1990 Thorn M58/D

In 1982 Thorn introduced its 50 W Lightstream M50, a breakthrough low-voltage display lamp derived from compact tungsten halogen reflector sources employed in film and diapositive projectors since the 1960s. The new lamp features a redesigned halogen capsule with a longer service life (2000 h vs. 20–100 h) suitable for the intended application, and a new mirror pattern with hexagonal facets providing a superior beam homogeneity. The new lamp proved highly effective and could replace standard incandescent reflector lamps of twice the wattage thanks to its more efficient optical design and operation. Consequently, the M50 (like similar lamps from the competition) became an instant success and soon Thorn expanded its Lightstream family of lamps with wide variety of different models (different sizes, wattages, and beam angles).

The M58/D shown here is one of the many offsprings of Thorn's original M50. However, the present lamp is highly unusual in the fact that this is a high-CCT type with a dichroic mirror designed to leak more red light that usual, resulting in a light color temperature of 4200 K instead of the 2900–3000 K of standard MR50 lamps. In the latter case dichroic mirrors are designed to transmit infrared light while projecting visible radiation, which allows highly intensive illuminations with a limited thermal load on the lighted products. The M58/D has the added benefit of emitting a cooler light color, ideally suited for the illumination of jewelry and crystalware as a higher light color temperature results in better sparkling effects from gems and lead glass (i.e., what is called "crystal" in the trade).

Interestingly, M58/D relies on a sand-blasted halogen capsule to achieve a 38° beam angle with a homogeneous distribution while using the same reflector as in the M50, which is 21° beam angle lamp. While the capsule's diffusing surface solves the problem of an uneven radial light emission due to the lamp's transversal filament, it also increases light spillage which reduces the peak intensity of the projected beam. Moreover, the loss of visible light to increase the light color temperature results also in a further 40–70 % reduction in beam intensity and in an overall lamp efficacy which is 1.8–3.2-times lower than that of conventional 3200 K MR50 lamps of similar wattage and beam angle. Finally, an intriguing feature of the present lamp is its GX5.3 base, which was the source of many issues with the original M50 and which was upgraded with the improved GU5.3 base with a grooved neck, patented by Thorn in 1984.