The dimensions of your phototool will change under the influence of changes in temperature, changes in humidity, mechanical tensions and ageing. Some dimensional changes are reversible others are not.
Most materials expand with increasing temperatures and phototools are no exception to that rule. The dimensions of all phototools in use today change with changing temperatures and humidity, and the amount of expansion is determined by the thermal and humidity expansion coefficient of the film.
Thermal Properties |
CT in mm/m °C |
Glass phototools |
9 |
Polyester |
18 |
Gelatine |
18 |
Silver halide phototools on PET base |
18 |
Diazo |
18 |
Most phototools absorb water and thus also expand with increasing relative humidity.
Humidity Properties |
CRH in mm/m % |
Glass phototools |
0 |
Polyester |
8 |
Gelatine |
Approx. 100 |
Silver halide phototools PET base |
Approx. 11 |
Diazo |
9 |
PET based phototools change rather fast when the temperature changes. The time constant for temperature changes (the time needed to reach 2/3 of the total change) is a few minutes. Glass phototools have a much larger time constant due to their higher thermal mass. So, a PET based phototool will reach its new size in about ten minutes when the film is exposed to the environment from both sides. When several films are piled up, you have to add ten minutes for every sheet. When a film is secured by a very strong vacuum that vacuum may hold the film and prevent it from changing size in response to changes in temperature or humidity. Although the temperature of the film changes the size will not for as long as the vacuum is applied. Once the vacuum is released, the film size will change instantly.
Silver halide phototools react in a more complex way to changes in relative humidity. The dimensional change is partly caused by the influence of the gelatines and partly by the influence of the PET. Gelatines have a time constant for relative humidity changes of only tenths of a second: they act like a sponge. They absorb and desorb much water and do so extremely fast. PET has a time constant for relative humidity changes of several hours: it absorbs or desorbs only a little water and does so very slowly. Approximately 50% of the dimensional change of a silver halide phototool due to fluctuations in relative humidity happens within a second. This is due to the gelatines. The remaining 50 % of the total dimensional change takes literally hours and hours. That’s why we recommend waiting for eight hours for a film to be acclimatised after a change in relative humidity. This timing is only valid when the film is exposed to the environment from both sides. However, when the film is piled up the inner sheets will never reach the new size. The outer films shield the inner ones from the influence of the humidity. The polyester of the outer films can be seen as a humidity barrier. Also here, when a film is secured by a too strong vacuum, that vacuum may hold the film and prevent it from changing its size.
