Test conditions

 

Conditions under which a vision chip is tested are very important. A mere statement of ``room condition'' or ``under sun light'' does not represent an engineering method of testing. More quantified measures are required.

The main function of vision chips is to acquire and process light intensity in space and/or time. Therefore, a measure of the lighting conditions should be provided at the first instance. Some simple tests and measures for this purpose are described here.

• Source of light. It is important to know the source of light. This can be either sun light or moon light for natural lighting, or fluorescent, incandescent, halogen, laser, UV, and many other types of artificial light sources. Specifying the light source is important for determining both the spectrum of the input light and additional components that might exists in a specific light source (for example AC light sources, like fluorescent and incandescent lamps, have an AC component present in them).
• Method of exposing the chip. Test patterns applied to the chip come either from reflected light from the patterns on Lambertian surface (for example a paper), or from transparent layers with light passing through them. In the latter, the light source should be located behind the transparency. Although both methods are acceptable, the first is closer to real situations, as almost all the objects in real environments are opaque with lambertian surfaces.
• Spectrum of the input light. Except for laser light sources, the spectrum of all other sources is broad. Some sources may have strong output light intensity at a wavelength, while producing light at other wavelengths as well (such as UV sources).
• Light intensity. Using any light source the amount of light being exposed to the chip should be known and measured. The light intensity can be measured either in lux or watts per unit area.
• Temperature. Like all other integrated circuits, the operation of vision chips is affected by temperature.
• Optical interface characteristics. The spatial mapping of the output world onto the surface of the vision chips is performed by the optical interface. The optical interface can range from a single lens to complicated lens structures. There are at least the following factors that are important when using an optical interface
  • Focal length
  • Depth of Focus
  • Aperture
  • Spectrum over which the lens does not show chromatic aberration, or alternatively the best spectrum for operation

There are instruments available for measuring each of the above tests. For many light sources, the manufacturers may provide basic data, such as spectrum and spatial distribution patterns. For almost all optical interfaces the manufacturers provide extensive data.

Another test condition of great importance is the biasing conditions of the chip. Vision chips usually have several biasing parameters to tune the functionality of the chip. Obviously, a chip may not function under all conditions, and tuning might be required for different tests. If the tuning (or adaptation) is a builtin or automated function, it does not need to be expressed in test conditions. However, if manual tuning should be performed to adjust the chip for different tests, it should be left at the ``best biasing conditions'' for all tests. This limitation is to simulate the operation of the chip if it is used for a real application, or in other words, the reliability and total dynamic range of the chip.