The EGG sensitometer used in this lab provides the exposure with

an xenon flash lamp. The lamp has a consistent output irradiance and

the exposure time can be controlled electronically. The light from

the flash lamp is diffused to provide a uniform exposure across the

entire surface of the step wedge. This provides an accurate and

simple means by which the film is exposed to a known energy. This

method is used to calibrate most photographic films. Unfortunately,

several problems arise when attempting to use this sensitometer for

holographic films. The xenon lamp is rich with many spectral lines,

giving a brilliant white appearance. The film response to this white

light is not necessarily the same as the response to a single laser

spectral line. This limitation can be somewhat eliminated by

incorporating a spectral filter between the lamp and the film. When

the filter is properly chosen, the output of the sensitometer can

approximate that of the laser source.

 A second limitation of the EG&G sensitometer is the extremely

short exposure time. The flash lamp emits light for a brief period

much less than a second. Considering that many holographic exposures

may last as long a minute, the time discrepancy between the use of the

film and the calibration of the film may be considerable. Reciprocity

is a term applied to the consistent response of film to the same

energy of light even when the exposure time is changed. When the

response does not depend on energy alone, but also exposure time, the

film suffers reciprocity failure. Reciprocity failure is common among

most films when attempting to use them at very long or very short

exposure times. It is important to calibrate the film with an

exposure time consistent with the intended use of the film.