08 TV Lighting Induction

7/29/2019 08 TV Lighting Induction

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TV LIGHTING

1. GENERAL PRINCIPLES

Lighting for television is very exciting and needs creative talent. There is always atremendous scope for doing experiments to achieve the required effect. Light is a kindof electromagnetic radiation with a visible spectrum from red to violet i.e. wave lengthfrom 700 nm to 380 nm respectively. However to effectively use the hardware andsoftware connected with lighting it is important to know more about this energy.

Light Source

Any light source has a Luminance intensity (I) which is measured in Candelas. Candelais equivalent to an intensity released by standard one candle source of light.

Luminance Flux (F)

It is a radiant energy weighted by the photopic curve and is measured in Lumens. OneLumen is the luminous flux emitted by a point source of 1 Candela.

Illumination (E)

It is a Luminous Flux incident onto a surface. It is measured in LUMENS/m2, which isalso called as LUX. A point source of 1 candela at a uniform distance of 1 metre from asurface of 1 square metre gives illumination of 1 LUX.

Luminance (l)

It is a measure of the reflected light from a surface. Measured in Apostils. A surfacethat reflects a total flux of 1 lumen/m2 has a luminance of 1 Apostils.

1m

1 LUMEN

1m

1m

1m

PointSource1 Cd.

Fig. 1 Illumination of 1 Lumen from a surface of 1metre/m2


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Induction Course (TV)

STI(T) Publication 58 003/IC(TV)/2004

Elementary theory of light also says that :

A

F

d

1EHence

AFor

AreafluxIncidentTotaltoequalalsoisThis

d

1

)sourcethefromcetandis(

1E,ationminIllu

2

22

Colour Temperature

One may wonder, how the light is associated with colour. Consider a black body beingheated, you may observe the change in colour radiated by this body as the temperatureis increased. The colour radiated by this body changes from red dish to blue and then towhite as the temperature is further increased. This is how the concept of relating colour

with temperature became popular. Colour temperature is measured in degree Kelvin i.e.)273C(

o. The table below gives idea about the kind of radiation from different kinds

of lamps in terms of colour temperature.

Standard candle 1930o KGas filled tungsten lamp 2760o KProjection bulb 3200o KFlash-bulb 3800o KHMI lamp 6500o KElectronic flash tube 6000o KAverage day light 6500o KBlue sky 12000 - 18000o K

By measuring the energy content of the source over narrow bands at the red and blueends of the spectrum the approximate colour temperature can be determined. All thecolour temperature meter are based on this principle.

2. COLOUR FILTERS AND THEIR USE

Colour filters are used to modify the colour temperature of lights and to match colourtemperature for cameras while shooting with different colour temperature, e.g., indoorand outdoor etc.

Generally it is normal to correct daylight to produce tungsten quality light by using

orange filters. However, when the amount of such light to be corrected is more, it maybe more practical to convert tungsten lamp to daylight, but with a considerably reducedlight output form the luminaries by using blue filters.

For certain situations like, sunrise and sunset the light is more yellow than midday, so acareful note should be made of the Transmission factor of each of the filters. Often acompromise has to be reached in terms of correction and light loss. For example full


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TV Lighting


correction may cause too much loss of light, so "half correction", although not ideal, maybe used.

Full colour temperature blue 37% 3200 - 5700 o K1/2 colour temperature blue 56% 3200 - 4300 o K2/4 colour temperature blue 69% 3200 - 3600 o K

Full colour temperature orange 55% 5700 - 3200 o K1/2 colour temperature orange 70% 5700 - 3800 o K1/4 colour temperature orange 80% 5700 - 4600 o K

Neutral Density Filters

In addition to colour temperature correction some times it may be necessary to reducethe intensity of day light at an interior location. Neutral density filters are available whichcan be used to attenuate exterior light. Three filters are available :

0.3 Density which has a transmission of 50%.0.6 Density which has a transmission of 25%

0.9 Density which has a transmission of 13%

Combination of CTC Filters and neutral Density Filters

Single filters exist which are a combination of full colour temperature orange and neutraldensity as follows :

Full orange + 0.3 N.D. with a transmission of 50%.Full orange + 0.6 N.D. with a transmission of 38%.

The HMI light source has a colour temperature of about 6000o K and can be used withexterior day light without the need for a colour temperature correction filter.

Mired Rating for Filters

The change of colour temperature of a filter is usually measured in mired (1 mired =1,000,000)/o K. This is because the same filter which reduces a colour temperature of3,200o K to 2,500o K would reduce a colour temperature of 7,000o K to 4,500o K.

Example : Suppose a C.T.C. filter used on a light source of 3000o K changed the colourtemperature to 4000o K then :

MIRED value of light source MIREDS333

3000

106

MIRED value of light source + filter MIREDS2504000

106

So the MIRED shift produced by the filter is -83 MIREDS

If the same filter were to be used on a light source of colour temperature of 5000 o K wecan estimate the new colour temperature as follows :


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MIRED value of light source 2005000

106

MIRED shift of C.T.C. Filter = -83

New MIRED value of light source plus the filter = 117 MIREDS

So, new colour temperature = 8500o K

The blue filter used in the above example, increased the colour temperature butdecreased the MIRED value.

If an orange filter had been used it would have reduced the colour temperature andincreased the MIRED value.

Important Points About CTC

Colour Correction Filters can be used on cameras, luminaries, and windows.The reduction of light is the same in each case, if the same kind of filter is used.

The difference between day light and incandescent light colour temperature isabout 144 mired.

A blue filter which raises the colour temperature for 90 mired absorbs about 70%of the light passing through. An orange filter which reduces the colourtemperature for the same amount of 96 mired absorbs only 35% of the light.

Since incandescent light with blue filters is very ineffective and uneconomic,carbon arc lamps (brutes) or HMI - Lamps are normally used under day light

conditions. These luminaries have a colour spectrum which comes very close tothat of sunlight and their light output per watt is much higher than that ofincandescent lamps.

If light of different colour temperature is desired in a scene, the followingprocedures are often applied :-

Consider a moonlight night exterior with natural light sources inside. Thedifference between the two will be about 140 mired. Luminaries to simulatemoonlight are filtered blue - (140 mired) and objects illuminated with thislight will appear deep blue while objects under natural sources can beunder white light. If an orange filter of about + 70 mired is used on thecamera, the white light of the natural sources will appear slightly orange,the moonlight will come but less blue.

Luminaries which are to imitate light of natural sources such as candles orkerosene lamps should be filtered slightly orange (about + 70 mired). Filllight must remain white or very lightly blue, as the orange hue of the keylights would otherwise remain unnoticed by the audience.


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TV Lighting


Filtered light in dark areas of a picture will render much higher colour saturation,in very light areas colours become whitish and de saturated. This fact should betaken into account when selecting filters for key lights or fill light.

Under natural sunlight the fill light contains usually slightly more blue than the keylight. If sunlight must be imitated, in a studio, it is advisable to give the fill light a

slight blue hue.

Colour television monitors in a studio have a colour temperature of about 6500o

K. If they are photographed by another camera their screens should be coveredby an orange gel of about - 100 mired.

3. LIGHTING EQUIPMENT AND INSTALLATION

Light intensity : The sensitivity of the camera determines the light intensitiesrequired. This sensitivity will be based on the camera pick up device tube(Vidicon, Plumbicon or CCDs).

Height of Studio grid : The maximum height of the luminaries and their throw isdirectly related to the height of grids. This factor coupled with camera sensitivityfixes the wattage of the luminaries.

Turn round time : The delay one can afford for changing a lighting set updecides the type of suspension system for a studio. More flexible systems offerthe quickest turn round.

Light levels : Required light levels can be obtained by keeping in mind thefollowing considerations. For a lighting level of 500 - 800 LUX, the wattagerequired is about 250 Watts/m2. Similarly for a level of 1000 - 1600 LUX, it is

about 500 Watts/m2.

The Incandescent Light Source

The most common form of light source used in television studios is the incandescent(hot filament) lamp. It is simple in operation and construction. It can easily be dimmed,and it does not require auxiliary equipment like chokes etc. it has a continuous spectrumoutput. The main disadvantages are that most of the power input is dissipated as heatand the lamps have a limited life. New forms of construction however have producedlamps with higher efficiency and longer life.

a) The Basic Tungsten Filament Lamp

Tungsten wire has resistance and is capable of dissipating power in the form of heat. Ifsufficient electrical energy is supplied to raise the filament temperature aboveapproximately 5000 C, light is emitted. Oxygen has to be excluded to avoid combustion.The filament is enclosed in an evacuated glass bulb. The tungsten filament has lightemitting characteristics similar to those of a black body radiator, where high temperatureprovides greater efficiency and higher colour temperature. But this lamp usually failsbecause of evaporation of filament, the higher the filament temperature the higher the


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rate of evaporation. This causes the filament to become thinner, thus reducing the lightoutput and colour temperature. The inside of the bulb is darkened by the deposition ofevaporated filament. This deposit absorbs some of the light thus reducing its intensity.

The evaporation can be reduced by having a suitable gas (which does not attack thefilament) into the bulb. However the gas molecules conduct the heat away from the

filament and in order to conserve the heat the filament is wound as a tight coil. This gasenables them to be run at temperatures several hundred degrees higher than those ofvacuum lamps. Evaporation and bulb blackening still take place in the gas-filled lampand in order to reduce its effect a large surface for condensation is required. Hencethese studio lamps has very large bulb. Even then towards the end of the filament life,this deposited layer reduces the light output and colour temperature to unacceptablelevels.

Increasing the pressure of the gas filling may suppress the ratio of evaporation, but thesizes used are not capable of withstanding more pressure. Use of smaller bulbs inharder glasses or silica permit the use of higher pressures but the advantage gained islost by virtue of the smaller surface area for condensation.

b) Tungsten Halogen Lamps (Tungsten Iodine or Quartz Iodine Lamps)

Tungsten halogen lamp is a major break through in lamp design. By the chemicalremoval of the deposited tungsten on the bulb the light output has been increased to100%. The useful life is at least double that of normal tungsten filament lamps and thelamp is physically smaller.

Fig. 2 Comparison of size and performance of normal Tungsten and tungstenhalogen lamp

Halogen is a general term for a family of very reactive elements like, fluorine, chlorine,bromine and iodine. Each of these combine with tungsten in a reversible reaction whichis controlled by temperature. At present the halogen used is iodine or bromine.

Halogen vapour is colourless and at bulb temperatures between 250 - 8000o C combineswith the deposited tungsten to form tungsten halide in vapour form. At temperatures


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TV Lighting


above 1250o C, encountered in the region of the filament, the tungsten halide dissociates- tungsten being deposited on the filament, and halogen being released to repeat thecycle. It would appear that we now have a lamp with a bulb which never blackens andan ever-lasting filament.The tungsten however does not deposit itself back from where it came but on the coolerpart of the filament. Because of the high bulb temperatures involved and small size the

glass has to be strong, so it is made fused silica (quarts). This also enable to have highgas pressure, which can be 4 times or more than the normal lamps. This reducesfilament evaporation and gives an extension to filament life.

Quartz bulbs should not be touched by hand since minute quantities of acid so depositedwill attack the glass when it is heated. This lamp has an efficiency of 20 Lumens perwatt.

Compact Source Iodide (CSI) Lamp

CSI is gas discharge light source. It produces white light at higher efficiency thenhalogens. This lamp is small and bright. It consists a glass envelope which contain

suitable gas vapour and two electrodes. When suitable voltage is applied between theelectrodes the gas gets ionized. This results in :

A discharge current which has to be limited by some external means, usually aninductor for ac. Operation.

Emission of radiation wave lengths is inversely proportional to quantity of energyreleased.

Increase in temperature and pressure.

The low pressure sodium lamp is a well known example of a discharge lamp. Theradiation produced in a low pressure mercury vapour discharge tube is mainly ultra -violetand use is made of this in the fluorescent tube, the ultra-violet energy excites the

fluorescent power coating of the tube to produce visible radiation. The radiation from amercury vapour lamp within the visible spectrum is increased if the gas pressure isincreased to about 20 x atmospheric pressure.

Fig. 3 CSI Lamp


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Addition of metal iodides in the high pressure mercury vapour improve the colourrendering properties of a light source. These kinds of light sources are called ascompact Source Iodide Lamp. The gas discharge raises the temperature and causesthe metal iodide to evaporate. This causes the metal atoms to release from the iodinemolecule. The metal atoms after collision with electrons in the arc and make asignificant contribution to the total radiation. Protection of the envelope from the

corrosive effects of the liberated metals is by a process like halogen cycle in tungstenfilament lamps. The metal atoms do not stay in the high temperature zones but migratestowards the cooler wall where, recombination takes place between the metal and theiodine atoms to form the metal iodine again. The metal itself is thus not present in thelower temperature zone close to the wall, and the envelope is thereby protected fromcorrosion.

Starting a CSI lamp : The pressure inside a CSI lamp when cold is belowatmospheric. At this pressure a starting pulse of about 10 kilo-volts is required tocause ionization. Once the lamp has been ionized the arc voltage falls to about80V. The warm up period of these lamps is about 30 seconds, during which time

the colour of the light changes from the purple-blue mercury radiation to whiteradiation indicating the evaporation and dissociation of the metal iodides. Whenused with ac. supplies the lamp will be extinguished when the cycle of voltagefalls below 80 V, but since between successive half cycles the gas does not havetime to de-ionize it will strike again as soon as the next half cycles rises above 80V. If the lamp is switched off for a few seconds de-ionization will occur and it isthen impossible to ionize the gas until the lamp cools for several minutes. This isbecause at the working temperature of the lamp the gas pressure is at about 15atmospheres, and would require about four times the available striking voltages.

The CSI lamp has a life of approximately 200 hours. The most common cause of failureis a seal leakage or fracture. Usually before complete failure the light output has falls to

a level which requires the lamp to be changed.

Important points

The CSI lamp when cold has a gas pressure below atmospheric (0.25 atm) and thereis no risk of explosive failure when handling. However, in operation the gas pressurebuilds up inside the lamp (10-25 atm) and there is a risk of the bulb exploding. It isimportant, therefore, that the lamp is always used in an enclosed fitting. The maindanger occurs when a short term mains failure causes the lamp to extinguish, thelamp will not start and gives the impression of a lamp failure. Any action to changethe lamp in these conditions, i.e. when the lamp is hot, is dangerous. It is thereforerecommended that if failure is suspected the lamp is allowed to cool for at least ten

minutes to allow the internal pressure of the lamp to reduce to a safe level beforeany attempt is made to change the lamp.

The CSI lamp emits ultra-violet radiation in addition to the visible light. This ultra-violet radiation is harmful as it can cause severe burning of the skin and damage tothe retina of the eye. To prevent this from happening, avoid looking directly at thelight source and use a glass plate over the open luminaries.


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Dimming Control : The CSI lamp is not suitable for use in applications where dimmingof the light source is required. This is because of the 30 second warm up period and there-strike of 2-5 minutes. 1 kW CSI lamp gives a lamp throw of 75 feet, the illuminatedarea is a 10 foot diameter circle at an illumination of approximately 2000 LUX.

The equipment associated with the CSI lamp namely the current limiting inductor, is

remote from the luminaries. This light is mostly used as a follow spot or an effectprojector.

The HMI Metal Halide Discharge Lamp

Sodium and Mercury vapour were the two main gases used with gas discharge lights.Though these light were more efficient than tungsten lights but these are not popular inTV because of their colour and quality of light.

The improvement in the discharge light spectrum came with the development of themetal halide. In these lamps rare earth elements are added in their halide form toproduce a more continuous visible spectrum and a "whiter" light source.

The HMI Lamp : H stands for mercury, M for medium arc and I for Iodide. The CSI andthe HMI lamps operate on the same principle. They use mercury vapour as the basicgas but the spectrum is largely determined by the rare earth metals that are added intheir halide form. The principle metals used are thallium, dysprosium and holmium, thelatter two being the metals used in current HMI bulbs.

Fig. 4 HMI - Bulb

Wattage rating of HMI bulbs : HMI bulbs are available in a range of sizes 200W, 1200 W, 2500 W and 4000 W. They have a luminous efficiency of about 90lumens/watt and are therefore much brighter than an equally rated tungsten bulb.

Operation of HMI bulbs : A striking voltage of 24 kV is used with HMI to ensurethat the bulb can be instantly restarted when the lamp is hot. This voltage isnecessary to ensure the restrike with the high gas pressure inside the hot bulb.This removes the major problem of CSI lamps

Like all other discharge source a choke is used to limit the current when the lamp isrunning. HMI has a colour temperature of 60000oK. it takes several minutes after firststriking for the lamp to reach this colour temperature. The initial colour is blue due tothe mercury vapour, but gradually the light "whitens" as the metal halides dissociate toadd more red and green content to the spectrum. When viewed with the naked eye orcolour camera the lamp could possible appear more magenta then daylight. In this case


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a white flame green filter will assist in removing the magenta appearance of the lamp.Filters may also be necessary to equalize the colour temperature of HMI bulbs.

With an orange filter the HMI bulb can be made to approximate to tungsten light, but themain use of a high colour temperature source is to mix in with daylight.

High Frequency Fluorescent Light Sources

High frequency Fluorescent light sources emits 70 - 100 lumen/watt as compared to 25lumen/watt of tungsten halogen lamps. Tungsten halogen has very poor efficiency andconverts lot of input into heat in the studios, whereas these lights convert 33% of theinput power into visible light as compared to 10 to 15% of the Tungsten halogen.

A firm of VIDESSENCE in USA has developed this system of high frequency fluorescentlights. The range of fixture available is from 17 watt to 832 watt, with a range of colourtemperature from 2700o K, 3000o K, 3500o K & 4100o K.

Each fixture incorporate an integral dimmer and control electronics. The control

electronics produce app. 33 kHz drive for the lamps. 100% to 20% dimming is possiblewithout reduction in colour temperature. These lights are free from radiations and flicker.

Light Fitting Mountings

a) Stands - portable with varying height.b) Grids - hanging them with the help of clamps and safety chain on grids.c) Various kind of grids :

- Fixed- Moveable, mechanically or motorised- Teleclimbers i.e. motorised grid- Motorised Telescopic bar

- Movable vertical rods with varying heights mounted on a cat walk systembelow ceiling.

Special Effects Light

- Sun/Moon projector- Water roller- Strobe flicker light- Cloud projector- Running water project- Sequential chaser with manual/music control- Par - par lights having parabolic glass reflector to give confined beam.

- Multi ten/Multi twenty - open face light. Size of the broad beam can bechanged with the movement of lamp.

- Follow spot with a circular concentrated beam


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Fresner lenses

This lens has series of sloped concentric ring cut away from its convexed surface. Eachring thus acts as a small prism gathering energy from the lamp and reflecting it intoparallel rays. This lens is used on most of the hard spot lights.

Different Lighting Techniques

- Eye light, Low intensity light on camera itself to get extra sparkle to anactor's eye.

- Rim light, to highlight actor's outline. It is an extra back on entire body atcamera level.

- Kickkar light, extra light on shadow side of the face at an angle behindand to the side of the actor.

- Limbo Lighting, Only object is visible, no back ground light.- Silhouette lighting, No light on subject, background is highly lit.

Fig. 5 Different types of Light Fittings


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Fig. 6 Component of a hard Light

Fig. 7 Different ways of Controlling Lights

Fig. 8 Lighting equipment in side a studio


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4. LIGHTING CONSOLE

In a television production, each scene will require its own lighting plan to give the desiredeffect. In order to assist in setting up a particular lighting plan, a console should provide:-

a) One man operation and a centralised control desk with ability to switch anycircuit.

b) Facilities to obtain good balance with flexibility to have dimming on any circuit.c) With all controls for power at low voltage and current.

Modern lighting consoles also provide file and memory to enable the console operator tostore and recall the appropriate luminaries used for a particular lighting plot. Theseconsole also provide Mimic panels to show which channels are in use and whichmemories or files have been recalled.

Dimmers

Thee basic methods for dimming are :-

a) Resistance : This is the simplest and cheapest form of dimmer. It consists ofa wire wound resistor with a wiper. It is used in series with the load.

b) Saturable Reactor (System SR) : The basic principle of the saturable reactoris to connect an iron cored choke in series with the lamp. The inductance ofthe choke can be varied by saturating the iron core by means of D.C. supplyon a separate control winding. When the D.C. control current is high the ironcore gets saturated causing the change in inductance in series with the load.This results in change in voltage and hence the dimming effect.

c) Auto Transfer : A wiper sliding along the winding varies the turns ratio andthus the lamp voltage.

These systems has the following disadvantages :

Low efficiency i.e. more power loss.

Operator to handle high voltages, not very safe

Costly and bulky

Advances in the technology have resulted in improved reliability, more efficient andcheaper design of dimmers. The operators now have to handle low control voltage witha maximum value of 20 V. These dimmers are called SCR (Silicon controlled rectifier)based dimmers. SCR is basically a semi-conductor rectifier which does not conduct untilit has been "switched on" by a gating pulse. It is a 4 layer semi-conductor, with threeelectrodes - anode and gate. It is connected in series with the load. Dimming isachieved by varying the timing of the gate pulses to control the average load current.


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Thyristor or SCR

It is a unilateral device and conduction take place from anode to cathode under properconditions of bias. The other member of this family is called "Triac" which is a bilateraldevice with three terminals and conducts in both directions. Thyristor and its action canbe considered by seeing it as two transistors connected back to back.

The presence of a positive gate pulse or potential on the base of TR2 turns it on andcurrent flows through TR2 via the base of TR1. This turns TR1 on and current flowthrough TR1 and into the base of TR2, thus holding TR2 on. The gate pulse can now beremoved and the thyristor conduct until the supply is removed and the regenerativeaction of TR1/TR2 is destroyed. The internal feed back makes the thyristor an extremelyefficient and fast switching device.

Fig. 9 Silicon Control Devices

Working of SCR circuit

Consider a variable control voltage applied at the gate of SCR device. At a particularcontrol voltage the SCR will get "on" thus passing the AC voltage to the connected load.The AC to the load will be through only during the time this SCR is on. This will happenafter a control voltage has switched on this device. It can be seen from the waveformthat the power given to the load is only during the part of the positive half cycle. Thisduration can be changed by changing the position of control voltage with respect to areference.

LOAD

AC SUPPLY

VC

LOAD VOLTAGE(HATCHED PORTION)

LOADCURRENT

SUPPLYVOLTAGE

Fig. 10 Working of a SCR Controlled Dimmer


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With electronically controlled dimmers, light output between Zero to maximum can beobtained by changing the position of control or trigger pulses. The trigger pulse areobtained by differentiating the output of a comparator. The comparator gives a pulse atthe moments of intersection of especially shaped ramp voltage, which is synchronizedwith the line voltage, and the control voltage Vc which is variable and controlled by theoperator.

To improve the efficiency of the system a second thyristor is connected across theoriginal, so that both half cycles are utilised. Dimming is thus achieved by varying thetiming of the gate pulses to control the average load current.

SCR SOR

TRAIC S

RAMPVOLTAGE

GENERATOR

PULSEGENERATOR

UNIT

AC LINE

VcVr

Fig. 11 Basic Thyristor Control System

Block Diagram of complete light Dimmer Control System

This block diagram shows three dimmer. Each dimmer is on different phase. Number of

such units may be connected in the actual system, depending upon the loadrequirement. An additional block labeled time base unit provide a reference waveform tothe trigger pulse generator which then controls the dimmer units. Rest of the blocks areself explanatory.

DIMMERUNIT

TIME BASE

UNIT

TRIGGER

PULSEGEN. UNIT

TO OTHERDIMMER

UNITS

R

TO OTHERDIMMER

UNITS

TO OTHERDIMMER

UNITS

DIMMERUNIT

DIMMERUNIT

POWERSUPPLY8V / 15V

CONTROLDESK

CONTACTORS

T

R

S

TREF. R

REF. S

REF. T

CONTROL D.C. VOLTAGE 0-10V

Fig. 12 Block Schematic of a Typical Dimmer System


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Microprocessor Based DTL Data lite Systems used in CPC Doordarshan

The DTL Modular Memory Control System is capable of controlling up to 256 channels.The DTL systems use multiple Z80 processors to calculate the required level for eachdimmer circuit.

The operational control panel surface is scanned by a local microprocessor whichreports any button pushes or fader movements to the processing electronics. Theprocessing electronics may be located away from the control surface, communicationbetween the two units is via a high speed serial link (56 kbit/sec). Each type of controlpanel (memory controller, playback controller, channel controller, or group mastercontroller) has an associated CPU unit in the main electronics rack. The CPUsassociated with a given controller do the limited range of calculations for all of thechannels in the system. e.g., the playback controller handles all playback calculations forall channels in the system. In block schematic the dotted lines show the logicalconnections between the control surface and the CPUs in the processing electronics.Because each processor handles only a part of the calculations needed for eachchannel there must be some way for information to be shared and exchanged between

the CPUs. This data exchange is provided by the common memory block.

In addition to the controller processors two other CPUs make use of the commonmemory - the Output Greatest CPU and the Video CPU. The output Greatest processorscans the working stores set up by the controller CPUs and where a given channel isselected in more than 1 controller the highest level is chosen to be output to the dimmer.The video CPU scans the common memory manager CPU handles the storage of filedata and also manages the serial link to the operational control panels. Data from thecontrol surface is written into the common memory by the memory manager CPU. Anychanges required to the mimic displays on the control surface are written into thecommon memory by the controller CPUs, the memory manager CPU reads the datafrom the common memory and transmits it to the control surface.

How does the system avoid a two CPUs trying to access different addresses in thecommon memory at the same instant in time ? The basic mechanism is to time divisionmultiplex the accesses. A central timer unit feds interrupt pulses to each CPU thatneeds access to the common memory. As soon as the interrupt is received the CPUtransfers data between the common memory and its private workspace.

The frame interrupt signal is connected to all the processor cards in the main processingunit, and synchronises the processes in the CPUs. All of the CPUs are able to executetheir software in the time interval between frame interrupts. Within the memory managerCPU the first action on receipt of the frame interrupt is to transfer data received from theserial interface into the common memory. The various line interrupt pulses cause further

accesses to the common memory. The first line interrupt pulse is used to initiate thetransfer of data between the CMOS file memory and the common memory whenever afile operation is requested. The second line interrupt is used to ensure that anybackground activities (flashes, link operations etc.) are kept active. The third lineinterrupt causes the memory manager CPU to transfer mimic data, tallies etc. from thecommon memory to the serial interface for transmission to the control surface. There isa similar interrupt structure for the other CPUs in the system.


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SERIALINTERFACE

MEMORYMANAGER

CPU

CMOS RAM(32k)

CMOS RAM(32k)

SYSTEMTIMER

TO OTHERPROCESSORS

INTERNAL ADDR BUS

DATA BUS

COMMON MEMORY ADDR BUS (16 BIT)

FRAME INT

LINE INT

LINE INT

PLAYBACKCPU

DATA BUS

COMMON MEMORY

TO OTHER CMOS RAM CARDS

Fig. 13 System Busses and Interrupts in a Datalite System

SERIALINTERFACE

CPU

PLAYBACKCONTROLLER

CMOS RAM(32k)

CMOS RAM(32k)

SERIALINTERFACE

(PANEL)

TO DIMMERS

GROUPMASTER

COMMON MEMORY

PROCESSING ELECTRONICS

SERIAL

& HOLD(D to A)

MEMORYCONTROLLER

FUNCTIONS

MEMORYMANAGER

PROCESSOR

PLABACKPROCESSOR

CHANNELCONTROLLER

CHANNELCONTROLLERPROCESSOR

GROUPMASTER

PROCESSOR

MEMORY

MANAGERPROCESSOR

RS422

OPERATORS DESK

VIDEO

PROCE-SSOR

VIDEO

GENE-RATION

R

S

S

R

Fig. 14 Block Diagram of Datalite Modular System


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The fade systems of the Datalite consoles are implemented entirely within the softwareof the controller units. The fade algorithm used is the digital version of a dip less crossfade :

o/p = A + (B-A) / 256)* n

The system timer generates regular pulses which are counted to generate the currentvalue for n. The processor then does a fast multiply to calculate (B-A)/256)*n. Theresult is added to a copy of the initial level A and the total left for the output Greatestprocessor to consider in conjunction with the output from the other controllers in thesystem.

The file data in the Datalite system is held in battery backed CMOS memory. EachCMOS card holds 32 kbytes of memory, and up to 192 kbytes of file memory can befitted (i.e. 6 cards). With a channel capacity of 1000 channels this means that about 200lots of file data can be held in the CMOS memory. The operator is free to use any filenumber up to 999 and therefore the storage system is divided into a heater area and adata storage area. The header block contains the file numbers a header area and a

data storage area. The header block contains the file numbers of all the files that arestored plus pointers to where the associated file data is to be found within the filestorage area.

5. OPERATIONAL PRACTICE

The incident light level used in any television studio depends on the light required b y thecameras in that studio to give depth of field consistent with types of programmes. Adance sequence for example will need higher f - stop no. for more depth of field hencerequiring more light level in the studios.

Contrast Ratio

Contrast is the ratio between the luminance of the lightest and darkest part of thesubject or image. The scene contrast out-doors is on average 160:1 or even higher.The use of lighting to create areas of light and shade, increases the overall contrastratio. In television the contrast should be restricted to about 20:1 and to ensure thisthe colours used by Scenic Designers are selected from a range which havereflectance between 3% and 60%. A television 'White' is a surface which has areflectance of 60%.

Illusion of Depth

This is the first artistic consideration, otherwise pictures will tend to look flat and

uninspiring. Subjects (artists and sets) are modeled by light, using shadows toreveal the shape, form and texture. The greater the shadow areas the more solidand three dimensional will the subject appear. Texture is revealed to a maximumwhen the surface is edge lit. Tonal contrast can be used to create the illusion ofdepth by arranging a recession of tones from the background (light) to foreground(dark). In order to achieve control over the scenic contrast the scenery should,whenever possible, be lit separately from the artists.


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TV Lighting


Script

Script requirements have to be satisfied to create atmosphere and mood. The scriptrequirements are basically of two types.

a) Explicit - Day, Night, Interior, Exterior, etc.

b) Implicit - Mood or Artistic Impression visualized by Director and LightingSupervisor.

Compatibility

Lighting should be compatible with B/W sets.

Types of Luminaries

There are 3 basic types of luminaries :

i) A soft-edged spotlight - Hard Source (Fresnel spotlight)

This is a focussed light source in which the position of the light source relative tothe lens can be increased or decreased to 'SPOT' or 'FLOOD's. Normally it is usedin the fully FLOODED condition to give the widest and most uniform illumination.The beam has a soft edge which gives the name to the luminaries as soft edgespot light and the lens used is a Fresnel lens.

Barn doors are fitted to these light to restrict illumination to specific areas. Usuallythe luminaries are used in the fully flooded condition because this gives moreuniform illumination and makes the barn doors most effective.

ii) A hard-edged spotlight - Hard Source (Effects spotlight)

This is a focussed light source in which the exist circular aperture of the lighthousing is projected by a lens system to give a disc of light with a hard edge to it.Alternatively, patterns cut from metal can be projected by inserting them near thecircular aperture.

iii) Soft Source

Usually a large area diffused source of light, producing as few shadows aspossible. These light sources are not usually focussed, so often they are fitted withlouvers to reduce the sideways spill of light.

Basic Three Point Lighting

Key light : This is the principle of key light source of illumination. It gives shape andmodeling by casting shadows. It is treated like "sun" in the sky and it should cast onlyone shadow. Normally it is a hard source.


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Fill Light : Controls the lighting contrast by filling in shadows. It can also provide catchlights in the eyes. Normally it is a soft source.

Back light : Separates the body from the background, gives roundness to the subjectand reveals texture. Normally it is hard source.

Background Light : Separates the person from the background, reveals backgroundinterest and shape. Normally it is a hard source.

In three point lighting the ratio of 3/2/1 (Back/Key/Fill) for mono and 3/2/2 for colourprovides good portrait lighting.

Position of Artist

At least 4', preferably 6' away from back ground to avoid artist's shadow on thebackground and to ensure that the back light angle will not be too steep. To ensurecontrol of background lighting, whenever possible use barn doors to keep key light off

background. Similarly avoid background light catching artist. In colour, plainbackground are often used care must be taken to ensure that on the monochromepicture there is a difference in tonal value between the face and the background.

Lighting Balance

The Key or Mood of the picture is determined by the ratio of the relative intensities ofKey Filter, and the tonal distribution in the picture.

Low Key Predominance of dark tones, large area of shadow.Facial contrast approx. 5 : 1. Dramatic effect.

Medium Key Facial contrast approx. 3 : 1.

High Key Predominance of light tones, small areas of thinshadows. An almost 2D effect giving a gay, lightmood. Contrast ratio less than 3:1.

The intensity of the backlight should be approx. the same as the key light but will alsodepend on the subject.

Lighting the Set for Drama

Openings such as windows within a set should be highlighted without overstating them.

Where the walls having such feature should be lit to reveal these features but care mustbe taken to ensure that there is only one shadow. The top of the set should be darkenedoff by using the barn doors, this puts a ceiling on the set by giving the feeling of a roof.If more than the top of the set is darkened, that gives enclosed feeling.


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TV Lighting


Indoor day time

If there is a choice in the direction of the sun (Key) take the shortest route inside theset to a wall, and if possible throw the shadow of window bars onto a door it usuallyis in shot.

A patch of light on the floor inside the set, backlight from outside using a soft source

at steep elevation adds realism.

When a set does not have a window, a window pattern can be projected onto a wallto produce a suitable window effect.

Roof and Ceiling Pieces - if they make lighting impossible, check if they can beremoved at the planning state. Light any ceiling pieces from outside, use a softsource at ground level. If the ceiling has plaster moulding or ornamentation, a hardsource may be used.

Indoor night time

The outside of the window should be dark, except for a possible dim skyline if theroom is well above adjacent streets, or lit by an outside practical lamp i.e. streetlighting.

The wall with the window in it should be lit at night to be brighter than for the daycondition. Subjectively the walls appear brighter at night than at day time.

Practical lamp should be placed such that they are in shot i.e. a standard lamp bya settee, table lamp by a wall. Where possible make the light from this practicalsource.

Often a completely different feel to the set can be obtained by reversing thedirection of lighting in the set compared to that used for day.

General for night effects it is not a good plan to just simply dim the set lightingwhen changing from day to night. This is because the excessive change incolour temperature of the light source and the apparent increase in saturation ofsurfaces at low luminance.

Outdoor daylight and Moonlight

The direction of the light is dictated by the position of the sun or moon . As a generalprinciple one should remember that sunlight (hard source) is accompanied by thereflected sky light (soft source) whereas moon light is a single hard source. One of thebiggest problems when lighting exteriors is the maintenance of single shadowphilosophy double shadows on a long shot will quickly destroy the apparent realismcreated in the set. A very large area filter light is ideal for exterior day light scenes. Thiscan be achieved by using a suspended white screen 12 x 8 where the filler would bepositioned then lighting it with hard light.


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The exact lighting treatment will depend on the situation but as a general rule, moonlighteffects are normally achieved by back lighting to give a more softer, romantic mood thanwould be achieved than a frontal key.

In colour, to obtain a night effect, blue cinemoid is used over the luminaries. This givesa stylized effect. An alternative is to use much more localized lighting than for day light

and light only the artists and odd part of the set.

Key light

The frontal key light may not suit both the demonstrator and his subjects. Its angle mightnot provide optimum modeling for items on the table. Subjects may shadow each other.The day light may also over illuminate the front of the table. The solution to theseproblems is to use barn door on the main key, restricting it to the demonstrator, andintroduce additional localized lamps to the front and side of the table-top to light thedisplay more effectively.

Where an overload mirror is being used to provide a camera with top shots, we may also

be able to utilize it to redirect a spot light downwards, projecting appropriate modelinglight into items on the table.

Backlight

Backlight is always liable to cast the demonstrator s shadow forward across the centralworking area of the table, causing obscuring or distracting shadows over the subject heis discussing. We may be able to avoid this by using a lower angle backlight, that isbarn doored to illuminate only the demonstrator s head and shoulders. Shadows from alow-intensity backlight may become sufficiently diluted by key lights to be quiteunobtrusive. Another approach is to use a soft backlight in order to avoid theseshadows altogether, but it can give less effective lighting, and its spill light is not so

readily controlled.

Reflections

Sometimes backlight bouncing forwards the camera, or multiple reflections of studiolights, can prove troublesome. The only practical remedies, apart from removing orcovering the offending surface, are to try repositioning. Occasionally it may be possibleto switch off a lamp from an isolated shot, to prevent reflection.


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08 TV Lighting Induction

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