|The TDC 116 Projector|
with lamps dimmed
When I joined the Victorian 3D Society in 1984, I recall that there were only two club members with projectors. However over the last 3 or 4 years there has been a dramatic increase in members with projectors, up to around 7 or 8.
As most projectors coming from the USA need some form of modification to operate on our mains voltage, and given that it would also be an opportunity to bring such projectors up to date by using better lamps, it is worth considering factors that can affect the overall light out put of a projector.
In an associated article that is a companion to this one, the modifications made to a TDC-116 stereo projector are discussed an illustrated.
Figure 1 shows a typical configuration for the layout of a projector lamp-house, slide gate and projection lens. There are variations from this configuration between projectors, but it is general enough to allow an examination of the main factors affecting the overall efficiency of a projection system.
|Figure 1: Typical Condenser Configuration in Slide Projectors|
The primary function of the arrangement in Figure 1 is to collect as much light as possible from the projection lamp, and direct the collected output (evenly) through the slide and projection lens. The affect of various components on overall efficiency is discussed below:
The projector lamp in older projectors comprised a tungsten filament suspended in a vacuum maintained by a glass envelope. As with any tungsten filament lamp, the light output that can be obtained from these lamps is determined by the operating temperature of the filament. More light output can easily be obtained by applying more power to the lamp, and thus raising the filament operating temperature. However the increased temperature causes the metal filament to "boil away" or evaporate more quickly thus shortening the life of the filament dramatically (the evaporated tungsten condenses on the glass envelope making is appear black as the lamp ends its life). In addition, the maximum temperature is limited to the melting point of the glass envelope (it is not uncommon to see glass projector bulbs deform as they near the end of their life).
These tungsten filament lamps were superseded many years ago by quartz-halogen lamps which still feature a tungsten filament, but the vacuum is replaced with a halogen gas such as iodine. In addition the glass envelope is replaced by a quartz envelope, or more usually these days by advanced types of glass such as Vicar that has a high melting point. These lamps can operate the tungsten filament at a higher temperature. The problem of the filament boiling away is reduced by the halogen gas which causes the evaporated tungsten to be placed back onto the filament, rather than on the quartz envelope of the lamp.
Quartz halogen lamps give more light output per Watt of electrical input, and are to be preferred for stereo projection. In addition the higher colour temperature of the lamps gives a whiter light which subjectively looks brighter, even when a light meter might say that two projectors have the same output. Table 1 shows a comparison of various lamp types.
|Lamp Type||Lamp Code||Volts||Watts||Colour Temperature||Output in Lumens|
It can be seen that for the two 150 Watt lamps the Q-H lamps gives more output, however the whiter light makes this difference look larger than measurements would indicate. For the other two lamps, equal output is obtained from the lower powered Q-H lamp, but the whiter light would make the DDB lamp look superior (this lamp is used in many overhead projectors).
The improvement in light output for Q-H lamps is helpful, however what is not readily apparent is that low voltage Q-H lamps can be made with filaments smaller filaments which has an efficiency benefit when comes to designing a condenser system (see below).
The function of the mirror is to collect the light output from the rear of the lamp and redirect it to the front. A properly designed mirror system can increase the output of a lamp with out a mirror by nearly a factor of two. (Some special lamps with built-in mirrors and specially designed condenser systems can give even more increase in output).
The amount of light collected by the mirror will be larger if the angle-a in Figure 1 is made larger however there is no advantage in making angle-a bigger than angle-b. Thus for an efficient mirror design with a large angle-a, either the mirror is large in diameter, or it is short in focal length (or a combination of these). Mirrors should be highly polished, although in some cheaper designs satin-finished mirrors are seen. Mirrors can also be made to reflect light, but transmit heat these are called dichroic mirrors, and are desirable because the slide gate temperature can be reduced.
|Figure 2: The First Condenser in the |
TDC 116 is actually two convex
lenses in series.
The mirror is a curved surface with radius-r, and the lamp is placed distance-r from the mirror surface. This causes an inverted image of the filament to be formed.
For older style tungsten lamps (that have large filaments) the image of the filament coils are interlaced with the real filament coils for highest efficiency. However even with this arrangement some of the light from the mirror is blocked by the actual filament.
For Q-H lamps with small filaments, the complete inverted image of the filament is placed just above the real filament. Thus all the light from the mirror is able to enter the condenser system.
The first condenser collects light from the real and inverted filaments. A crucial factor that affects the efficiency of the first condenser is the angle-b over which the light from the lamp is collected. As shown in Figure 1, light beams travelling outside the angle-b are lost. Therefore it is desirable to make the angle-b as large as possible to maximise the light output.
Since the lamp is placed near the focal point of the first condenser, a short focal length is desirable so that angle-b can be made large. Modern projectors generally feature an aspherical first condenser that have a very short focal length. Older style projectors usually have a longer focal length first condenser, as aspherical lenses were too expensive to make in the past.
|Projector||1st Condenser Diameter||d1 (see Fig1)||Angle-b|
|Liesegang 150W QH Lamp||50mm||12mm||128 degrees|
|TDC 116 500W||60mm||23mm (orig'l lamp)||105 degrees |
In Table 2, the Liesegang projector has an aspherical first condenser with a short focal length compared to the TDC 116 which has two plano-convex lenses of longer focal length, but larger diameter. For each projector, the angle-b over which the condenser system collects light has been calculated. The TDC condenser system collects light over a smaller area, and is thus less efficient.
All tungsten filament lamps mainly create heat - only a small percentage of the output is visible light. To reduce the amount of heat reaching the slide gate, heat absorbing glass must be included in the light path. This glass has a slight green-tinge, and affects the colour of the light slightly. The loss of light in the heat filter could be between 0.25 and 0.5 stops. Heat filters must have adequate airflow across them to stop them from melting.
In most condenser system, the light rays travelling from the first condenser to the second are essentially parallel, and cover an area roughly the diameter of the largest slide diagonal to be used. However if all the light rays collected by the condenser are to pass through the aperture of the lens they must be focussed to form an image of the lamp at the narrowest part of the projection lens at the aperture as shown in Figure 1.
The position at which the lamp image is formed should be set in accordance with the focal length of the projection lens if maximum light transfer is to occur. This means that the focal length of the second condenser should be longer for a long projection lens, and shorter for a short projection lens. However most commercial projection system make some sort of compromise, allowing for projection lenses to be used over a given range (say 85mm to 125mm) without significant loss in efficiency. (The Kodak SAV series has provision for changing the second condenser when using focal lengths exceeding 150mm).
Note that the position of the slide is usually quite close to the second condenser. If the slide is moved too far away from the second condenser then uneven illumination of the slide usually results.
In stereo projection the use of polarising filters results in a significant loss of light. Typically a high quality polarising filter loses between 60% and 70% of light (or around 1.3 stops of loss) Note however that when two polarisers of 1.3 stops loss are place over each other and aligned, the resultant loss is about 1.6 stops, not 2.6 stops. This is the case when viewing stereo through glasses - the total loss from the projector filters (1.3 stops) and glasses (say 1.3 stops) combined is around 1.6 stops. Fortunately the sliver screens used for stereo projection tend to counteract this loss.
The main concern for obtaining high light efficiency from the projection lens is to ensure that the image of the filament formed at the aperture of this lens actually fits within the aperture. In other words, if the lamp image is roughly a square of 30mm side length, then the aperture must equal or exceed 42mm diameter for all the light to "fit through" the aperture. (If this were a 120mm lens, an aperture of 42mm would correspond to an f2.8 lens.)
Note however that if the speed of the lens was increased (to say an f2.5 lens instead of f2.8), then there would not be any resultant increase in light output. In this respect projection lenses appear to behave differently to camera lenses, but this is only because of the shape and concentration of the light source. In contrast, if the speed of the lens in this example was reduced from f2.8 to f3.5, then some portion of light would not pass through the aperture, and so the image on the screen would be duller.
A factor that is not immediately apparent, but which affects the efficiency of a condenser system, is the size of the lamp filament.
If the projection lamp has a physical filament 5mm wide and 2.5mm high, then when the image from the mirror is taken into account, the effective filament size will be 5mm wide x 5mm high.
If we use a projection lens of focal length 120mm and f2.8 aperture, the aperture will measure 42mm in diameter.The largest filament image that will inside the 42mm diameter aperture is 30mm square. Since the effective lamp filament is 5mm square, the condenser system has therefore magnified the lamp filament by a factor of 6.
If the projection lamp has a physical filament 10mm wide and 10 mm high, the magnification required of the condenser system is now only 3, instead of 6.
|An impression of a |
TDC-116 Stereo Projector in Action
The increased magnification required for the small filament is gained by increasing the power (ie: decreasing the focal length) of the first condenser. This automatically means that the small filament lamp is place much closer to the first condenser, thus dramatically enhancing the angle-a over which light is collected by the first condenser.
Thus a small filament lamp of less power than a large filament lamp, can give superior brightness.
Low voltage lamps feature smaller filaments than higher voltage lamps of equal power. Therefore the most desirable tungsten filament lamp to use is a low-voltage-quartz-halogen lamp. Some popular types are: FCS 24V/150W; EHJ 24V/250W and EVD 36V/400W.
Part 2 of this article
describes in detail the modifications made to a TDC-116 projector to fit
low voltage halogen lamps to it.