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Photopic and scotopic vision

The visual phenomenon caused by light stimulation of different wavelengths acting on visual organs in two luminance ranges. When the brightness of light stimulation is more than 3 candela (CD), vision obtained mainly by human cone cells is called light vision or cone cell vision; when the brightness of light stimulation is below 10-3 nit, that is, under dark adaptation, vision mainly obtained by rod cells is called dark vision or rod cell vision. There are most pyramidal cells in the fovea of human retina, and only a few pyramidal cells are mixed in the rod cells at the edge of retina. Rod cells were mainly distributed in the edge of the retina. There were no rod cells in the fovea. The density of rod cells per unit area was the highest when the deviation was 20 degrees from the fovea. Light vision is mainly central vision, while dark vision is edge vision. Therefore, in low light conditions, if you want to find dim stars, keeping the target in the visual focus center is not as clear as the edge vision.

In the case of bright vision, the human eye can distinguish the details of objects and color, but the sensitivity of different wavelengths of visible light is different, so different colors of light with the same energy show different brightness. Generally speaking, yellow green looks the brightest, while the red and purple at both ends of the spectrum are much darker. This kind of relative luminous efficiency of light with different wavelengths is usually called spectral relative luminance function (V (λ) function for short) or relative luminous efficiency function, visibility function, etc., which can be expressed by spectral relative apparent brightness curve (see Figure). The V (λ) function is the reciprocal of the energy needed for people to feel the same brightness when they look at different colored lights, that is, V (λ) = 1 / E (λ). Where V (λ) is the spectral brightness function value of corresponding wavelength λ; E (λ) is the monochromatic light energy of wavelength λ. At present, the commonly used V (λ) function is the average value of K.S. Gibson and e.p.t. Tindall and W. Koblenz and W.B. Emerson respectively. It was adopted by CIE in 1924. The peak is at 555 nm.

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The CIE V (λ) function is determined according to the material of the white eye. Later, several scholars measured the V (λ) function of different races (Egyptians, Caucasians, Central Africans, etc.). The results show that the luminance function of non white people is lower than CIE V (λ) in short wave band. In recent years, Chinese psychologists and physiologists have measured V (λ) function with scintillation method

(1) The V (λ) function of Chinese eyes is consistent with CIE V (λ) function. At present, there is no sufficient evidence to prove that the difference in ethnology affects V (λ) function;

(2) With the increase of age, the V (λ) function of the short wave side of the spectrum tends to decrease, which is mainly due to the yellowing of the crystal.

In the past 60 years, there have been people's objections to the CIE V (λ) function, and the opinion is that the short band is low. In 1951, D.B. Judd proposed the correction value of CIE V (λ) function in short wave band. With the development of gas discharge light source and monochromatic light source, CIE V (λ) function can not meet the needs. Chinese Academy of Metrology and Institute of psychology, Chinese Academy of Sciences, studied V (λ) function with the method of heterochromatic lightness matching. The experimental data have been adopted by the International Commission on illumination and included in the international average values of V (λ) 2 ° field of view (VB12 (λ)) and V (λ) 10 ° field of view (vb110 (λ)) recommended by CIE Publication No. 75 in 1988.

In addition to age, the experimental conditions and research methods affect the V (λ) function. For example, under the condition of bright vision, the V (λ) curve slightly changes compared with the V (λ) curve of 2 ° field due to the different influence of macular pigment and the participation of rod cells.

As far as normal human eyes are concerned, rod cells themselves can not produce color vision. They only produce achromatic white, gray and black vision. On the contrary, under low light conditions, all objects are neutral color. The curve of spectral relative luminance function (V '(λ) function) of dark vision deviates from V (λ) curve to short wave, as shown in the figure above. This shows that the sensitivity to long wave is decreased, but the sensitivity to short wave is improved. This phenomenon is called Purkinje phenomenon.

CIE V '(λ) function is recommended by CIE in 1951 based on the results obtained by B.H. Crawford's direct comparison method and g. Wald's threshold method. The peak is at 507 nm. This curve represents the average spectral receptivity of rod cells when the stimulus leaves the fovea more than 5 ° in a completely dark adapted observer under 30 years of age. The shape of V ′ (λ) curve is mainly determined by the absorption characteristics of photosensitive chemicals at different wavelengths. The absorption curve of rhodopsin is similar to V '(λ) curve. In recent years, Chinese psychologists have measured the V ′ (λ) of Chinese by direct comparison method

(1) The shape of V ′ (λ) curve is close to CIE V ′ (λ) curve, and the peak value is slightly shifted to long wave;

(2) Age also affects the function.

The human eye's perception of light stimulation with a brightness of about 10-3-3 nits is called intervision. In intervision, rod cells and pyramidal cells act and interact at the same time, and their corresponding relationship is constantly changing, which makes people's color judgment very unreliable.