XPost: sci.electronics.basics, sci.engr.lighting   
   From: oj@invalid.invalid   
      
   "Frederick Williams"  wrote in message   
   news:512E90AC.163CAAFC@btinternet.com...   
   >I know one can dim filament bulbs with triacs (or similar).   
   > I think I know that one used not to be able to dim fluorescent lamps by   
   > the same or similar means.   
   > But can one dim low power lamps _of some kind_ by _some means_?   
   > What is a low power lamp?  Is it (at present) a fluorescent lamp?   
   > Etc.   
   >   
   > The questions are prompted by strip lights outside a brach of Sainsburys   
   > that are on all day even in bright sunlight.  I would have thought that   
   > lamps could be installed the brightness of which could be controlled (in   
   > an inverse fashion) by the brightness of the incident sunshine.  But   
   > would that require the strip lights to be replaced as well?   
   >   
   > Please not that my knowledge of things electrical is limited.   
   > --   
   > When a true genius appears in the world, you may know him by   
   > this sign, that the dunces are all in confederacy against him.   
   > Jonathan Swift: Thoughts on Various Subjects, Moral and Diverting   
      
   Pretty much any commonly used lighting technology (incandescent, fluorescent   
   (T8, T12, T5, T5HO), compact fluorescent, metal halide, LED, high pressure   
   sodium, etc.) has means to do dimming.  The  costliness varies.  Sometimes   
   special 'dimming ballasts' are needed wiht the fixtures, and very often,   
   special dimmers are needed.  And yes, lighting can be controlled based on   
   daylight (search 'daylight harvesting').  While I do not know what   
   Sainsburys is (and I am assuming that 'brach' is just a typo for 'branch'),   
   in a scenario with outdoor fixtures mounted on a building exterior it would   
   be typical to do less intelligent controls.  At least it would be typical in   
   much of north america.  Either photocell control, or a timeclock, or a   
   combination of photocell and timeclock, might be used.  The lights would   
   just be turned off and on in a more or less 'dusk til dawn' manner.   
   Astrological timeclocks are available that can accommodate changing   
   sunrise/sunset times with the seasons, and beyond that they can be   
   programmed with routines for weekdays, weeknds, holidays, and all sorts of   
   stuff.  And they're not that expensive.   
      
   'Low power' may not be a well defined term, maybe 'efficient' is a better   
   term (depending on what you are actually asing, of course).  Certain   
   technologies are more efficient than others, but if you want lots and lots   
   of light out of an efficient luminaire, you still need lots of power.  All   
   of the lighting technologies mentioned above are OK for efficiency, except   
   for incandescent which is not good for efficiency.  As rough numbers, I'd   
   say you can get 70 lumens per Watt to 140 lumens per Watt out of the   
   efficient technologies above.  Fluorescent is probably around 100, and   
   widely used in normal indoor commercial applications.  High pressure sodium   
   and metal halide are often used outdoors, with efficiencies between say 80   
   Lumens/W and 140.  Incandescent is probably 10-20 lumens per watt.  LED   
   lighting is changing fast.  There is a ton of research money and effort   
   going into LED and it may well be the future of lighting.  I don't think it   
   is king of efficiency yet for bulk lighting, generally speaking.  Though for   
   some people it is the go-to technology.  And the marketing hyping it is at   
   times perhaps a little ... zealous.   
      
   There are other newsgroups where you might get a better response on a   
   question like this.  I'm a little rusty on them but   
   alt.enegineering.electrical might be one, sci.engr.lighting another.  I'll   
   crosspost them in this reply.   
      
   j   
      
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