Light Measurement for Holography
One of the important parameters for transmission holography is the ratio between the light intensities of the reference beam and the object beam. It has been found that having the reference beam brighter by the ratio 4:1 to 10:1 works well for holography. You can perhaps visualize the reason for a stronger reference beam by saying that you need a strong carrier beam and a smaller modulating beam from the object to produce the image.
For our holography experiments, it is important to optimize exposure for the film used. The film emulsion was Agfa 8E75HD which has optimum exposure at 25 microjoules/cm2. The Thorlabs detector shown above is rated at 0.47 amps/watt at 640 nm, so this sensitivity was used in calculating exposures. Since the detector area is 0.01 cm2, the exposure time in seconds is calculated from
When we measured the light directly from the reference beam at the film position, and then added the light from the object, we found that we got better exposures by increasing the exposure 2 stops. That is, we multiplied the calculated time by four.
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