The amount of light shining on a specific region of an object's surface is expressed as illumination. This value is based on how brightly different wavelengths of light, which correspond to different colors, are seen by humans. Because humans perceive light with a wavelength of around 550 nanometers and close by (green) as brighter than light with longer or shorter wavelengths (purple, blue, red), illumination calculations are adjusted for each wavelength. It's common to compare illumination to how bright something seems to the human eye.
The area across which the light is scattered has an inverse relationship with illumination. Accordingly, illuminance for a given light source would be higher for a smaller area and lower for a larger area.
Illuminance, Luminance, and Brightness: What They Mean
Illuminance can be thought of as the light coming at the object, or more specifically, as the amount of light that touches the surface. Luminance, the quantity of light reflected from the surface, is distinct from this. If you consider the following, it is simple to recall:
Incident Light plus equal brightness illumination.
One of the reasons the term "brightness" is not frequently used in science is this. Our sense of brightness can be based on how much light is coming from the surface and being reflected (luminance), coming from the surface and being reflected (illuminance), or even how much light is present overall in the space. Depending on the circumstances, it might also be a combination of some or all of these. Additionally, we could consider the area, the object, or the light source as being bright.
Illuminance, as photometry uses, is the total light flux incident on a surface, divided by its area. It is a measurement of incident light intensity that has been wavelength-weighted using the luminosity function to correspond to how bright people perceive things. The luminous flux per unit area radiated from a surface is the same as luminous emittance. Another name for luminous emittance is luminous exitance.
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