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Glossary FAQ

Determination of exposing time of holograms

All the holographers familiar with the situation, when the hologram is not properly exposed - brightness of the hologram is low, there is a veil on all its surface etc. We offer the simple and rather precise method of definition of optimal exposing time of the hologram. For this purpose it is necessary to measure illumination intensity E of a photoplate. As the hologram is recorded by monochromatic laser light, the measurements of illumination intensity are necessary to carry out in energy units, for example, in mkW/cm2 [4]. It is necessary to refuse usual photometer devices such as luxmeters, because they are designed for measurements in white light.

The silicon photodiodes are most suitable for measurements in holography, that have a high light sensitivity, good linearity in a broad band of photocurrents, big size of a photosensitive surface, opportunity to work in a photovoltaic mode, without the power supply. We recommend as a measurer of a photocurrent the microammeter, that have broad band of measured currents - from 50 nA up to 50 mA (figure shows FD-7k photodiode, which is popular in Russia). The special measurings have shown very good linearity of recording of photocurrents on all bands for a pair: such special photodiode - microammeter. It allows one to determine illumination intensity and, accordingly, the time of exposing of the hologram depends on value of a photocurrent under the simple formula with use of scaling coefficient K.

Figure shows the diagram of relationship between sensitivity S(l) and wave length for the special photodiode, which is recorded in a band of wave lengths including the basic line of generation of lasers: 488 nm, 514 nm and 633 nm.Scaling coefficient can be calculated from the expression:

K = 1/S(l)*A, mkW/(mkA*cm2),

Where S(l) is spectral sensitivity of the photodiode, mkA/mkW, A - photosensitive surface of the photodiode, cm2.

There are in the table below the coefficients calculated for three wave lengths of the laser at a diameter of a diaphragm on a surface of the photodiode 3 mm. The complete diameter of a photosensitive surface of the photodiode is equal 8 mm and it not always suitable for precise measurements of illumination intensity from an object beam.

Wave length, nm S(l) K










The determination of illumination intensity of a photoplate E is multiplication of value of a photocurrent I, measured in mkA on scaling coefficient K.

E = K*I, mkW/cm2

The exposing time T, corresponding to necessary energy of exposing Q, mkJ/cm2, is determined by the formula:

T = Q/E, s

Where E is summary illumination intensity of beams, recording the hologram, in a sectional point of a photoplate expressed in mkW/cm2.-->