| Table 1: Key Photometric Units | |
| 1 footcandle | = 1 lumen/ft2 |
| 1 footcandle | = 10.764 meter candles |
| 1 footcandle | = 10.764 lux |
| 1 candle | = 1 lumen/steradian |
| 1 candle | = 3.142 x 10-4 Lambert |
| 1 Lambert | = 2.054 candle/in2 |
| 1 lux | = meter candle |
| 1 lux | = 0.0929 footcandle |
| 1 meter candle | = 1 lumen/m2 |
Often, a customer struggles with contrast and resolution problems in an imaging system, while underestimating the power of proper illumination. In fact, desired image quality can typically be met by improving a system's illumination rather than investing in higher resolution detectors, imaging lenses, and software. System integrators should remember that proper light intensity in the final image is directly dependent upon component selection.
Correct illumination is critical to an image system and improper illumination can cause a variety of image problems. Blooming or hot spots, for example, can hide important image information, as can shadowing. In addition, shadowing can also cause false edge calculations when measuring, resulting in inaccurate measurements. Poor illumination can also result in a low signal-to-noise ratio. Non-uniform lighting, in particular, can harm signal-to-noise ratios and make tasks such as thresholding more difficult. These are only a few of the reasons why correct illumination for your application is so important.
The pitfalls of improper illumination are clear, but how are they avoided? To ensure optimal illumination when integrating a system, it is important to recognize the role that choosing the right components plays. Every component affects the amount of light incident on the sensor and, therefore, the system's image quality. The imaging lens' aperture (f/#) impacts the amount of light incident on the camera. Illumination should be increased as the lens aperture is closed (i.e. higher f/#). High power lenses usually require more illumination, as smaller areas viewed reflect less light back into the lens. The camera's minimum sensitivity is also important in determining the minimum amount of light required in the system. In addition, camera settings such as gain, shutter speed, etc., affect the sensor's sensitivity. Fiber optic illumination usually involves an illuminator and light guide, each of which should be integrated to optimize lighting at the object.
The light intensity for our illumination products is typically specified in terms of footcandles (English unit). Lux, the SI unit equivalent, can be related to footcandles as follows: 1 lux = 0.0929 footcandle.
| Table 2: Illumination Comparison | ||
| Application Requirement | Object Under Inspection | Suggested Type of Illumination |
| Reduction of specularity | Shiny object | Diffuse front, diffuse axial, polarizing |
| Even illumination of object | Any type of object | Diffuse front, diffuse axial, ring light |
| Highlight surface defects or topology | Nearly flat (2-D) object | Single directional, structured light |
| Highlight texture of object with shadows | Any type of object | Directional, structured light |
| Reduce shadows | Object with protrusions, 3-D object | Diffuse front, diffuse axial, ring light |
| Highlight defects within object | Transparent object | Darkfield |
| Silhouetting object | Any type of object | Backlighting |
| 3-D shape profiling of object | Object with protrusions, 3-D object | Structured light |
Types of Illumination
Since proper illumination is often the determining factor between a system's success and failure, many specific products and techniques have been developed to overcome the most common lighting obstacles. The target used throughout this section was developed to demonstrate the strengths and weaknesses of these various lighting schemes for a variety of object features. The grooves, colors, surface deformations, and specular areas on the target represent some of the common trouble areas that may demand special attention in actual applications.
