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High Pressure Mercury Lamps |
candlepower [candela], hence the name (fig. 6-13). A
candle radiates light equally in all directions. If you
imagine such a source surrounded by a transparent
sphere of one foot [meter] radius (figure) than by
definition, the amount of luminous energy (flux)
emanating from one square foot [meter] of surface on
the sphere is one lumen [lumen], abbreviated lm.
Since there are 40 square feet [meters] of surface
area in such a sphere, it follows that a source of one
candlepower [candela] intensity produces 40 or 12.57
lm (a lumen is a unit of light quantity), and in terms of
power is equal to 0.0015 watt. It therefore also follows
that 1-cp [cd] source produces 12.57 times 0.0015 watt;
that is, 0.0189 watt or approximately 1/50 watt of
luminous energy. The lumen, as luminous flux, or
quantity of light, is comparable to the flow of current in
an electrical circuit
One lumen of luminous energy occurrence on one
square foot of area produces an illumination of one
footcandle (fc). When the area is expressed in square
meters, the illumination is expressed in lux (lx). If you
were to consider a light bulb to be comparable to a
sprinkler head, then the amount of water released would
be the lumens and the amount of water per square foot
(meter) of floor area would be the footcandles [lux].
The metric unit, lux, is smaller than the corresponding
order to change footcandle to lux, you would multiply
by 10.764.
Restating what you have just learned
mathematically, it would look like this:
footcandles =
lumens
square feet of area
Or
lux=
lumens
square meter of area
High Intensity Discharge Lighting
Efforts to improve the power efficiency and reduce
the maintenance costs led to the development of a new
family of lighting that has been generally categorized as
high--intensity-discharge lamps (HID). These lamps
all have a negative-resistance characteristic. This
means that the resistance decreases as the lamps heat up.
As the resistance decreases, the current increases. In
fact, the current will increase indefinitely unless a
current--limiting device is provided. All gaseous
conduction HID lamps, therefore, have current limiters,
called "ballasts." Lamp life and more light per watt are
two main advantages that HID lamps have over
incandescent bulbs. The basic types of HID lamps used
unit, footcandles, by a ratio of approximately 10 to 1. In
in area lighting consist of three groups of lamps:
Figure 6-13.—Relationship between a light source of one candlepower and the illumination produced.
6-12
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