Why is it necessary to control the light intensity ratio of reference light and object light from 4:1 to 15:1 in holography?

If the light intensity ratio between the reference light and the object light is too large, it will cause poor contrast and unstable images.

The object light and the reference light should meet the following requirements: the optical path difference between the object light and the reference light should be as small as possible. It is best for the optical path lengths of the two beams to be equal, and should not exceed 2cm at most. Use a string when adjusting the light path. Measure it well; the angle between the two beams of light should be between 30° and 60°, preferably around 45°, because if the angle is small, the interference fringes will be thin.

This has lower requirements for the stability of the system and the resolution of the photosensitive material; the light intensity ratio of the two beams of light must be appropriate, generally between 1:1 and 1:10. Than measured using silicon photocells.

The reference light is emitted by the light source and is reflected and expanded directly onto the dry plate. The light emitted by the object light source is reflected by the object and then emitted onto the dry plate. Its phase and amplitude are reduced to the surface of the object. The influence of the two beams of light will be different from the reference light. When these two beams of light hit the dry plate, the interference is related to the object, so the fringes can record the phase information of the object, thereby taking a holographic photo.

Extended information:

Reference beam The laser beam used to read and write holograms. It is one of two laser beams used to create holograms. In order to read a hologram, some parts of the reference beam (i.e. its angle of incidence and spectrum) must be propagated as precisely as they were used to write the hologram. Therefore, typically reference beams are Gaussian beams or spherical beams (beams radiating from a single point), which are easily reproducible.

Considering an object as a collection of coherent point sources, the object light wavefront is the linear superposition of the spherical waves emitted by all point sources. The spherical wave emitted by each point source interferes with the reference wave.

Consider the object light wave as a linear superposition of many plane wave components propagating in different directions. Each plane wave component interferes with the reference plane wave and records the elementary hologram.

Baidu Encyclopedia--Reference Beam

Baidu Encyclopedia--Holographic Photography