Since the TS exhibits a photographic negative effect and vapours, dyes, paint, powders and stains are all ruled out, another theory is it is some sort of proto-photograph. This is proposed by Nicholas Allen.

A comparison between the 1978 Shroud of Turin Research Project (STURP) data concerning the qualities of image formation on the Shroud of Turin and the inferences of the author’s recent investigation into shroud-like image formation techniques employing technology readily available to medieval cultures as far back as the eleventh century strongly suggests that the negative image as found on the Shroud of Turin was the product of a form of primitive photography employing either silver nitrate or silver sulphate as a light sensitive agent.

Briefly, these characteristics of the image, as found on the so-called Shroud of Turin, may be listed as follows:

Superficiality: Although it is quite likely that the areas associated with the stigmata are formed from blood, 2 the negative image itself is essentially the enigmatic discolouration of the uppermost fibres of the linen threads which constitute the Shroud’s fabric. This image has not `penetrated’ the threads in the sense that it is not visible on the underside of the Shroud. In addition, the image is not visually coherent to the naked eye at close range.

Detailed: The Shroud’s negative image, once transformed to a positive state by means of modern photography, is highly detailed, which has allowed medical experts to claim that they are able to detect the presence of such details as rigor mortis, contusion wounds, excoriations and a variety of facial wounds (Barbet 1950:23–45). It should also be considered that without the medium of modern photography it is uncertain if anyone living before c 1898 could have seen these details 4 (that is when Secondo Pia made his historic photographic negatives of the Shroud).

Thermally stable: The Shroud’s image was not affected by the intense heat of a fire which nearly destroyed it in 1532.

No pigment: It is quite certain that no pigment was applied to the Shroud and the image is not caused by pigment, dye or stain, either.

Three-dimensional: The intensity of the image varies according to the distance of the body from the cloth, strongly suggesting that the body did not in fact come into direct contact with the Shroud. The mathematical ratio is so precise that Jackson and Jumper were able to create a three-dimensional replica from the image.

Negative: The image is a negative which is as visually coherent as a positive photograph when its tonal polarity is reversed.

Directionless: The process that formed the image operated in a non-directional fashion. It was not generated according to any directional pattern as it would have been if applied by hand. A painting, for example, shows strong directionality, that is, the direction by which the medium was applied is evident from the brush strokes.

Chemically stable: The straw-yellow discolouration composing the Shroud image cannot be dissolved, bleached, or changed by standard chemical agents.

Water stable: The Shroud was doused with water to extinguish the fire of 1532. Although this has caused a visible water stain, the image itself does not appear to be affected.

If one accepts that water stability and chemical stability may both be covered by the same nomenclature, then there exists a total of eight conspicuous attributes of the image which are peculiar to the Shroud of Lirey-Chambéry-Turin. … _Turin.htm

Using materials that are available in the 14th century, here was how the image could’ve been produced:

More specifically, if a piece of linen, permeated with a dilute solution of either silver nitrate or silver sulphate, is positioned inside a camera obscura , it can record (in the negative) the details of a sun-illuminated subject situated outside the camera obscura. It must be stressed that this image can only be obtained if it is focused onto the linen cloth by means of a quartz (optical quality, rock-crystal) bi-convex lens. In addition, for this image to be life-sized (for example the dimensions of an adult human corpse), it is necessary for the combined image conjugate and object conjugate distances to total about 8,8 metres. In other words, the subject to be `photographed’ must be positioned (that is outside the camera obscura) some 4,4 metres from the aperture, whilst the screen supporting the prepared linen cloth must correspondingly be placed at a similar distance from the aperture (inside the camera obscura). At these long distances it is essential that the lens should have as large a diameter as possible (for example, well over 60 mm 9;) so that as much light as possible enters the camera obscura. It is important to emphasise here, that a pinhole and/or lens made from optical quality glass will not suffice for this purpose. Indeed, only optical quality quartz will permit the passage of UV radiation from the subject (corpse) to the specific silver salt which impregnates the linen material, and both silver nitrate and silver sulphate are particularly sensitive to the UV end of the light spectrum (particularly 195 to 240 nm). The image thus obtained is in the negative, and (surprising as it may seem) after immersion in ammonia becomes chemically stable. In fact, by immersing the cloth in urine or dilute ammonia it is possible to remove all traces of silver (reduced or otherwise), and the cloth together with its encoded negative image may be brought out of the camera obscura into the light of day. The image is only visually coherent at a distance of some two to three metres, appears only on the upper fibrils of the cloth and is a record of the illumination of the subject over a period of days. For this latter reason the visual record contains a negative encoding of the three-dimensional characteristics of the original subject. In this context at least, the image is unlike a modern photographic negative in that it is not a `snap-shot’of a particular moment in time, 10 but rather the record of the original subject according to the physical distance of a particular feature of the subject from the prepared organic support (for example linen cloth). If a photographic negative is made from this cloth, then a highly detailed, positive image of the original subject will result. Readers should compare this image with the positive image of the head from the Shroud of Lirey-Chambéry-Turin.

From the preceding evidence alone it is possible to postulate that somebody in the late thirteenth or early fourteenth century may have had the necessary knowledge and materials to have taken either a human corpse or even a life-like bodycast and have suspended it vertically in direct sunlight for an unspecified number of days such that it (the corpse) received an equal amount of morning and afternoon illumination. This subject (corpse or bodycast) would have had to have been situated opposite an aperture (containing a simple bi-convex quartz lens) of a light-proof room (camera obscura). Inside this room or camera, it would have been necessary for a large screen to support the linen cloth (Shroud), which had been previously treated with a very dilute solution of either silver nitrate (0,5%) or silver sulphate (0,57%). The inverted image of the corpse would have been focused onto this prepared support and after a few days the UV sensitive silver salt would have turned purplish-brown, forming as it did a negative photographic image of the subject. To achieve the twofold image which now appears on the Shroud of Turin, it would have been necessary for this operation to have been repeated twice to obtain an impression of both the frontal and dorsal images of the sun-illuminated corpse. After both exposures had been completed the linen cloth would have been soaked briefly in a dilute solution of ammonia (5%) or possibly even urine. This latter action would have ostensibly removed all silver (both exposed and unexposed) from the linen cloth and also would have allowed it to be exhibited outside the camera even in direct sunlight, without further discolouration occurring. Even though the silver salt had been removed, the cloth would have still contained a faint negative straw-yellow image — one which seemed to be encoded in the very structure of the linen itself, albeit on the upper fibrils. … _Turin.htm

It might be theoretically possible to create a photographic image during the 14th century. But realistically, this is not really tenable. There is no evidence photographic technology existed at that time. It would be hundreds of years later the first documented photograph was taken. And how could photography come immediately into history and then disappear as a lost technology? Modern photography required decades of incremental research and discoveries to arrive at true photography. For a single person to come up with photographic technology from scratch in his lifetime would be miraculous. And why would someone even bother to invent a totally new technology just to take one picture? How could such a groundbreaking discovery immedidately fall into obscurity? How many attempts are necessary to take such a photo on a linen cloth? Why even bother to do it on a cloth with an expensive weave?

Features the proto-photo theory does not explain: depth encoding, angle encoding, second degree distortions, x-ray effect, rigor mortis, faint image on back of cloth, and uncolored fibers next to colored fibers.

STURP photographer Barrie Schwortz comments on Allen’s theory:

In this paper, I will review the “proto-photography” theory proposed by Prof. Nicholas Allen of South Africa. This theory concludes that the raw materials to produce photography not only existed in medieval times, but that a brilliant medieval “photographer” actually used them to invent photography 500 years before the documented creation of the first photographic negative by Joseph Niepce in 1818.

To his credit, Allen has actually achieved what he set out to accomplish. He has, without question, used medieval raw materials to create a faint but good quality photographic image on linen cloth. As I will show however, his own results provide the best evidence against the validity of his theory. In the end, any attempt at duplicating the image on the Shroud of Turin must match all of its physical and chemical properties, not just a select few. It must also withstand the scrutiny of careful, side-by-side comparison to the original.

He goes on to claim that one half of the Shroud image was exposed at a time, first the ventral and then the dorsal half. He further concludes that it would take about four days to properly expose each half of the cloth, needing at least eight days to complete the entire task. Recently, he modified his theory to include a third exposure for the face, made with a different lens.2 To prevent the decay of the body during more than a week of exposure to the bright sunlight necessary for adequate exposure of the “film,” Allen suggests that the camera obscura was located in a cold climate.

Allen has not been able to provide even one example of this medieval proto-photography process anywhere in art or photographic history, although he has carefully and extensively documented early historical references to lenses and cameras obscura.

Allen also expressed to me his more recent belief that the Shroud is actually a composite of three different exposures, now concluding that the facial image was made as a distinctly different and third exposure onto the cloth.

He supports this claim by stating that he has recently detected “spherical aberrations” in the facial image on the Shroud which leads him to this conclusion.5 Obviously, this would make the process of creating the image even more complex for a medieval photographer and even harder to accomplish. Today, even with the advanced state of modern digital imaging techniques, such a perfect composite image could only be accurately accomplished by a highly trained photographic expert. To conclude it was produced by a medieval photographer truly stretches the imagination.

Allen’s photographs contain a strong directionality of light. This is obvious from the deep shadows cast on his subject by the strong overhead sunlight he used to create his images (Figure 1). These are clearly seen in the eye sockets, under the nose and chin and below the hands and is unlike the image on the Shroud (Figure 2), which demonstrates no such directionality of light at all. It is further confirmed by the “washing out” of detail in certain parts of the image, most notably the tops of the feet, which received far more light and cumulative exposure than the rest of the body (Figure 3).

Since the densities on a photographic negative are not dependent on the distance between subject and film, there is no way that this density information can be incorporated into an image photographically.

There is one additional facet of Allen’s image that is considerably different from the image on the Shroud. The Shroud image has no distinct or sharp edges, yet Allen’s body image has a very distinct and sharp edge, much as one would expect from a properly focused photograph.

The proto-photography theory proposed by Prof. Nicholas Allen was able to create an image on linen cloth, but not one that duplicated the image properties of the Shroud of Turin. When attempting to provide a viable image formation mechanism for the Shroud, one has to account for all of the image properties, not just a few of them. Allen failed to understand certain important facets of the image on the Shroud of Turin. Much as it truly takes a professional artist to properly evaluate a painting, so too must photography be evaluated by the professional photographer. In the case of the proto-photography theory, other professional evaluations of Allen’s theory have reached similar conclusions.

Admittedly, Allen was able to create a viable photographic image using medieval raw materials, but he did so from the perspective of 21st century science. Surely raw materials must exist on our planet today that may eventually lead to the development of interstellar travel, but their mere existence is not enough to actually provide us with the technology. That will have to wait until our technological development advances to a much higher level than exists today.

If we accept the argument that the mere existence of certain raw materials is reason enough to believe someone actually used them to invent a technology that was still 500 years in the future, we should start searching archaeological sites around the world for the remains of medieval cellular phones, microwave ovens and nuclear weapons! Just because the raw materials for these highly advanced technologies existed, doesn’t mean someone actually created them, particularly before human knowledge advanced enough technologically to truly make this possible.

The third theory was that the Shroud image was created in medieval times using a photographic process, even though the first documented photographic negative still exists and was created in 1826 by Joseph Nicéphore Niépce in France! There is not one shred of evidence in the historical record that any photo graphic images were created before then. Since the primary light sensitive material used in the photographic process is silver, the STURP team looked for it using spectral and chemical analyses and not one trace was found. According to proponents of that theory, all the silver was removed during the fixing process. However, the chemicals used to fix the image and make it permanent only remove the UNUSED silver and there would be plenty of it remaining in all of the image areas. Also, such a light sensitive emulsion would have to be coated onto the entire Shroud in a semi-viscous liquid or gelatin form which would have penetrated deep into the fibers of the cloth and some silver would have remained there until this day. Yet not one trace was found. Again, we proved the Shroud image was not created by a photographic process. Of course, if one takes the time to thoroughly analyze the Shroud’s global image properties (and I am a professional photographer that has had 38 years to do so), one can easily see that, other than the negative-like light to dark reversal, the Shroud image is absolutely nothing like a photo graphic image. One cannot encode spatial or topographic 3-D information into an image using normal photography. … us-christ/

Stephen Jones notes Allen did not completely use Medieval technology for his replication attempt. His lens was made using modern materials and modern techniques.

Last night (1 September) I found in Allen’s book, “The Turin Shroud and the Crystal Lens,” where he indirectly admitted that he had used “synthetic quartz” (pp. 94, 96-97, 207, 240, etc.), that is “fused quartz,” made from quartz sand heated to ~2,000°C in a modern furnace, not rock crystal, the latter being what was only available in the Middle Ages to fashion a quartz lens. Therefore Allen’s “medieval photography” theory fails through lack of experimental support. And, as we shall see, unless Allen can explain (he has read my blog in the past, even emailing me about something I wrote about his theory) where he has clearly stated and shown that the materials and methods he used were entirely medieval, then I will have no alternative but to assume that Allen has indeed in this committed scientific fraud (although presumably due self-deception). … turin.html

In terms of matching the number of features of the shroud, I believe the proto-photograph does a better job than any other explanation of the image involving an artist. However, it still lacks explaining many features of the shourd as I noted above.