GB/T 3978-2008Standard illuminants and geometric conditions (English PDF)

3.1 Illuminant

Radiation with a certain relative spectral power distribution in the wavelength range that affects the color perception of an object.

3.2 CIE standard illuminants

CIE defined illuminant A and illuminant D65 based on the relative spectral power distribution.

3.3 CIE sources

The relative spectral power distribution of artificial light sources specified by

CIE which is similar to the relative spectral power distribution of CIE standard illuminants.

3.4 Geometric conditions

The geometric relationship between the illumination source of the color measuring instrument and the detector and the color sample of the object, which is different from the illumination observation conditions in the visual evaluation.

3.5 Reference plane

The plane on which the measured sample or reference standard is placed during the measurement.

3.6 Sampling aperture

The illumination area on the reference plane or the area where the receiver detects the flux, whichever is smaller, shall prevail. When the area of the illumination area is larger than the area of the detection area, it is called

"overfilled"; when the area of the illumination area is smaller than the area of the detection area, it is called "underfilled".

3.7 Forty-five degree directional geometry

The incident light illuminates the reflective sample in a direction at 45° from the normal, the symbol is 45°x.

3.8 Forty-five degree annular geometry

The light source illuminates the reflective sample at an angle of 45° to the normal from all directions at the same time, the symbol is 45°alpha.

3.9 Zero degree directional geometry

4.1.3

The second column of Table 2 shows the tristimulus values and

chromaticity coordinates of standard illuminant A (380 nm ~ 780 nm, 5 nm interval) under the CIE 1931 standard chromaticity observer; the second column of Table 3 gives the tristimulus value and chromaticity coordinates of the standard illuminant A (380 nm ~ 780 nm, 5 nm interval) under CIE 1964 standard chromaticity observer.

4.2 CIE standard illuminant D65

4.2.1

The relative spectral power distribution S D65 (lambda) of the CIE standard illuminant D65 is determined by the value given in the third column of Table 1.

4.2.2

The relative spectral power distribution of CIE standard illuminant D65 is extrapolated on the basis of the measured data of daylight. It represents daylight which has a correlated color temperature of approximately 6500 K (also known as the nominal correlated color temperature of daylight illuminants).

4.2.3

The third column of Table 1 shows the relative spectral power distribution of the standard illuminant D65 between 300 nm ~ 780 nm, at 5 nm interval; this set of values can meet most of the actual calculation needs. Other intermediate values can be calculated from published values using linear interpolation.

4.2.4

The third column of Table 2 gives the tristimulus values and chromaticity coordinates of the standard illuminant D65 (380 nm ~ 780 nm, 5 nm interval) under the CIE 1931 standard chromaticity observer. The third column of Table

3 gives the tristimulus value and chromaticity coordinates of standard illuminant

D65 (380 nm ~ 780 nm, 5 nm interval) under CIE 1964 standard chromaticity observer.

4.3 Other illuminant D

4.3.1

When D65 cannot be used, it is recommended to use one of the daylight illuminants D50, D55, or D75. Their spectral power distributions are as shown in the fourth, fifth, sixth columns of Table 1, respectively. Their tristimulus values and chromaticity coordinates under CIE 1931 standard chromaticity observer and CIE 1964 standard chromaticity observer are as shown in the fourth, fifth, sixth columns of Table 2 and Table 3, respectively.

4.3.2

When none of these daylight illuminants can be used, formulas (2) ~ (7) can be used to calculate the daylight illuminant under a certain nominal correlated color temperature (Tcp ). The correlated color temperature of the illuminant given by these formulas is approximately equal to the nominal value.

4.3.3 Chromaticity coordinates of illuminant D

5.1

CIE light source A

5.1.1

The gas filled tungsten lamp with correlated color temperature of 2856 K

(c 2 = 1.4388 x 10

-2 m·K) simulates the artificial light source of standard illuminant A.

5.1.2

When light source A is used in the ultraviolet region, a lamp with a fused silica glass bulb or window shall be used.

5.1.3

When the spectral power distribution of the light source is required to be more accurate, the actual light source used shall be calibrated for spectral radiance.

5.2 Artificial light source representing standard illuminant D65

5.2.1

It is not recommended to use artificial light sources to realize CIE standard illuminant D65 or other illuminant D.

5.2.2

By calculating and changing the special metamerism index of the illuminant, the quality of the daylight simulator used for chromaticity can be described and evaluated. When the spectral power distribution of the light source needs to be more accurate, the actual light source used shall be calibrated for spectral radiance.

6.1 Geometric conditions of reflection measurement

6.1.1 Diffusion

8°, including specular components (symbol is di:8°)

The sampling aperture is uniformly illuminated from all directions by the inner surface of the hemisphere bounded by its plane; the measurement area is overfilled. The response of the detector to the sampling aperture area is uniform.

The reflected beam axis and the sample center normal are at an angle of 8°.

The radiation reflected by the sampling aperture is uniform in all directions […]

6.1.6

Alternative geometric conditions of diffusion (symbol is d:0°)

An alternative geometric condition for diffusion is that the exit direction is along the sample normal, which is a strict geometric condition that does not include specular reflection.

The light whose apex is located at the center of the sampling aperture, central axis is located on the normal line of the sampling aperture, from all directions between the two normal cones with half angles of 40° and 50°, respectively, illuminates the sampling aperture uniformly. The detector whose apex is located at the center of sampling aperture, central axis is along the normal direction of the sample, in a normal cone with a half angle of 5°, receives the reflected radiation uniformly. This geometric condition can minimize the effect of selective reflection on sample texture and direction. If this lighting geometric condition is approximated by multiple light sources arranged in a shape close to a circle, or approximated by a fiber bundle with multiple light exits arranged in a circle and illuminated by a single light source, the resulting geometric condition is called a annular/vertical geometry condition (symbol is 45°c:0°).

6.1.8

Vertical/45° annular (symbol is 0°:45°alpha)

The angle and space conditions satisfy the condition of 45°alpha:0°, but the light path is opposite. Therefore, the sampling aperture is illuminated vertically; the reflected radiation is received by the annular with the center at an angle of 45° to the normal.

The angle and space conditions satisfy the condition of 45°alpha:0°, but the radiation is only emitted from one azimuth. Eliminate mirror reflection, highlight the influence of sample texture and directionality. The symbol x indicates that the incident beam illuminates the reference plane from an arbitrary direction.

Figure 2 is a schematic diagram of 45° unidirectional/vertical geometric conditions.

6.2.2

When the geometric conditions meet the requirements of 6.1.1, 6.1.2,

6.1.6, 6.1.7, 6.1.8, 6.1.9, 6.1.10, the measured result is the reflection factor.

When the measurement angle is small enough, the magnitude of the reflection factor is the same as the magnitude of the radiance factor; when it conforms to 6.1.3 and the integrating sphere is an ideal integrating sphere, the measurement result is the reflectance.

Note: The measurement result under the condition of 45°x:0° is the radiance factor beta45:0 ; the measurement result under the condition of 0°:45°x is the radiance factor beta0:45 ; the measurement result under the condition of di: 8° is the radiance factor betadi:8, which is close to the radiance factor betad:0 ; the measurement result under the 8°:di condition is the reflectance rho.

6.2.3

Measuring certain types of samples (such as retroreflective materials) requires different geometric conditions or tolerances. If special lighting and measurement conditions are used, special instructions are required.

6.2.4

When using the integrating sphere, in order to block the direct light between the sample and the entrance hole or measuring hole on the spherical wall, a white-painted screen is required. The total area of the opening of the integrating sphere shall not exceed 10% of the reflective area inside the sphere.

6.2.5

The diffuse sample can scatter radiation to a direction close to parallel to its surface; this part of the radiation shall be included in the diffuse reflectance measurement.

6.2.6

When the integrating sphere is used for the measurement of fluorescent samples, the relative spectral power distribution of the illumination system will be replaced by the reflected power and emission power of the sample.

Therefore, the conditions of 45°alpha:0°, 45°x:0°, 0°:45°alpha, 0°:45°x are more suitable for this application.

6.3 Geometric conditions of transmission measurement