Astronomical Society of Edinburgh
Journal 38

Hypersensitisation of Photographic Film

Recently, the Society purchased equipment to enable us to make hypersensitised film. For long exposure deep-sky photography, this is essential and this article will attempt to explain why we need it, how it's made and how to use it.

Any literature on this subject will tell you that hypersensitisation is a process which removes a phenomenon known as low light intensity reciprocity failure from photographic film, combined with an actual increase in film speed. To understand what this means, we need to know what reciprocity itself is. It has been stated that the density of an image recorded on photographic film is simply the product of time of exposure and the intensity of the light striking it i.e. time and intensity are reciprocals of each other. However, this was found to be incorrect in practice. Unfortunately for astrophotographers, this relationship only holds for exposure times that are short, typically faster than 1 or 2 seconds. For exposure times longer than this, the relationship breaks down and we have reciprocity failure. If we double the exposure time, we ought to double the image density on the film - reciprocity failure means that this does not happen and in fact in my own experience of black and white film, about 50% of the image density in a 40 minute exposure is generated in the first 5 minutes or so and the last 10 minutes may only be contributing 5 or 10%. Hypering reduces or completely removes this effect, and the image density increases in a linear way with exposure time. In addition, a significant increase in film speed can be achieved by a process termed reduction sensitization.

There are several documented ways of hypering film, but we are involved in two - vacuum treatment and hydrogen soaking. These methods were developed by Eastman Kodak researchers in the 1970s in response to the demands of professional astrophotographers.

It has been established that reciprocity failure is almost entirely due to the presence of oxygen and water vapour absorbed into the photographic emulsion. These so-called contaminants are removed by subjecting the film to a vacuum for a period of time. The time this takes depends on the depth of the vacuum, which in turn is obviously dependent on the vacuum pump and how gas-tight the system is. We are using a Jigtool mechanical pump with a modified paint-spray pressure pot to hold the film, and 2 or 3 hours appears to be sufficient. Once the film has been treated in this way, the hypering gas can be admitted to the pressure pot, to chemically sensitize the film. The procedure that follows depends on several variables, and while guide-lines can be found, each individual system needs to be optimised. Properly hypered film is dependent on time, temperature, pressure and percentage of hydrogen, and these differ for different films. With our system, using 10% hydrogen/90% nitrogen as the hypering gas, Kodak Technical Pan 2415 should exhibit a speed gain of around x8 after 8 hours at + 15psi at 60 C.

With clear skies at a premium in Scotland (particularly this winter), we don't want to be wasting dark sky time testing film by trial and error, so it is fortunate that other astrophotographers have established that TP 2415 is sufficiently "cooked" when a developed strip of unexposed hypered film shows an increase in chemical fog over base fog of 0.3 to 0.7, base fog being measured on a strip of unexposed, unhypered film. We can measure this approximately against a standard step wedge.

TP 2415 can be stored after hypering for a few days at room temperature without losing its speed, or for several weeks in the deep freeze. Colour films are less forgiving and should be used fresh.

Exposure times are typically 30 - 60 minutes at f 6.3.

TP 2415 can be developed in a number of different ways, but the accepted and almost universally used developer for hypered film is Kodak D-19. Typical times for stock solutions are 5 to 8 minutes at 20 C. It has been reported that D-l9 gives film speeds between x2 and x4 faster than Kodak HC110 developer when using hypered TP 2415.

At least until we have the method established in a routine way, I will be doing the hypering myself. Courtesy of Lorna McCalman, Castle Photographic at the Mound are now keeping TP 2415 in bulkloaded form, and a cassette of 12 exposures can be bought from them at a very reasonable price. The Society will be charging. around 50p a roll for hypering to cover the cost of gas etc. Once dark skies are with us again, I intend to establish a methodology for hypering a range of colour print and slide films too. So get those cameras out, and make it snappy!

Charlie Gleed