrate that your particluar luminaire avoids this problem, we're here to help. Use our enquiry form to request a quotation for a colour goniometric measurement.