The light output of HMIs are:
(1) close to daylight colour temperature, This means an HMI can be used to augment daylight without filters, or be the sole lighting when using daylight film stock. When used with tungsten film stock orange filters (e.g. 3/4CTO) are generally used. Note: the filters should be high temperature, flame retardant: cheaper theatre gels do melt!.
(2) for a given power rating (watts) more efficient than tungsten/halogen bulbs. The improved efficiency means the lamp head is usually smaller than its tungsten counterpart and easier to move/operate.
HMIs have a purple tinge when first switched on, but turn white as the bulb heats up. Bulbs have a finite life: old bulbs have a reduced output and purple colouration to their output. The bulb glass should be clear, not discoloured or opaque. Bulbs are rather expensive, even 575W are around £100 each. All hire companies expect broken/damaged bulbs to be returned, or else the hirer gets billed for the new tube!
HMI bulbs operate not with a filament but by 'striking' and maintaining a DC plasma current between two electrodes. This is inherently more dangerous than AC current. The conversion to DC requires a 'ballast' between the lamp and mains. The ballast must be earthed for safety reasons. All ballasts have an earth monitor lamp, and the lamp should not be operated if there is no earth detected. The wire wound ballasts have a permanent orange lamp that is a power to earth connection (either loss of earth or mains 'live' will extinguish it). It must be noted this may trip some modern earth leakage detectors. Electronic ballasts usually have a momentary action earth detector switch/lamp.
Wire wound ballasts are known to flicker at 100Hz, at normal camera shutter aperture and speeds (24/25fps) this is not a problem. At fast shutter speeds flicker free electronic ballasts are recommended.
Bulbs should not be touched by hand under any circumstances, else they will discolour, overheat and shatter. The gas in the bulbs is under low pressure when cold but very high pressure when hot. In use the bulbs are actually red hot, and potentially dangerous. The bulbs should be allowed to cool for 10 min. or so before attempting to open the lamp unit. Either the safety glass or full scrim should always be fitted if people are to be in front of the lights.
'Double ended' HMI lamps must only be operated with the bulb in a horizontal position. The 2.5k lamp in particular comes with warnings not to operate it at more than 5 degrees off horizontal.
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Soft
boxes give a spread of light suitable
for background lighting or fill.
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Fresnel
lensed lamps are very directional and can be focused better than reds or blondes.
eg 2k or 2.5k pole operated "Castor".
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Tungsten
lights
Bulbs should not be touched by hand, else they will discolour and overheat.
Halogen bulbs emit harmful Ultraviolet light, for work in close proximity to
the bulbs the safety glass, which also absorbs UV must be used. Either the safety
glass or full scrim should always be fitted if people are to be in front of
the lights in case the bulb shatters. Filters: CTB with daylight film stock.
Fuse rating for tungsten lamps
at 240v (UK):
300W:2A, 500-650W:4A, 750-1kW:6.3A, 1200-2kW:13A, 2500-3250:16A, 5kW:25A, 10kW:50A
(note: amps (current) = power/240 (ie watts or VA/240))
Pulsars
(baby red heads), usually supplied with case comprising:
3 Quartzcolor mod 3130 adjustable spot/flood heads, 3 Barn doors, 2 full mesh
scrim, 2 1/2 scrim & 3 safety glass. Bulbs are Thorn, GEC CP81 FSK, (Halogen
300W) or similar. Fuse rating 2 amp.
Red
Heads, the standard film/video general purpose tungsten filament lighting,
usually supplied in threes, with case comprising:
3 Quartzcolor mod 3142 (or "Arri 800s") adjustable spot/flood heads,
3 Barn doors, full mesh scrim, safety glass. Bulbs are Thorn , Ushio JPD 240v
650w-C (Halogen 650W) or similar. Bulbs around £10 each. Fuse rating 4 amp.
Blondes,
general purpose tungsten filament lighting, usually supplied singly
or in pairs,
Quartzcolor model 3150 2k adjustable spot/flood head, Barn doors, full mesh
scrim, ½ scrim and safety glass. Bulbs are Thorn , Ushio JPD 240v 2kw-C (Halogen
2kW) or similar. Bulbs around £20 each. Fuse rating 10 amp.
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Blue Screen:
1. Bear in mind that the DV formats are not the best for blue & green screen
since it is compressed 5:1 with either 4:1:1 or 4:2:0 rather than 4:2:2 colour
system (technically: some of the chroma colour information is missing). DV pictures
also tend to smear edges over 4 or 5 pixels so the edge of the object in front
of the blue screen will never completely separate from the blue, selecting a
best will always be a compromise. BetaSP, DigiBeta or Digital-S works better
in this context. If you can, do the blue screen straight from camera into a
mixer (ie before the signal is compressed). Most professional video mixers use
RGB inputs, the Y/C output from most consumer to DVCam/Pro will need to go through
a decoder first.
2. Use white light to flood the blue screen - just ordinary tungsten light - although some people like to blue a blue or green gel on the light - it depends a little on what software or hardware you are using for keying (Ultimatte recommend uncoloured light). U.V. is not going to help.
3. Basic rule is to keep the light HARD but EVEN with NO SPILL on the subject. Soft light bounces around more and produces a less saturated colour on the blue screen. Use a couple of blondes (2kw) or red heads (800w) set to flood with some light diffusion (light tough spun) to even out the light. Use a light meter to make sure the light is even across the screen.
4. Light the actors with a normal key/fill/back set up - but make sure there is no blue or green light from the screen spilling into the shadows. Make sure they are far enough away from the screen that they can be lit independently (i.e. the light on the screen is not hitting them and vice versa). Some people put a light complementary colour gel (i.e. yellow with blue screen, magenta with green screen) on the back light to compensate for spill but this may create more problems than it solves. It is often easier to set your exposure for the blue screen (which obviously needs to be bright but not too bright so it is over exposed and begins to bleach out) and then light your actors to match the exposure you've already set. You can judge this on a monitor - the screen should be as bright as it can be without losing colour saturation.
Radio Mics:
Radio mics allow more freedom of movement for presenters/actors but are more expensive to hire and prone to reception problems. There are 2 frequency bands:
(1) VHF (for which no UK licence is required, 173.8MHz, 174.5MHz, 175MHz and less common 174.1MHz and 174.8MHz). in an open field range is 200m but in a steel and concrete building this will reduce to 10-30m. Receivers can be single freq. or dual (diversity) receivers. Single receivers are more prone to drop outs. VHF frequencies limit the no of mics in use at one time to 3 (or 5 if your system has the 174MHz bands too).
(2) UHF or Wideband FM (channel 69). Requires licence to use. Signals go further for the same power output. Over 50 mic channels available, so less chance of interference. More expensive to buy/hire.
Many microphones require a power supply to 'bias' the signal (to eliminate
crossover distortion). Some unbalanced
mics get power from the camera, or an internally fitted battery. In situations
requiring long cables and balanced lines typically batteries are used eg with
AKG B18 phantom adapter (for XLR connected mics).
connections: XLR: (1) screen & neg. earth (2) blue (3) red.
balanced/unbalanced mics. Long cables to mics increase the possibility of noise via magnetic or radio interference. This is reduced by using balanced line cables (2 wires and an overall screen). the signal wires being electrically isolated from the mics amp by a transformer. The balanced line requires a voltage across it, usually supplied by a 'phantom power' unit.
pick-up hum/noise on balanced line cables: usually caused by incorrect wiring such that the screen is either not earthed (ie 'floating' and acting as an arial) or has a voltage drop along its length (as occurs if it is connected at both ends to a supply voltage). Ideally the screen should only be conected to earth at one end (pin 1 being the conductor of earth through the cable, not the conector shell).
video
printer. An under utilised piece
of equipment! Enables an immediate hard copy picture from a video camera or
video assist. Usually as an aid to framing 'clever' cuts, or for continuity.
