Manufacturer: Thorn Lighting
Model: A1/8 240V 500W P283/25 (Replacement for DMX type)
Application: Specialist - Projection
Wattage: 500W
Diameter (max): 60mm
Length: 130mm
Tube Length: N/A
Bulb/Tube material: Clear glass
Colour Temperature: 3200K (Based on some internet searches - May not be the same for all lamps of this type)
Peak output wavelength: Broadband
Total light output: Unknown
Rated lifetime: Unknown
Cap: Pre-Focused
Operating voltage: 240V
Operating current: 2.1A
Warmup/restrike time: None
Cost (original): 14.10 (from label on lamp packaging)
Value (now): 5-15 approx (average across various online retailers)
Place of manufacture: United Kingdom
Date of manufacture: Unknown.  Date code B7H on lamp cap
Lamp Status: New, untested
Notes: Projector lamps have always been some of the most nicely engineered, and looking at some of the UHP mercury vapour examples in use today, this is a trend which appears to stand to this day.

Early projection equipment made use of carbon arc lamps for a light source.  These had a number of disadgantages: They were hugely power hungry (requiring a high current, low voltage supply too - which made things even more awkward), flickered, produced copeous amounts of ultra-violet radiation in addition to visible light, were very inefficient, and just to top it all off, required almost continual attention while running.

More recently, professional movie projectors, LCD projectors and the likes have almost exclusively moved over to UHP (ultra high pressure) mercury lamps of various sizes as these provide the necessary high lumen outputs, but in a very compact package without many of the disadvantages of the carbon arc - though they're not without their own challenges!  They produce huge amounts of UV, and require either forced air or water cooling - in comparison though they're downright user-friendly.

In the domestic market, the high cost and complex control gear meant that as far as slide and film projection was concerned that discharge lamps (until LCD projectors started to become commonplace in the last few years) never really gained a foothold.  Instead, some rather specialised incandescent products were designed.

A projector needs a very specific light source, especially in terms of the shape of said light source, so that all of the light can be correctly injected into the optical system.  Smaller, lower wattage lamps tended to do this by using very specially designed reflectors and compact filaments.  Larger lamps such as this however tended to make use of a filament arranged in a flat grid.  Effectively giving a small, intense square of light.  The high loading of these lamps meant that many lamps employed quite complicated mechanisms to prevent the filament sagging during its lifetime, which would have taken it out of the focal point of the optical system.  These in conjunction with specified burning positions and carefully designed envelopes to encourage any blackening to occur outwith the area that light is actually needed from resulted in lamps which had quite acceptable lifetimes for their intended purposes.  A few hundred to a thousand hours doesn't sound like much as far as a light in your house is concerned, but for providing the light source for watching the odd reel of cine film, it's actually quite a lot.  I imagine there are a lot of domestic projectors out there which have long since been retired - and have still got their original lamps in.

As with most lamps intended for use in projection equipment, both new and old, this lamp represents a beautifully well engineered bit of technology.
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This lamp added to the Virtual Display Shelf on the 12th August 2010 at 20:48.
Exhibit Number: 76.


References: Lamp packaging and a Google search.


Acknowledgements: Many thanks to the website reader who donated this lamp (amongst many, many others) for display.