||The Warmtone /N mercury lamp was a short-lived colour, having been introduced by Sylvania USA around 1975 and discontinued ten years later. It has a relatively low colour temperature and offers greatly improved colour rendering properties. This was, of course, achieved at the expense of luminous efficacy. The colour was developed to provide a match with Deluxe Warm White fluorescent tubes, and has a similar appearance to incandescent light.
The phosphor is based essentially on the usual europium activated yttrium phosphate vanadate material, but with finer grain size particles, and a tighter control over grain size distribution. The effect of this is to increase the efficiency of the material as a converter of UV radiation into red light.
In addition, the powder contains a second phosphor, this being cerium activated yttrium aluminium garnet. Normally this phosphor produces a yellowish-green emission, but at the high bulb wall operating temperature its emission peak is down-shifted to longer orange wavelengths. Another characteristic of the Ce:YAG phosphor is that it has a slight yellowish body colour. This is because it is activated by long wave UV and its absorption band extends slightly into the blue end of the visible spectrum. Consequently it absorbs deep blue light, and this accounts for its yellow colouration.
Final colour correction is achieved with a top coat of yellow dye, which serves to reduce the intensity of the 404nm and 436nm blue mercury lines, shifting the spectral balance to the warmer red end of the spectrum. /N lamps are therefore rather inefficient, but do achieve a red ratio of around 14% and thus make a good colour match with incandescent.