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Foreword:
In computer graphics, concepts such as Radiance and Luminance are not distinguished. Radiation is a concept of irradiance, while brightness is a concept of photometry.
Strong irradiance does not mean that the brightness is strong. For example, ultraviolet light is invisible to the human eye, so no matter whether its irradiance is low or weak, its brightness is 0. Then in computer graphics, the human eye
sees Things that are not available are not within the scope of the discussion, but here we can briefly understand this concept. If the concept of irradiance appears, we can directly treat it as a concept in photometry.
1. What is Irradiometrics
The science that takes the electromagnetic radiation energy of the entire electromagnetic wave band as the research object is objective and independent, and observers do not see it.
what is light Light is an electromagnetic wave. Light has different wavelengths and spectra and not all light is visible to the naked eye, and light has wave-particle duality. Factors such as temperature can affect irradiance, which can change the final visual effect.
In computer graphics, the "visible spectrum segment" is the object of research and calculation, and it is based on geometry and propagates in a straight line.
Electromagnetic waves with a spectral wavelength of 0.1nm to 0.1cm are within the scope of the "optical spectrum band". If it is less than that, it is a gamma ray, and if it is greater than it, it is a category of microwaves and radio waves.
Optical spectrum segment: Divided by wavelength: X-ray, far ultraviolet, near ultraviolet, visible light, near infrared, short-wave infrared, long-wave infrared and far infrared.
Visible spectral segment: the spectral segment that produces visual stimulation to the human eye and produces brightness and color perception. The wavelength of visible light usually refers to 0.38~0.76 μm (micrometer)
Radiant flux: light energy of all wavelengths radiated by surface element dS per unit time, radiant
energy, radiation intensity, etc. There are a large number of articles describing its calculation method, unit, influence on materials, and measuring instruments. Here, it mainly describes the visual correlation. In This is not for discussion but for record.
2. What is Photometry
The science of studying the intensity of light is based on the "standard human eye"'s perception of light.
The perception intensity of different colors of light in the human eye is different, and the brightness that the human eye can receive at the same time also has a certain range. The different cells in the human eye and what functions they play will finally allow us to feel the difference in light.
The human eye receives light information through the retina, which is full of "cone cells" and "rod cells". When the luminosity is sufficient for photopic vision, the human eye uses push cells to receive light and distinguish colors. When the luminosity is not enough, the object contours are separated by the visual frame rod cells.
The figure below is a graph of the intensity of stimulation of rod cells at different wavelengths:
It can be seen that different wavelengths stimulate rod cells differently.
The characteristic that the human eye has different sensitivities to light of different wavelengths is also called visual sensitivity. Personality has an impact on the ability to visually distinguish objects.
| noun | Interpretation |
|---|---|
| cone cells | Better at judging colors |
| rod cells | Only works in darker environments, but cannot distinguish colors and details. |
| visual acuity | The human eye has different sensitivities to light of different wavelengths. With the same irradiated power, the yellow-green light is the brightest and the red-purple light is the darkest. |
| Luminance perception characteristics | The brightness that the human eye can perceive at the same time has a certain range, usually 109:1. |
| color vision | Cone cells in the human eye are most sensitive to red, green, and blue light. When light enters the eyeball, the corresponding cells are stimulated differently, enabling people to distinguish colors. |
| resolution | Refers to the ability of a person to distinguish the details of a scene. |
| Photopic vision | Refers to vision under sufficient light conditions, the pendant cells of the retina can be activated to clearly perceive colors |
| Scotopic vision | When the light is very weak, the rod cells of the retina are activated, and the perception of color is weak but the outline of objects can be perceived. |
| Mesopic vision | Between photopic vision and dark vision, everyone feels differently. At this time, both cone and rod cells are activated, and they can express color and brightness to a certain extent, but the clarity will be relatively weak. |
Differences in light and dark vision:
| physical quantity | unit | Interpretation |
|---|---|---|
| Luminous flux | Lumens (Φv) | Lumen refers to the luminous energy received by the light source on a plane with a size of 1 square meter and a distance of 1 meter from a uniform point light source with a luminous intensity of 1 cd. |
| Luminosity (luminosity) | Candela (Iv) | The light source emits light radiation with a unit of 540X1012Hz in a given direction, and emits radiation with an intensity of 1/683W/Sr in this direction, the unit (candela). Human eyes also have different perceptions of light intensity in different wavelength bands. The same power can usually adjust the wavelength to change the perception of brightness. |
| Illumination | Lux (EV) | The luminous flux received per unit area. |
| Light output | Lux (Mv) | Luminous flux emitted by a light source per unit area. |
| Brightness (brightness) | Candela per square meter (Lv) | The total light energy emitted by a light source per unit area in a given direction in each unit. |
| Exposure | Lux sec(Hv) | |
| Luminous Efficiency/Light Efficiency | lumens per watt ( η ) | Refers to the ratio of the light source to the input function of the light source, and sometimes refers to the ratio of light energy to irradiated energy |
| Light source energy utilization rate | Pure Quantity (V) | The ratio of the light radiation energy to the input electric power of the light source. |
