Photometric Stereo (PMS)

Generate edge and texture images using shape from shading

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Photometric Stereo (PMS)

Generate edge and texture images using shape from shading

Photometric stereo is a computational imaging function which allows the user to separate the shape of an object from its 2D texture. Its primary purpose is to accentuate the three-dimensional surface structure of an object. It works by firing segmented light arrays from multiple angles and then processing the resulting shadow images in a process called “shape from shading”.

It is useful for the detection of small surface defects and 3D surface reconstruction. PMS is a height driven process which can enhance surface details like scratches, dents, pin holes, raised printing, or engraved characters. Because the final image is a computed surface based on the shading information, surface coloring or features without height are removed. This can make visually noisy or highly reflective surfaces easier to inspect. PMS is especially effective on surfaces that have 3D structure but little to no contrast. This capability is rapidly becoming popular in the MV market.

Principle of CCS LED illumination being used within Photometric Stereo computational imaging applications

A basic PMS system uses lighting from four or more directions to cast a directional shadow around raised or sunken features on an object. The illumination may be a ring light with four 90 degree quadrants, an array of four bar lights, or any other arrangement that produces directional lighting. The feature map can be applied through different algorithms to show surface details that are hard to find or can't be detected in pure visual or machine vision images.

Tire inspection with PMS

The images in this example were captured with four long bar lights arranged in a square pattern around the tire.

The quadrants were fired in sequence to create the East, South, North, West images on the left.

Tire E

Tire S

Tire N

Tire W

Tire Texture

Tire Shape

Tire Texture and Tire Shape images are then created by combining these images in software as shown. The PMS routine removes the visual noise and leaves only the features of interest. The contrast of the sidewall printing is greatly enhanced, simplifying OCR or OCV and increasing accuracy.

The Texture image is often useful for showing surface coloring without interference from surface structure or for removing glare from reflective surfaces. In this example, only the Shape image shows useful information.