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Chapter twelve

Transparent positive images

The need for positive transparencies has increased in the modern era, due to their frequent use as a visual means for teaching purposes and explaining practical and technical lectures in particular. Such images require special methods to be viewed, including the use of projectors on a screen display. To avoid scratching or damaging these positive transparent images, slides are usually pressed between two sheets of glass to protect them if the image is on a support. of celluloid, or a sheet of glass is placed against the layer of gelatin if the image is on glass. There are two main methods for obtaining transparent positive images:

(a) By printing or enlarging transparent negative images on a sensitive film or glass with a positive sensitive layer on it. This method is known as the Negative Positive Process L.

(b) By showing the film after filming it in another way known as reverse projection or by the most common name, which is “reversal development”, which is a method that leads to a positive image immediately after showing instead of the negative image... and it talks about the following about the “positive negative method” to obtain on positive photos, provided that the next chapter is devoted to the process of “digital display”

Positive negative way to get transparent positive images:

You may want the positive transparent image to be in one of the following formats:
(a) It should be the same size as the negative image (of course the touch).

(B) or that its size is larger than the negative image (by enlarging it).

(H) Or that its size is less than the negative image (reduced).

It discusses each of the previous cases in detail as follows:

The transparent positive image from printing negative images, of course, touching:

It is possible to obtain a transparent positive image of the same size as the negative image if we follow the contact printing method, whether by means of a contact printing frame (Fig. The work methods in this case do not differ from what we have previously mentioned regarding printing negative images on sensitive positive paper, except that in this case – the sensitive paper is replaced by a film or a glass plate covered by a positive sensitive layer.

The factories have prepared various devices for printing the 35 negative films fried on 35 mm meal films as well (and we show in Figure 97 one of them that is easy for amateurs to use).

It is also possible to obtain a positive transparent image of the same size as the building negative image by reimaging the latter with a 35 mm camera also after illuminating the negative image with transmitted light, and on condition that the image’s distance from the school is exactly twice the focal length (Fig. 98) so that the transmission scale is 1 : 1 Thus, the positive and negative images have the same area.

Transparent positive images from enlarging small negative images:

To obtain a magnified transparent meal image from small transparent negatives, the magnifier is used using the same method previously explained in the magnification section, with the replacement of the sensitive paper with a positive sensitive film or glass.

Transparent positive images from reducing negative images:

We may have a negative transparent image and ask to reduce it or reduce a part of it to make a small transparent positive image. Here, follow one of the following methods:

1- Either re-photographing the large negative by means of a camera whose size corresponds to the size of the required transparent positive image, and with the negative image illuminated by penetrating rays.

2- Or by using a magnifying device with a very long bellows, the distance between the negative image and the lens of the magnifier is twice the focal length of the lens. In this case, the negative image (which is required to be reduced) is placed in the place of the negative carrier in the amplifier, with the positive film placed in the place of the sensitive paper. Then the exposure begins in the usual ways after setting the distance very precisely.

It is noted that the image can be greatly reduced whenever the focal length of the magnifier lens is shortened. If the magnifier is not equipped with a bellows, the distance between the lens and the negative image can be increased by installing “extension tubes” between them. It is also possible to reduce the image using the usual magnifying devices by placing additional lenses in front of the original lens of the magnifier (this method is not recommended due to the decrease in sharpness of the image).