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Positive slide projectors and vision machines ..
Selection and use of materials .. Book of photography

Selection and use of materials
Positive slide projectors and vision machines

Slides are made to be viewed by transparency, i.e. by transmitted light. The slide, i.e. positive images, gives a more complete image than an image on paper because the light passes through the film instead of being reflected on the white paper substrate, but this slide requires special equipment to examine it.

- Vision machines:

Some of these machines are so small that they can be carried in a pocket, but their magnification is weak. Simple vision machines lead to distortion in the image, especially at its edges. They are easy to use, but only one person can look at them and through them.

- Projectors:

They give bright images with large dimensions, but they usually require a dark room.

- Projection from behind:

Most projectors are used for frontal projection, i.e. from the same side as the opaque screen. However, we can also project from behind on a semi-transparent screen, which allows the views to be projected into the interior without making the room completely dark. Some projectors are specially designed for rear projection: folding models with a lens that are easy to carry and can be put in a small bag. But the small screen limits the number of viewers and does not allow the resolution of the finest details of the images.

- Projection lamps:

Projectors are usually classified according to the power of the lamp, the less expensive models have a lamp operating at 100 or 150 watts and can use the maximum power without the need for electrical ventilation.

(Vision machines: The simplest vision machines are not equipped with lighting, the positive slide is illuminated by an external light source, the image is enlarged by a magnifying lens. Some of these machines are equipped with an electric lamp. Most of the time these devices are made of plastic.)

The more powerful lamps (from 150 to 300 watts) have ventilation, but the general trend these days is to use the small gas lamp (quartz-halogen) with low voltage. This lamp has a longer life and does not blacken, but it requires a step-down transformer, which leads to an increase in the price of the projector. The most popular lamps are 100 watts 12 volts and 150 watts 244 volts. The filament of gas lamps is more compact and strong than that of conventional incandescent lamps: the small dimensions of the light source give great luminous efficiency. We can consider that a quartz-halogen lamp of the 100 or 150 watt class gives as much light as a classic 300 watt lamp. Some projectors are equipped with a device that reduces the luminous power when you want to show a slightly brighter slide.

- Slide projectors mounted on a piece of cardboard 5 × 5 cm

(Slide in English and Diaportive in French):

Images of 24 × 36 and 28 × 28 (126 film) are displayed in a 5 × 5 cm frame. The same machines can display "super slides", i.e. large slides measuring 4 × 4 cm and also placed in a 5 × 5 cm frame.

The simplest projectors are equipped with a back and forth viewer in which the slides are placed one after the other. Most machines use stores containing 50 slides: some are operated by hand and most are motorized, which allows them to be operated from a distance, connected to the machine by a multi-pole tape. The drive can be either forward or backward.

It is also possible to calibrate from a distance, as a very small motor ensures the movement of the lens in both directions. This allows for recalibration when we project images mounted in a frame without glass or frames of different thicknesses. Automatic calibration is present in the most advanced projectors. In this case, a photocell adjustment device is provided that continuously moves the lens holder according to the position of the light-sensitive film surface.

- Installing a 35 mm projector:
A projection table allows for the appropriate height and stability of the projector. On the lower part of the table, the change baskets can be placed. A table like this one is foldable. The projector in the opposite picture is a model with a horizontal circular basket and the possibility of remote control.

- Loading the slides in the projector:
Many projectors use longitudinal baskets where the slides are placed. This model uses a standard basket for 36 or 50 pictures.

- Other machines use a vertical circular basket that can be replaced by a longitudinal basket. This model contains 100 pictures.

- Some rare models accept single images. We place the slides directly under their cardboard frame in the machine compartment.

- The 110 projectors have a circular magazine or the images are pulled in by a collar.

- Other 110 machines have back and forth image paths, no preparation for packing is required.

- The 6 x 6 projectors are delivered with image paths for 35 mm slides.

- The screen can be wall mounted or mounted on a tripod.

There is also an automatic timer that ensures that the images pass without stopping and at regular intervals: if the machine has a circular store, the display continues without stopping. The projection lenses have different focal dimensions. For the 24 x 36 mm size, the most popular focal length is 85 or 90 mm, which ensures a display with a base length of one meter in a room of normal dimensions. As for large rooms, a longer focal length is used. As for the shorter focal lengths, we only use them if we lack space and cannot go back. There is also a projection zoom that allows us to choose the size of the projected image without changing the distance between the projector and the screen.

- 110 projectors:

110 images mounted in a 5 x 5 cm frame can be displayed on a 35 mm projector. However, the image is very small under normal conditions, but if we enlarge its length by reducing the focal length or by moving the projector back, the image becomes very straight and unclear.

In this case it is preferable to use a special projector designed for this size because the 110 image is mounted on a 3 x 3 cm frame. These models of projectors use a lamp with a reflector that gives maximum illumination to the small images. The images are pulled by a hoop or placed in a small circular storage.

- Slide projectors 6 x 6 cm:

There are few models that accept this size: there is a model that passes the images back and forth and an automatic projector that works like a 35 mm machine. Of course, the lens burner is longer: a 140 mm burner for 6 x 6 gives the same size as a 90 mm for 24 x 36 mm.

- Screens:

We can display images on any white space such as a wall, but a foldable screen is preferred. Some screens fold automatically thanks to a spring mechanism. They come in all sizes. For square slides, a square screen is required, which also allows for vertical and horizontal images to be displayed. The image does not necessarily fill the entire screen area. The screen surface is either silvered, sparkling or matte white, and the difference between them lies in the way the light is reflected.

Sparkling screens are made of very small balls of glass (or other transparent material) that send light vertically towards the screen at a narrow angle: the display is very bright, but viewers must be close to the axis of the display. Most often, a matte screen is used because it allows viewers to be distributed. The silvered screen gives an effect intermediate between

sparkling and matte white, but high lights have a less beautiful metallic appearance. We obligatorily use a metallic silver screen for projection in polarized light because the metal surface does not adjust the polarization of light.

The screen can be fixed to the wall like a board but it is often preferred to have a screen supported on a stand because it is easier to place and arrange. The tripod stand is very common. The choice depends on the available space and the desired projection. Make sure that the screen is perpendicular to the projection axis so that the image is perfectly rectangular and its clarity is evenly distributed.

- Light box:

Slides with large dimensions cannot be projected and must be examined and viewed on a light box. Its surface is made of opaque glass or semi-transparent plastic and the light comes to it from behind. The light source is usually made of neon tubes of the "daylight" type, the advantage of which is that it does not emit strong heat. The light box is also useful for examining large negatives and comparing a set of small slides.

- Automatic display with sound and image gradation:

It has become customary these days to accompany the slide show with a commentary and background music. A tape recorder can be connected to a projector by means of a timer socket which most projectors are equipped with. A simple method of controlling the passage of images through a tape recorder involves the use of a metal contact on the magnetic tape which presents the next image at the appropriate time according to the previously recorded commentary or music. More modern machines use a time signal recorded on one side of the magnetic tape and the other side stores the sound. The linked image changer (pictured above) uses a pair of identical projectors which are fired alternately and progressively, giving a continuous display. The images merge into each other without breaking the continuity. There are several types of manual or automatic, with mechanical or electronic control.