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Talking Photometry: Why High CCT Boosts Scotopic Lumens

Simple photometric lux and lumen measurements will misrepresent the true bightness of white light LEDs under conditions of low ambient illumination. The blue light peak of phosphor white LEDs boosts scotopic performance and - in effect - gives "free lumens" under low light conditions as the following article explains.

Scotopic Photometry   Photometric measurements are performed using apparatus that is designed to spectrally match the daylight adapted response of the eye, this being the CIE photopic observer function which peaks at 555nm (green). This applies when the source luminance is above 3cd/m2. However, when the eye is fully dark adapted, the photopic response no longer applies - instead the eye adopts the scotopic response which peaks at 507nm (blue/green). The scotopic response applies when luminance is less than 0.01cd/m2. Human vision doesn't immediately switch from daylight/photopic to dark/scotopic vision – it takes about 30 minutes for the eye to transition from fully light to fully dark adapted. At luminance levels between 0.01 and 3cd/m2, vision involves some degree of both photopic and scotopic response. We call this the mesopic region.

As the light level drops, we transition from full photopic vision to photopic vision with some scotopic to scotopic vision with some photopic and then finally we get to full scotopic vision in the dark. The peak sensitivity of the eye when fully dark adapted (scotopic vision) is about 1.8x that of the fully daylight adapted eye (photopic vision), comparing photopic at 555nm with scotopic at 507nm. Although a simplification, we can say that the human eye is better tuned to blue wavelengths at low light levels than under conditions of high ambient illumination. This explains why we perceive white light LED lamps (which are based on a blue LED emission with a phosphor coating) as being “brighter” than alternative light sources even though photopic lumens or lux readings may say otherwise.

This phenomonon is explained by the fact that the lux meter (photometer) is measuring the light source with a daylight-adapted photopic response. In other words, the simple lux meter is giving a misleading reading. We hear anecdotal reports of LED street lights appearing brighter than alternative sources even though the lux meter is saying the opposite. In these cases, we should remind ourselves to trust our eyes - after all, as human beings, we are the perfect photometer (although we are not calibrated for absolute readings!)

Using a spectroradiometer, Photometric Testing is able to measure both photopic lumens and lux as well as scotopic lumens and lux. Our equipment allows us to report both photopic and scotopic values and hence establish the true scotopic to photopic gain (or multiplication factor). Some manufacturers will quote both photopic and scotopic values in their datasheets as this helps to explain a real benefit of LED technology, however for standard photometric data files (.ldt, .ies) we do not recommend using anything other than photopic values for now. To use scotopic values would cause confusion and potentially lead to issues when the end customer verifies the lighting scheme using a simple (daylight response) light meter. We say that it is better to stick with photopic values for the lighting scheme but explain separately that the actual, perceived brightness will be to some degree higher based on the eye’s tendency to favour blue light in lower light level conditions.

The spectral sensitivity of the eye transitions gradually from photopic to scotopic depending upon the actual light level. It is not currently possible to perform mesopic light measurements, however CIE have published Technical Report 191:2010 as a reference for mesopic photometry. We expect an international ISO standard for a unified system of photometry will be developed based upon the CIE report. The actual improvement in the perceived “brightness” of an LED over a competing technology will depend upon the light level and spectrum of the two sources. The light level will determine what proportion of photopic lumens or lux and what proportion of scotopic lumens or lux are used to calculate the effective mesopic lumens or lux.

To learn more about the CIE's unified system of photometry, please read the article CIE AND MESOPIC PHOTOMETRY published by Professor Liisa Halonen and Dr Marjukka Puolakka, Chairman & Secretary of CIE TC1-58.