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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.


Keywords: Lamps

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.

Philips_SON21C_50W_Sapphire_-_NL_l1970s.jpg Sylvania_MS40021PS21BU-Only_-_USA_2000.jpg Philips_SOI-45W_DoKa_-_NL_1967_a.jpg Xycarb__255860_2000W_120V_-_FR_1995.jpg Thorn_M5821D_-_EN_1990.jpg
Lamp/Fixture Information
Manufacturer:Philips
Model Reference:Philips SOI-45W DoKa
Lamp
Lamp Type:Low pressure sodium
Filament/Radiator Type:Nonthermal discharge in neon and sodium vapor
File information
Filename:Philips_SOI-45W_DoKa_-_NL_1967_a.jpg
Album name:Max / Nonthermal discharge lamps
Keywords:Lamps
Filesize:190 KiB
Date added:Aug 13, 2024
Dimensions:1200 x 403 pixels
Displayed:9 times
DateTime Original:2018:01:12 16:16:05
Exposure Time:1/34 sec
FNumber:f/8
File Source:Digital Still Camera
Flash:No Flash
Focal length:39 mm
ISO:200
Model:X-T1
Software:Adobe Photoshop CS5 Windows
White Balance:0
URL:https://trad-lighting.net/gallery/displayimage.php?pid=232
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Comment 1 to 5 of 5
Page: 1

Tuopeek   [Aug 13, 2024 at 06:32 PM]
Interesting to see a dedicated darkroom sodium lamp. As you mention, I have also seen a darkroom lighting unit using a SO/H version but just a standard 140W version. Seemed like a large wattage for a darkroom light but it had a very heavy filter with a similar orange coloured filter on the inside and a dark red on the outside sandwiched together.
Ria   [Aug 13, 2024 at 07:52 PM]
We have a 35W SOX version of this, must find the picture of it. Also one of the Metrovick SO/H versions. Still lots to upload Very Happy
Max   [Aug 13, 2024 at 09:10 PM]
@Tuopeek - It's no wonder they needed the output of a 140 W SO/H with that much optical filtering...
Sammi   [Aug 17, 2024 at 06:28 PM]
Between us we've got the three ages of LPS safe lights, the SO/H, SOI/H and SOX. LPS Love
Max   [Aug 18, 2024 at 08:54 AM]
Yes, we do indeed, which is rather interesting!

Comment 1 to 5 of 5
Page: 1