As an optical sensor to measure surface shape, a Moire 3D camera has to project a grating pattern onto the surface to be measured. Then the deformed grating image is taken into a computer to undertake the 3D surface shape calculation. The following are several fundamentals of optics.

1. Light Wavelength: Light is one of the electromagnetic waves, with its wavelength ranging from UV (l >250 nm) to Infrared (l <1 mm) rays. The visible light, which is visible to human eyes, has wavelength from 400 nm to 700 nm. Common wavelength dimensions (metric) include angstroms (Å,, 10^-10 meters), nanometer (nm, 10^-9 meters), and micrometers ( µm, 10^-6 meters). The lights used by Moire cameras are visible strobe light, red laser light (continuous light), or infrared laser light (continuous light). In case of long working distance and strong light intensity needed, laser light is usually employed. However, as a conventional illumination lighting, halogen light emits lots of heat, which is detrimental to measurement accuracy. So, it is not used directly.

   

2. Basic of light
   • Reflection

     When a light ray traveling in air encounters a opaque surface, part of the incident ray is reflected back into air. Actual object surfaces are rough, which consist of numerous sub-surfaces oriented to various directions. As shown in Fig. 1-2, the reflection pattern shows that the most intensive reflection rays are in the main reflection direction. For smooth surfaces like mirrors, the reflected light rays are parallel and in the main reflection direction. In case of diffusive surfaces like paper or plaster, the light rays scatter in different directions (see Phong's formula). When using a Moire camera to measure a mirror-like surface, the best orientation for the camera is close to but not exactly in the main reflection direction, since there is a halation problem when exactly in the reflection direction, and on the other hand the contrast is poor from any direction far from the reflection direction. Another solution is to use a white spray to turn the smooth surface into a diffusive surface.

   

   

   • Interference

     Considering the light as a wave, multiple light waves can add together constructively or destructively. In constructive interference, the amplitude of the resultant wave is greater than that of either of the individual waves, while in destructive interference, the resultant amplitude is less than that of wither of the individual waves. This phenomenon is called interference, and it is used for sub-micron, high accuracy optical measurement, like Mikleson Interferometers. OPTON has developed a Digital Surface Flatness Measuring Apparatus with oblique-incident laser rays to cover a range from sub-micron to several microns. This range is very important for industries. In order to let two light rays interfere with each other, the following conditions have to be satisfied:

     1) has phase difference
     2) from same light source
     3) single wavelength.

     Therefore, interferometers have 2 light rays coming from one light source to pass through different routes, and then to add them together. The coherent light sources are usually lasers. So with the OPTON's Digital Surface Flatness Measuring Apparatus.

Two lenses are used for the CCD camera and the projector. Due to lens aberrations and image plane distortion, there are some extents of blurring for the image formed, and this could be an error source of optical measurement machines. The following are OPTON's solutions to this problem.

• Lens Aberrations:
  • Aberrations occur when the light rays pass through the lens far from the optic axis of the lens. Lenses can produce various forms of aberrations, like spherical aberration, astigmatism, coma aberration, and chromatic aberration. When different wavelength light rays pass through a lens, color aberration also can happen. Here are OPTON's solutions:
    • (a) cross-axes layout of the CCD camera and the projector results in using mostly the center part of the lens;
    • (b) combination of different lenses to compensate the aberrations for each other;
    • (c) finally, the calibration procedure will ensure the measurement accuracy.

• Image Plane Distortion
  • The object is not in the best focus position of the lens, and one point on the object can create a blurred circle in the image. The size of the blurred circle can be calculated by the following formula.

    

The following are the OPTON's solutions to this problem

(a) to increase the intensity of the light source and to increase the aperture of the lens
(b) to increase the working distance (the S in above formula will be increased).
(c) to adjust the object into its best focus position by program with laser pointers.
(d) to automatically delete those data taken out of the depth of view, which ensures the minimum measurement accuracy.

CCD camera is one of the video cameras. Instead of film, a CCD (Charge Coupled Device) chip is positioned behind the lens, which can convert light intensity into electronic signals, and then transfer into a computer directly. A CCD camera has the following features: small, accurate, stable, low-price, and long service life. That's why they are used in Moire Cameras. The following are some fundamentals of CCD cameras.

• The number of pixels and the size of CCD chips
  • The unit used for converting light intensity into electronic signal is called pixel. In a CCD camera, there is an 2D array of pixels, usually 760 x 480 pixels. The size of a CCD chip is from 1" in few years ago to 2/3"�, 1/2", and 1/3" in present. Since the size of a CCD chip is getting smaller and smaller, so is the CCD cameras. Therefore, small low-price lenses can be used. Required by the development of high vision TV and other industries, the number of the pixels on a CCD camera is getting increased. Now, 1000 x 1000 and 3000 x 3000 high-resolution CCD cameras are available on the market. Due to relatively high price of the high-resolution cameras, conventional 760 x 480 CCD cameras are used in Moire cameras. However, 1000 x 1000 CCD cameras can be employed to cover big field of views, and to keep same accuracy in the meantime.
• Sensitivity of CCD camera
  • CCD imager is sensitive to the light rays from visible to infrared (400-1000 nm). However, for the lens, the image position will vary dependent on the wavelength of the light. This is the reason of image distortion if the light has a broad range of wavelength. In order to overcome this problem, different filters are used from different light source to filter out other wavelength components. For example, IR filter is used for Kesenon lighting, band-pass filter is used for red laser, and high-pass filter is used for IR laser.
• Digital CCD camera
  • Both analog video cameras and digital video cameras are used in Moire 3D cameras. The following is the comparison of these two kinds of camera. The price of a digital camera is relatively high currently. However, the digital cameras will become the main-flow, and in the meantime the price will be reduced in near future since it is easy to build the interface with a computer.

  Camera Type	Contrast	Accuracy	Dimension	Shutter Speed	N/S Ratio	Price	Note
  Digital	Good	High	Big	1/10,000 sec Max.	Good	High
  Analog	Worse	Intermediate	Small	1/256 sec Max.	Worse	Low

• Synchronization of strobe lighting and CCD camera
  • The strobe lighting lasts about 1/10,000 second to project a grating pattern onto the surface to be measured. A CCD camera has its own cycle of charging and reading out. It is needed to synchronize the timing of both the strobe lighting and the CCD camera cycle. Fig. 3-1 shows the synchronization control of Moire 3D cameras.