When the light achieve to the object it divides into Reflected light / Scattered light / Transmitted light and some of absorbed. The ratio of each effects are depend on the object details such as material / surface shape / flatness and also wavelength of the light. Generally shorter wavelength has higher scattering rate. It’s mean short wavelength such as Blue is suitable for small defect inspection such as scratch or crack of the object than Red color. We recommend to imagine these factors before choose lighting unit for vision system.
It is important to understand the relationship between lens angle and dimension of lighting unit especially the case need to use direct light such as direct ring light or diffused ring light and back light etc. Normally the light unit dimension should be double size of FOV (Files of View) except the combination with Telecentric lens. Because the incident light angle will the be same as reflect light angle (in case of flat surface). So, in order to cover the entire FOV, we need to consider lens angle. And if the lens or lighting working distance will be longer, the required light unit dimension will be bigger accordingly.
Choosing suitable lighting unit, it is better to imagine which type Dark Filed or Bright Filed is suitable image for the project purpose. In dark filed case, camera receive scattered light, reflected light is away from the camera. So the defect or mark will be shown as white color in the image with Mono chrome camera. On the other hands, bright filed case, camera receive reflected light and able to get high contrast. But specular reflections on shiny or reflective surfaces. Also lighting unit should be double size of FOV. Optional diffuser plate or Diffused light (such as VL-RD / VL-LRD) should be good option in order to avoid hot spots or dark spots in the camera image.
The most important things to make the best vision system is find out the lighting and lens which provide enough contrast between target point and others in the image. In order to get the contrast, we need to find out suitable shape and size, color, brightness from variety of machine vision lighting. Because these conditions will be different in every project by the object difference such as surface shape, material, color, and mechanical condition, inspection purpose etc.. Sometime, application may need multi-angle or multi-color, mixed-color lighting in order to get contrast. VAL has wide-range of line-up and also customized model capability.
It is very important to choose correct light to fix machine vision system because color will also effect the contrast between target point and others in the camera image. When we consider “light” it will helps if understand the difference between Additive colors and Subtractive colors. Lights such as Green / Blue / Red are Additive colors, ex. Green + Blue + Red = white, Green + Blue = Cyan etc. So the result will be determine by addition. On the other hands Pigments such as Cyan / Magenta / Yellow are Subtractive colors, ex. Cyan absorb Red, Magenta absorb Green, Yellow absorb Blue, then CMY absorb all lights and become black. So the result will be determine by subtraction.
When consider lighting brightness, many people compare the “maximum power consumption”. Of course, power consumption will be one of a key factor but it can’t be only one factor to compare lighting brightness. Following image is a comparison between 2.5W Spot lighting and 3.0W Spot lighting at exactly same condition (camera shutter speed / Telecentric lens / LED power supply).
The 2.5W spot light provides much brighter image than 3.0W even lower power consumption. It is better to unerstand the other factor which is related lighting brightness as following chart. If there are two lightings which has the same power consumption with different angle of the light emitted, A is 120 degree / B is 60 degree, and if the 60 degree illumination able to cover entire FOV, image brightness will be brighter than 120 degree one. Thus power consumption should be just one of the factor when you consider lighting brightness.
Sample image of the angle of the light emitted difference. Narrow angle light provides more higher intensity than Wide angle light. But the lighting area is smaller than Wide angle type. Both light is the same power consumption with 24V design.
This is an introduction of lighting method which is common in current machine vision market.
Continuous mode and Strobe mode are famous in current machine vision market. But it is better to separate on more mode to avoid misunderstanding. It is 3 mode, Continuous, ON/OFF (or Non-over drive strobe) and Strobe (or Over Drive). Over drive mode is the method to increase lighting brightness twice of 100% brightness at continuous mode.
It’s not so rare case that the project needs customized lighting even though there are so many kind of standard lighting products in worldwide market. VAL is capable to design and provide customized light solution from small to large quantity. The following points are important factor when we consider customized light. Please try to fill in the blank as much as possible when give us customized or semi-customized opportunity.