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Definitions
of Terms
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What
is a Fluorescent Lamp?
A fluorescent lamp is a glass tube within which an electrical discharge
is maintained through a cocktail of mercury and various inert gases.
The bombarding radiation produces excited atoms and these emit photons
as they fall back to ground state. A thin layer of phosphor on the
inside surface of the glass tube is then caused to fluoresce in
the visible light spectrum. The choice of gases determines the colour
of light. Most photographic fluorescent lamps are designed to match
daylight colour temperature.
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What
is an Incandescent Lamp?
An incandescent lamp is a source of light that contains a solid,
such as an electrically heated filament. These can range from normal
household lamps to photographic tungsten halogen lamps. All incandescent
lamps will require some degree of colour correction to match daylight.
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What
is an HMI Arc?
The term HMI is a trademark of Osram. They
own the various patents. The original inventors are two German scientists:
Bernard Kuhl and Werner Block. The "H" represents the
chemical symbol for mercury (Hg, Hydrargyrum in Latin). The "M"
is Metal Halide from the Rare-Earth Group (Dysprosium, Thulium and
Holmium). The "I" refers to the Iodine combined to create
the Halogen cycle, a very common "cleaning cycle" in Tungsten
incandescent lamps that prevents the Tungsten from the electrodes
or filaments from depositing a black residue on the inside walls
of the lamp and lowering the output.
Confusion regarding the term HMI arises from the fact that in the
beginning the above components were kept as a trade secret and the
temporary answer was that "M" represented, not the above,
but 'Medium Arc', to define the length of the arc gap in comparison
to other discharge lamps in production.
A fluorescent has a long arc gap and a Xenon lamp has a short one.
So it all sounded logical and convincing, but as a result we now have a situation where other manufacturers arc lamps are now referred to as HMI.
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What
is Inverse Square Law?
The closer your light is to the subject,
the brighter the illumination will be. The rule is, for a fixed
beam angle, when you double the distance the same light is spread
over four times the area.
This is known as the Inverse Square Law, and it was for this reason
that we designed the Paglight with a spot-to-flood facility.
This gives you the ability to change the beam angle and recover
light that may have been lost outside the frame area. Conversely,
you can spread the light to reduce the exposure, even when using
the Softlight Diffuser Kit. This technique is especially useful
when balancing a foreground subject with respect to a surrounding
or background scene, and unlike using a dimmer it does not affect
colour temperature (see 'Colour Temperature & Dimmers').
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