Technologies Incandescent bulbs The development of the incandescent bulbs has come a long way since 1802, when Humphry Davy first lit a platinum wire by passing an electric current through it. These bulbs lit the path to the industrial development and we are currently witnessing their dusk.

The abilities of classic incandescent bulbs to resemble sunlight are limited. Their Colour Rendering Index is ultimate: 100. But the trouble is the CCT, the colour temperature, which is only about 2900 K. We see that as a warm yellow light.

This limitation comes from the melting point of tungsten, the metal from which the filaments are produced, which is 3695 K.

The filament of an ordinary bulb operates at a temperature of about 2900-3000 K. Increasing the temperature leads to a dramatic shortening of the bulbs life. For example, projector bulbs with filaments operating at 3300 K are rated for just several tens of hours of service.

A revolutionary approach here is using a filter to reduce the excess yellow light seen. A suitable material to do this has been discovered: neodymium oxide, Nd₂O₃. This powder is mixed into the glass of the bulb, giving it a blue-violet tint. The unique properties of the Nd^III+ are also used enhance contrast in astronomy optics and, most prominently, in infrared lasers.

The filter absorbs part of the light energy at wavelengths around the colour yellow - which we see predominantly in the traditional bulb. The result is a "less yellow", whiter light that can be very well combined with fluorescent lamps.

The Nd₂O₃ glass fluoresces at near infrared (~1300 nm). Part of the yellow light is converted to heat. The filter absorbs 20-25% of the total luminous flux produced by the filament.

The yellow colour is a transitional stage between red and green. As the Nd filter reduces yellow it increases the colour contrast between red and green.

As the filtered bulb is a "coloured" light source (off-Planckian locus), the CRI evaluated by the classical formula only reads about 75-80, which is pretty misleading. A new formula called CQS is being worked on for evaluating the colour quality of LED diodes that would follow the viewer's impression of the given light more closely than the CRI formula currently in use.