One important clarification in determining the color temperature of white LEDs is understanding the difference between Color Temperature and Correlated Color Temperature (CCT). Hence, some combination of these three primary colors – red, green, and blue – will produce white light, and this is the basis for color temperature as perceived by human vision and machine vision cameras. The spectrum of colors also has a direct correlation with light wavelength, and Figure 1 demonstrates the result of combining light wavelengths to produce the same perception of color – again modeled after the three human vision receptors. This definition lends itself well to the standard incandescent tungsten light bulb filament that works similarly, since it emits a specific color temperature related to its resistive heating state. Black body radiators emit a spectrum of wavelengths due to an increase in temperature as a result of agitated electrons as a black body radiator heats up, the color it emits changes from red, through yellow, to white, and finally blue. The emitting body is called a “black body radiator” after the idealized color of the object (black) which makes the emission equations the simplest. The color temperature of light is expressed in Kelvins (K) and is modeled after black body radiators, which produce all light frequencies. As mentioned earlier, external factors can affect the performance of the human eye in perceiving visible wavelengths, which can reduce inspection accuracy and drives the demand for machine vision inspection.
Photometric measurements are often expressed in lumens (lm) for a source or in geometric luminance (lm/m^2) or illuminance (lm/m^2/sr) at a distance from the source. These measurements of light are typically gathered using a spectrophotometer.Īlthough the same source or geometric radiant power conceptualizations are applicable, photometric measurements are based on the human perception of light radiant power relative only to visible wavelengths. Radiant power may be conceptualized and specified as source power in Watts (W) or in geometric units measured away from the emitting source, often expressed as radiance (W/m^2/sr) or irradiance (W/m^2 at a specified distance). Radiometric measurements are the physical measurements of optical radiation power over the entire electromagnetic spectrum. One aspect is the power generated, expressed in either radiometric or photometric units, and another is color temperature (typically applied to white light). There are multiple ways to characterize and specify light. Characterizing Light: How We Measure Radiant Power and Light Color Temperature Using the coordinates from the diagram allows users to develop a color that is closest to what is perceived by human visual receptors, resulting in a more consistent way of analyzing and repeating colors – particularly beneficial in machine vision. This CIE color space is patterned after the three color receptors (red, green, blue) of the human eye and is the pinnacle gauge for color modeling. This is illustrated by the Trimistus Ternary Chromaticity diagram, which plots the values of color using known X, Y, and Z coordinates (Figure 1).įigure 1 – Trimistus Ternary Chromaticity Diagram. The human perception of the color can be dependent on many factors, including ambient light in a room, the texture of the object’s surface, object reflectivity, how the observer’s brain processes color (or does not, in the case of color blindness), and many other factors.Īs a way to specify the exact color temperature of a light source, the Commission Internationale de l’Eclairage (CIE) developed a color model mapping system. It should be noted that color is not a physical or intrinsic property of an object. Background on Color Temperature in LEDs What is Color Temperatureīefore diving into the more complex aspects of color variation, it’s important to begin with the basic understanding of color: the color of an object is generated from the wavelengths an object reflects or transmits, which is then perceived by the observer.
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For this reason, it’s important to understand the concept of color temperature: how it’s measured, its impact on the success of your vision system, and how to ensure you source the best white LED light for your application. When utilizing white LEDs, it’s important to understand that the radiated emissions are a combination of most of the wavelengths from the visible spectrum for white LEDs, the color temperatures range from the warm, yellowish end of the spectrum to the cool bluish end and may ultimately impact the accuracy and robustness of your inspection. When specifying LED lighting for machine vision applications, the LED wavelength can have a significant impact on the success of the vision inspection.
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