Daylight in a Box
For imaging applications that need to measure multiple colors, white LED lights have supplanted halogen and fluorescent lights for a number of reasons, including LEDs’ ability to offer a wider range of colors and varieties of “white” light, also referred to as the color temperature.
“We offer white LEDs that run from cool blue-whites around 6500 Kelvin to neutral whites around 4000 K to warm whites at 2200 K that have more red in them,” says Oliver Szeto, President of Metaphase Technologies Inc. (Philadelphia, Pennsylvania). “While we see a lot of machine vision applications use the 6000 K cool whites because of high output, there’s also a trend toward lights that more closely mimic natural sunlight, around 3500 K.” For color machine vision applications that require accurate color measurements, a light’s color rendering index (CRI) is important because it quantifies how closely the light will reproduce colors compared to a reference light source, usually daylight.
White LEDs are made one of two ways: by applying a phosphor coating over a blue LED light that produces a broadband light closer to white light, or by mixing different-colored LEDs to make a broadband light source. Both methods result in a spectral continuum that is higher in some narrow wavelength bands within the white light spectrum compared to others. For the most challenging color vision applications, designers need to carefully match these “spikes” to the specific wavelengths. This is where choosing a lighting supplier with in-house engineers can really help, adds Metaphase’s Technical Sales Manager, Mark Kolvites. A quality supplier will make sure that the actual red, green, and blue (or more) LEDs mix to create a white light, or the blue LEDs with phosphor coating provide uniform illumination without hotspots that can cause trouble for automated inspection systems.
As the information above shows, color machine vision solutions can require in-depth knowledge of the physics behind machine vision. The good news is that by choosing the right supplier and partner, designers can solve applications where success isn’t just black and white.
refer to:http://www.visiononline.org/vision-resources-details.cfm/vision-resources/Is-Your-Machine-Vision-System-Color-Blind/content_id/4333
