A beam of light is projected onto an opaque object, part of the light will be reflected, and the remaining light will be absorbed by the surface of the object. The percentage of reflected light flux to incident light flux is not affected by the amount of light flux. Suppose the incident luminous flux is Φ0 and the reflected luminous flux is Φr. For a certain measurement surface and a specified incident angle, the ratio of the incident luminous flux and the reflected luminous flux is fixed. If this ratio is β, the β value is called the reflectance: 1. Geometric conditions. The transmission measurement must satisfy the following geometric conditions as far as possible: the incident light beam must be able to be evenly projected from the half-space body onto the object under test, and only the light beam passing vertically through the object under test should be measured. Using Ublich sphere as the device for transmission measurement can make the measurement meet the above geometric conditions. Ublich ball is a hollow sphere (Figure 1-3), the inner cavity is coated with matte white barium sulfate, two small holes with the axis line orthogonal to the center of the ball are opened on the spherical surface, and the total of the two small holes The area does not exceed 2% of the inner wall area of ​​the sphere. So why stipulate such geometric conditions? Because if the geometric conditions do not meet the above requirements, it may affect the measurement effect. To prove this, compare two completely transparent objects Mt and Mc (Figure 1-4), that is to say, the transmittance of these two objects is 1.0. The object Mt is transparent, it does not diffuse the directional beam, and the object Ms causes the directional beam to form a uniform diffusion completely in the half space. If the incident light is diffuse light, that is to say, the incident light is the light emitted by the Ubrich sphere, then the transmitted light flux is uniformly distributed in the half space for both transparent objects and scattered objects. Even if the measuring instrument can only measure the luminous flux within a certain solid angle, the measured value is the same. The geometric conditions of transmission measurement are determined according to this principle. [next] 2. Transmission density and Carrier effect. Transmission density is defined as the logarithm of the inverse of transmission. If the transmission density is D and the transmission rate is τ, then the transmission density is expressed by the following formula: When measuring white paper with a certain gloss, because the reflection characteristics of the measurement surface are between the first two, the light received by Sb contains only the diffuse component, while the light received by Sa is the combination of the diffuse reflection component and the total reflection component And, so the indication of Sa will be greater than Sb. The above example shows that the results of reflectometry are closely related to the geometric conditions and the gloss properties of the measured surface. So why does the reflection measurement stipulate that the latter geometric condition should be adopted instead of the former? This is because the latter geometric condition only theoretically measures useful image information. The former measurement result contains the largest noise information (the first surface reflection component). 2. The reflection density makes the measurement principle. It can be seen from Figures 1-5 and 1-6 that no matter which geometric condition can only measure a part of the reflected light flux, and the reflectance needs to be measured based on the total reflected light flux, so these measurement methods cannot be used to measure the reflectance. However, these measurement methods can be used to compare the brightness L of the object under test, and the brightness coefficient α represents the ratio of the brightness LM of the measurement area of ​​the object to the brightness L0 of a reference area. When measuring the brightness coefficient (or reflectivity) of various white surfaces (such as various paper surfaces), this standard white can be used as the reference area. If you want to measure the absolute value of the brightness coefficient of a certain surface, just compare the measured surface with the brightness of the white standard. The reflection density Dr is similar to the transmission density Dt, which is equal to the logarithm of the reciprocal of the reflectance, namely: Sometime Easy take from the Folding design products, but not limited by folding. Reversible Umbrella,Rain Proof Reversible Umbrella,Double Layer Reversible Umbrella,Easy Hold Umbrella NINGBO DAC ARTS & CRAFTS COMPANY , http://www.dacpets.com
β = Φr / Φ0
There are three different reflection forms of incident light on the surface of opaque objects: directional reflection, diffuse reflection, and reflection forms in between. If a directional beam is projected onto an ideal white diffuse reflection surface, all incident light will form a uniform reflection in the half space above the surface, and the reflection brightness at all angles is exactly the same (Figure 1-1). An ideal diffuse reflective surface that is completely white and matt does not exist, but a standard white calibration plate made of magnesium oxide or barium sulfate can be approximately regarded as a matt white surface. The mirror surface is different from the matte white surface, and its reflection of incident light follows the law of reflection. The mirror surface of mercury-coated glass is close to the ideal total reflection surface, but even such a mirror surface is not complete. Reflection, any surface will absorb part of the incident light, the difference is that some surfaces reflect more light, and some surfaces reflect less light. Figure 1-2 shows the reflection of matte paper, glossy paper and glass mirror, respectively. The length of the arrow indicates the intensity of the reflected light in the corresponding direction.
Figure 1-1
Figure 1-2
Similar to the reflection of light, the ratio of the transmitted light flux to the incident light flux of a certain object is also fixed, and the ratio is called the transmittance. If the incident light flux is Φ0 and the transmitted light flux is Φr, then the transmission τ is expressed by the following formula:
Ï„ ï¼ Î¦r / Φ0
Figure 1-3
The parallel beam enters the sphere through the hole 1, and forms a light spot L on the inner wall of the sphere, and then the light spot L reflects the light uniformly onto the inner wall of the sphere. The test object is next to the hole 2. In order to prevent the hole 2 from receiving the direct reflection of the light spot L, an oval baffle S, which is also painted in dull white, needs to be placed between the light spot and the hole 2, so that the tight The test object next to the hole 2 is irradiated with uniform light from the half-space body.
Figure 1-4
In both cases, Φτ = Φ0 holds, however, the measurement results show that the measured value of Mt in direct light is greater than the measured value of Ms. This is because any luminous flux measuring instrument can only measure the light quantity from a certain solid angle, that is, only a part of the transmitted light quantity can be measured. Therefore, the ratio of these two measured values ​​is approximately equal to the ratio of the luminous flux contained in the two solid angles of the former and the latter, but since the former is greater than the latter, the result of Mt> Ms is obtained.
D = lg1 / Ï„
The following formula is used to convert between transmittance and density:
lg1 / Ï„ = D
1 / Ï„ ï¼ 10D
Ï„ ï¼ 10ï¼D
It has been pointed out earlier that if the transmission measurement does not meet the specified geometric conditions, even if two completely transparent objects (that is, the density D is 0.00), when illuminating with parallel light, objects with a diffuse surface are better than non-diffuse The measured transmittance is smaller, that is, the former shows a higher density value than the latter. If two objects are non-diffuse bodies with the same transparency, one is illuminated by parallel light and the other is illuminated by diffuse light. Even if the incident light intensity is the same, the measured transmission density value is also different. This phenomenon of the difference in the transmission density indication caused by the different diffusion properties of the two measured objects or the different geometric conditions is called the Carrier effect. The quotient of the measured density value when the same object is illuminated with parallel light and diffuse light is called the Carrier coefficient. The Carrier coefficient can be used as a measure of the size of the silver particles in the film. When processing and converting images on film, special care should be taken to minimize the Carrier effect. In the gray balance calibration of the electronic color separation machine, the reason why the use of non-silver salt gray scales is emphasized is to avoid the deviation caused by the Carrier effect.
1. Geometric conditions. Reflectance measurements (except for specular reflections) are mainly performed on diffuse surfaces, so this measurement is more complicated than transmission measurements. Due to the structural design of the densitometer, it is difficult to achieve uniform illumination of the measurement surface from the half-space body and vertical measurement of the surface, so 45 ° / 0 ° and 0 ° are often used in reflection measurement / 45 ° two geometric conditions (Figure 1-5). In the picture: Ms represents the photometric sensor.
Figure 1-5
In reflectometry, the measurement result is closely related to the geometric conditions of the measuring device and the gloss properties of the surface of the test object. An example is given to illustrate the necessity of specifying geometric conditions for measurement. Figures 1-6 show two measurement geometric conditions. Measure the mirror surface, matte white surface and paper with a certain gloss under these two measurement conditions. In the figure, Sa and Sb respectively represent two photosensors with the same specifications.
Figure 1-6
When measuring a mirror object, the sensor Sa displays the largest measurement value, because according to the law of reflection, the incident light is totally reflected on the measurement surface, and the sensor Sa is exactly arranged on the reflected light path. The value of sensor Sb is zero because it has deviated from the reflected light path by 45 °.
When measuring a matte white surface object, because the incident light flux is uniformly diffused in the half-space range, the indications of Sa and Sb are the same.
The characteristics of the surface of the reference area should be close to the ideal matte white surface, that is, it should meet the specified barium sulfate white standard.
The smaller the reflectance and the larger the reciprocal, the greater the reflection density. However, as can be seen from the foregoing, the specified geometric conditions can only measure a part of the luminous flux, and the reflectance needs to be calculated based on the total reflected luminous flux, so this geometric condition cannot be used to measure the reflectance. Fortunately, under the same optical conditions, the values ​​of reflectivity and brightness coefficient are approximately equal, namely:
β = α
The reflection density meter is based on this principle to determine the reflection density. The function of the reflection density meter is to compare the brightness of the measurement area with the brightness of a certain reference area.
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1. Reflectivity and Transmittance
2. Transmission measurement
Three, reflection measurement