::Exposure

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 the coefficients in the table below, that are calculated for three wave lengths of the laser at a diameter of a diaphragm on the 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.

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 sensitivity of a photoplate S, mkJ/cm², is determined by the formula:

T = S/E, s

Sensitivity S is mentioned on a label of a box with photoplates (see Lesson 2).

Let's consider concrete example for our recording scheme. We use the photoplate PFG-03 with energy of exposing 1000 mkJ/cm².

The measurement at a centre of the photoplate gives the value of a photocurrent - 3 mkA. We multiply this value on scaling coefficient (50) for a wave length 633 nm and it gains illumination intensity of photoplate:

E = 3*50 = 150 mkW/cm²

Next, we divide known value of exposing energy for photoplates PFG-03 by the received value of illumination intensity and it gives required time of exposing:

1000 (mkJ/cm2)/150 (mkW/cm2) = 7 s

The received time of exposing is optimum time for this illumination intensity of a photoplate PFG-03.

In summary we give several concrete recommendations:

• Find out beforehand the optimum energy of exposing for photoplates from the manufacturer, if it's not written on a box.

• Hold a photosensitive surface of the photodiode in a parallel plane of a photoplate (see fig.) and closer to it, if possible. Otherwise value of illumination intensity will be measured incorrectly.

• Keep uniformity of a beam on surface of a photoplate. More than 30% from illumination intensity at centre does not suppose wane of illumination intensity on edges of a photoplate. It is possible to do these measurements with the help of the photodiode - just on value of a photocurrent. Do not rely on your eyes - they can't compare soft illumination intensities.

• Try to test linearity of measurements of a photocurrent. For this purpose find a neutral light filter with well-known multiple factor of absorption (for example, 2 or 3). Measure the illumination intensity by the photodiode, then put the light filter in front of the photodiode (plane of a filter should be perpendicular to the direction of light). If a measurement is linear, a photocurrent will reduce by the same value. Otherwise, a photocurrent will reduce by a lesser value. In this case a measurement is nonlinear, use a microammeter with a lesser input resistance.

• All the measurements should be carried out in a dark room.

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