Colour adaptation properties of the eye

(Prism by Isaac Newton – Photo courtesy of Petrucci Marco Lighting Design)

The technical data sheet of each individual luminaire and light source contains a technical fact of fundamental importance: the colour rendering index. Light sources can have very different colour rendering characteristics, depending on the emission spectrum.  In order to achieve colour rendering similar to that which we are used to during daylight hours, the light source must have a continuous spectrum (see incandescent lamp spectrum), while light-emitting diodes have relatively irregular spectra.

The colour rendering index (CRI) varies on a scale of 0 to 100. Incandescent lamps generally have a CRI of 100, while most light-emitting diodes have a CRI of 80. Some have a CRI of 97 and LED strips have a CRI of 98.

The colour rendering index is a system derived from eye-vision experiments to assess the impact of different light sources on the perceived colour of objects and surfaces. Two different light sources that have the same colour in X and Y in the colour diagram, or the same colour temperature, may not have the same spectral distribution.

If you buy two light-emitting diodes of the same brand or series, despite being identical on the product sheet, they will emit slightly different light in terms of colour homogeneity and brightness. This phenomenon does not occur with classic incandescent or halogen bulbs.

When LEDs first appeared on the market, they were very different from each other despite coming from the same production line. Thanks to advances in technology, they have now improved considerably. However, there can still be small differences in colour temperature (measured in degrees Kelvin), in the amount of light emitted (measured in lumens), and in colour rendering (measured in CRI). These are caused by unpredictable factors, such as tolerances in the components used during the manufacturing process, depending on the quality of the phosphor layer deposited on the LED chip. Unfortunately, this means that differences in LED lights can be significant enough to be recognizable to the human eye.

One characteristic that affects the quality of light from an LED source is the proximity of the emission to the black body line, as shown in the CIE chromaticity triangle or diagram in use since 1931.

The evolution of production will undoubtedly lead to improvements in colour rendering but without ever achieving the results of an incandescent or halogen lamp.

So how can a halogen source be replaced by an LED source? 

If we look at a ‘naked’ halogen bulb (also devoid of the physical and historical presence of the incandescent bulb ampoule), the eye perceives a bright, dynamic and sparkling light. The sparkling light effect is difficult to find when looking at an LED source.