1967 Philips SOI-45W DoKa

This rare and unusual sodium lamp was made for a very few years by Philips in the Netherlands in order to cater for the specialized application of darkroom lighting. To this end, a standard 45 W SO-I lamp is provided with a thick outer layer of orange paint which filters out any blue light emitted by the noble gas fill and which could affect photographic papers and films.

Low-pressure sodium lamps made in the 1960s and earlier featured a particularly narrow discharge tube so as to obtain a sufficiently high power dissipation as required with their primitive thermal insulation (i.e., an internal glass sleeve which radiates parts of the infrared energy back towards the burner in the case of SO-I lamps). However, such design results in high current density and electric field strength in the plasma, which enhances the emission of Sodium’s spectral lines in the green and blue parts of the spectrum. Moreover, the electric field is strong enough around the electrodes to excite the noble gas atmosphere of the burner, resulting in additional neon and argon lines in the lamp’s emission spectrum. Although inconsequential in general lighting applications, this parasitic light emission can affect light-sensitive papers and films, and the use of sodium lamps in darkroom environments thus requires the use of a proper optical filter. In the present lamp this takes the form of a superficial coat of paint, which is thick enough to block all blue and green lights. However, scattering losses in the paint results in a light output which is only a fraction of that of standard 45 W SO-I, but still sufficient for the intended application.

Philips does not seem to have listed this particular lamp in any of its catalogs and datasheets and it is possible that the Dutch made this modified version of their standard sodium lamps for original equipment manufacturers (OEMs) only. Interestingly, this SOI-45W was not the first sodium lamp used in photographic applications since older SO/H lamps (with a detachable Dewar jacket) were employed in dedicated luminaires provided with colored glass windows.