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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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