Recently, researchers from the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences proposed a novel optical element made by picometer-differential twice-exposed holography, called picometer comb. Its diffraction field has the advantage of generation of a stable light field distribution with extremely-long depth of field and small divergence angle.

In the experiment, this diffraction field was used as the projection structured light of the 3D shape measurement system, and some objects were reconstructed successfully. The relevant result was published in Applied Optics.

Pattern blurring caused by limited focal depth of the digital projector is a common problem in 3D shape measurement based on structured light projection, especially in long-distance and long depth of field measurement scene, which seriously affects the accuracy of 3D reconstruction.

The concept of picometer comb was first proposed by Prof. ZHOU Changhe's group, referring to a diffractive optical element produced by the twice-exposed laser holography with its periods at picometer-scale difference. This method derived from the periodic measurement technique of picometer-scale accuracy of the exposure interference field.

The research group used the diffracted light of the picometer comb as the projection structured light to build a 3D shape measurement system.

They adjusted the beam angle of the interference field to regulate the periodic difference of the two holographic interference fields. Then, the substrate coated with photoresist was exposed with this exposure field, and the picometer comb was obtained after developing, fixing and drying.

The study showed that the superposed exposure field of picometer comb was the nesting of two envelopes of different sizes, and the period of the large one was far larger than the size of the single small envelope (see Fig. 2).

They found that when a beam of plane light was incident on the picometer comb at a certain angle, its - 1 diffraction light was essentially generated by the interference of two holograms at a very small angle, which has the characteristics of long depth of field, low divergence angle and high contrast.

The researchers used this special element to generate structured light in 3D shape measurement, and carried out a number of reconstruction experiments of objects. According to the results, compared with digital projection equipment, picometer comb could be utilized for 3D shape measurement of object with long depth in large scene without defocusing and dramatically-decreased resolution.

As the size of the picometer comb is very small and easy to integrate, so it is very suitable for massive industrial production and is expected to be widely used in 3D measuring equipment.

Fig. 1. Binocular 3D shape measurement system using a picometer comb. (Image by SIOM)

Fig. 2. SEM images of the picometer comb at different areas. (Image by SIOM)