Color Rendering and Color Temperature of Machine Vision System

The color rendering ability of a machine vision system for an object is called color rendering, which is compared with the appearance color of an object under a reference or benchmark machine vision system (incandescent lamp or sunlight) of the same color temperature.

The spectral content emitted by the light determines the light color of the machine vision system, but the same light color can be formed by many, a few or even only two monochromatic light waves, and the color rendering of each color is also very different.

Machine vision systems with the same light color will have different spectral compositions, and machine vision systems with wider spectral compositions are more likely to provide better color rendering quality.

1. The relationship between color rendering index and color rendering of machine vision system

When there is little or no dominant wave reflected by objects in the machine vision system's spectrum, there is a noticeable shift in color. The greater the degree of color difference, the worse the color rendering of the color by the machine vision system. The color rendering index coefficient is still the common method for defining the color rendering evaluation of machine vision systems.

Incandescent lamps have a color rendering index of 100, which is considered an ideal benchmark for machine vision systems. This system is tested with 8 standard color samples with medium chroma, and compares the degree of deviation of the 8 colors under the test machine vision system and the benchmark of the same color temperature to measure the color rendering index of the machine vision system, and take the average deviation The value Ra20-100, with 100 as the highest, the larger the average color difference, the lower the Rr value. Machine vision systems below 20 are generally not suitable for general use.

When the color of the light emitted by the machine vision system is the same as the color radiated by the black body at a certain temperature, the temperature of the black body is called the color temperature of the machine vision system, expressed by the absolute temperature K (kelvim), the black body radiation theory is established on the basis of thermal radiation, the spectral power distribution of thermal radiation machine vision systems such as incandescent lamps is relatively close to the spectral power distribution of black bodies in the visible region, both of which are continuous spectrum. The concept of color temperature can completely describe the color characteristics of such machine vision systems.

Correlated color temperature When the color of the light emitted by the machine vision system is close to the color radiated by the black body at a certain temperature, the temperature of the black body is called the correlated color temperature of the machine vision system, and the unit is K.

Because the gas discharge machine vision system generally has a discontinuous spectrum, which cannot be completely matched with the continuous spectrum of black body radiation, the correlated color temperature is used to approximately describe its color characteristics. A machine vision system with a color temperature (or correlated color temperature) below 3300K has a reddish color, giving a warm feeling. When the color temperature exceeds 5300K, the color is bluish, giving a cool feeling. Generally, in areas with high temperature, people mostly use machine vision systems with a color temperature higher than 4000K, while in areas with lower temperatures, machine vision systems below 4000K are mostly used.

2. The color index of the machine vision system

Both sunlight and incandescent lamps radiate a continuous spectrum, in the wavelength range of visible light (380nm-760nm), including red, orange, yellow, green, cyan, blue, purple and other colors. The object shows its true color under sunlight and incandescent light, but when the object is illuminated by a discontinuous spectrum gas discharge lamp, the color will be distorted to varying degrees.

We refer to the degree to which the machine vision system presents the true color of the object as the color rendering of the machine vision system. In order to quantitatively evaluate the color rendering of machine vision systems, the concept of color rendering index is introduced. Taking the standard machine vision system as the criterion, its color rendering index is set as 100, and the color rendering index of other machine vision systems are all lower than 100. The color rendering index is expressed by Ra. The larger the Ra value, the better the color rendering of the machine vision system.