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3nh
2024-12-17
As an optical instrument, the colorimeter has different measuring angles according to the different angles of light source illumination. The more common ones are 45/0, d/8 and multiple angles. Colorimeters with different measuring angles have different functions and effects. This article introduces the angles and functions of colorimeters.
In color measurement technology, existing color measuring instruments include colorimeters with different geometric light paths such as 0:45°, multi-angle and d:8° (also called integrating sphere type).
0:45° instrument has two main measurement modes, one is 0° incident light and receiving 45° reflected light, the other is 45° as incident light and 0° as reflected light. The measurement principle is shown in the figure below.
The optical path diagram of the integrating sphere measuring instrument is shown in the figure below. Its basic principle is: the light is irradiated vertically on the measured object, and the reflected light is evenly reflected in the integrating sphere (the inner surface of the integrating sphere uses a completely reflective diffuse material, and the light can be reflected back and forth) and then received at a single angle. It is equivalent to a uniform sphere emitting light, irradiating the surface of the measured object, and measuring its spectral reflectivity. Generally, a gloss absorption trap is installed inside the sphere at a position opposite to the receiving angle. When this gloss absorption trap is turned on, any specular reflection can be reduced to a minimum value. This measurement method is called "specular reflection exclusion" ( SCE ). The result is similar to the measurement result of the above-mentioned 0:45° instrument. When we measure the color appearance of the high-gloss surface and need to consider the gloss influence of the specular emission, the gloss absorption trap is turned off and the "specular reflection inclusion" method is adopted for measurement ( SCI ).
The multi-angle colorimeter mainly measures the values of five angles: 15°, 25°, 45°, 75°, and 110°. The principle is shown in the figure below.
An ideal diffuse reflector will reflect any beam of light in a diffuse reflection form, that is, in all directions. Therefore, the angle between the incident light and the observer is not important and will not affect the observation result. However, the surface of a real object is not an ideal diffuse reflector. Due to the different geometric positions of the incident light and the observer, there will be slight color differences due to the mirror reflection component and surface texture. Annular illumination can eliminate this effect. It is a 45° annular illumination and 0° (normal direction) observation, which can be abbreviated as: 45/0 : c . Since the light path is reversible, it is equivalent to 0° illumination and 45° direction annular reception. The colorimeter corresponding to this color measurement geometry condition is 3nh 's NR145+, etc. This structure of the instrument does not include specular reflection light at all, and the measured data is consistent with human eye observation. In addition, this instrument can only measure the color of opaque samples.
The integrating sphere is a hollow metal sphere, the inner surface of which is coated with a medium grey highly reflective diffuse material, such as barium sulfate or polytetrafluoroethylene. The light entering this sphere can only be emitted from the measuring hole or the light source hole after multiple reflections. A beam of light shines into the integrating sphere from any angle that does not pass through the center of the sphere, and after multiple reflections from the sphere wall, it will illuminate the sample from various angles and finally emit the integrating sphere through the measuring hole or the light source. The measuring hole is located at an angle of 8° to the normal line, and the detector is composed of a group of photoelectric tubes. For example, the TS7020, TS7030, TS7036 and other colorimeter of Sanen Company are colorimeter of this structure.
CIE does not specify the aperture size of the integrating sphere, but only requires that the aperture area of all apertures should not exceed 10% of the inner surface area. Therefore, the aperture position, size and incident angle of the light source of products from different manufacturers may be different. There is a light trap symmetrical to the observation hole about the normal. When the light trap is closed, the specular reflection component is included, which can be abbreviated as d/8:i (d represents diffuse illumination, 8 is 8 ° angle reception, and i refers to including specular reflection). It can also be abbreviated as SPIN. When the light trap is turned on, the specular reflection is removed. This condition is abbreviated as d/8:e, where e refers to removing specular reflection. This instrument can measure the color of not only opaque samples, but also transparent samples.
As the name implies, a multi-angle colorimeter is a colorimeter with multiple measuring angles. It can be divided into multiple measuring angles such as 15°, 25°, 45°, 75° and 110° according to the different angles of the light source. It measures scattered light at different angles. The multi-angle colorimeter mainly measures the values of the five angles of 15°, 25°, 45°, 75° and 110°. Here, we take the five-angle structure as an example. The light source is located at 45° to the normal line. There are five receivers in total. These five receivers are 15°, 25°, 45°, 75° and 110° to the mirror reflection line respectively. It has 4 more receivers than the single-angle structure instrument, and it measures scattered light at different angles.
When lighting at 45°, in order to distinguish these angles, they are expressed as: 45as15°, 45as25°, 45as45°, 45as75°, 45as110°, the number before as represents the angle between the light source and the normal, and the number after as represents the angle between the receiver and the mirror reflected light.
When the observation angle of the special effect sample changes, in addition to the change in brightness, the hue also changes, especially near 15°; and when the light source position changes, the hue at the 15° position will change significantly.
The multi-angle colorimeter is a detection instrument that simulates the human eye's sensitivity to red, green and blue light when measuring color. It can analyze the object being measured from multiple angles. Generally speaking, angles of 15°, 45°, and 110° are often selected for analysis. Multi-angle colorimeter is mainly used in the automotive industry. When using a multi-angle colorimeter to measure automotive color difference, calibrate the colorimeter and measure the sample under D65 light source illumination conditions, and select the measurement angles of 15°, 25°, 45°, 75° 4 angles, calculate the △E values of the 4 angles respectively to judge whether the color difference is qualified. For plain paint, the single angle 45° ΔE is required to be less than 1.7. For metallic paint and pearlescent paint, the four angles 15°, 25°, 45°, and 75° are required to have ΔE less than 1.7, which means the color is qualified, otherwise it is unqualified.
