By standardizing the luminous efficiency of the human visual system, the subjective nature of photometric measurement may be eliminated. This was done in by the Commission Internationale d'Eclairages or CIE, by performing empirical tests with over one hundred observers.
The CIE material was proposed in and adopted as an international standard in Radiometry is the science of measuring light from any part of the electromagnetic spectrum. In general, the term usually applies to the measurement using optical instruments of light in the visible, infrared and ultraviolet wavelength regions.
What you see on the this page you are reading is the luminance of the black letters compared to the luminance of the white screen.
Luminance is measured in candelas per square meter. Since luminance is what we see then light sources which we look at have luminance too. The luminance of the sun and the moon give us a good idea of the huge range of brightness which the human eye can handle.
That is why you should not look directly at the sun for very long. In the diagram below taken from the chapter on Luminance my book "Candelas Lumens and Lux" a light source illuminates a piece of paper, but your eye percieves the luminance. Defined as the quantity of luminous flux emitted uniformly into a solid angle, the basic unit of luminous intensity is the candela, equal to one lumen per steradian.
Several things are suggested by this definition. One, this measurement is not applicable to collimated light sources. Two, it is inaccurate for non-uniform emitters. From these, the solid angle can be calculated, and then divided into the flux reading 5. It is the ratio of luminous flux to power. The former sense is sometimes called luminous efficacy of radiation, and the latter luminous efficacy of a source.
The luminous efficacy of a source is a measure of the efficiency with which the source provides visible light from electricity. The overall luminous efficacy of a source is the product of how well it converts energy to electromagnetic radiation, and how well the emitted radiation is detected by the human eye. In some systems of units, luminous flux has the same units as radiant flux. The luminous efficacy of radiation is then dimensionless. In this case, it is often instead called the luminous efficiency, and may be expressed as a percentage.
The distinction between efficacy and efficiency is not always carefully maintained in published sources, so it is not uncommon to see "efficiencies" expressed in lumens per watt, or "efficacies" expressed as a percentage.
In general, the person using the tool gets the same illumination after locating the two sources, and the illumination is equal on two adjacent surfaces. The relative luminous flux of a source was compared with a standard source. The photometer is placed such that the illuminance from the source being investigated is equal to the standard source, as the human eye can judge equal illuminance.
The relative luminous fluxes can then be calculated as the illuminance decreases proportionally to the inverse square of distance. A standard example of such a photometer consists of a piece of paper with an oil spot on it that makes the paper slightly more transparent. When the spot is not visible from either side, the illuminance from the two sides is equal.
By , three types were in common use. As the intensity produced by the source to be measured at the photometer approaches that of the standard source the flicker which can be seen through the eyepiece begins to diminish until a point is reached where no flicker is apparent. At this point the intensities of the sources are proportional to the inverse of the square of their distance from the photometer. An advantage of the flicker photometer over the other types is that it is unaffected by a variation in the colour temperature between the sources as a position of minimum flicker can always be attained.
This Photometer consists of a metal box with two entrances and an eyepiece as well as a slot which allows a white magnesium carbonate disk to be mounted between the two entrances. Similar to the Weber Photometer this item was designed to compare the brightness of an unknown light source to the brightness of a standard light source. Light from each of the two sources enters through its respective entrance and illuminates one side of the white disk.
The light that is reflected off each side of the disk is directed through a right angled prism and towards a Lummer-Brodhun cube. This cube directs the light rays coming from both sides of the disk into the eyepiece so that the user can observe the light from both sources simultaneously.
A Lummer-Brodhun cube is made up of two right-angled prisms that are placed with their hypotenuse sides together to form a cube. Where the prisms do not touch, total internal reflection occurs. Where they do, all the light is transmitted to the other side. The observer can then see a circular disc of light from one source, surrounded by an halo from the other.
By matching the brightnesses of the two, the observer can use the known luminosity of one source and the distances to both sources to calculate the luminosity of the unknown source.
The spherical area is a projection of the object of interest onto a unit sphere, and the solid angle is the surface area of that projection. If we divide the surface area of a sphere by the square of its radius, we find that there are 4p steradians of solid angle in a sphere.
One hemisphere has 2p steradians. The symbol for solid angle is either w, the lowercase Greek letter omega, or W, the uppercase omega. I use w exclusively for solid angle, reserving W for the advanced concept of projected solid angle w cosq. Both plane angles and solid angles are dimensionless quantities, and they can lead to confusion when attempting dimensional analysis. A solid angle extends the concept to three dimensions.
It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness.
The luminous intensity in candelas is a measure of how bright the beam in a particular direction is. If a lamp has a 1 lumen bulb and the optics of the lamp are set up to focus the light evenly into a 1 steradian beam, then the beam would have a luminous intensity of 1 candela.
The resulting beam is narrower and brighter, however the luminous flux remains the same. The Inverse square law defines the relationship between illumination from a constant-intensity light source and its distance from a surface. It states that the intensity per unit-area on the surface varies in inverse proportion to the square of the distance between the source and surface.
This range corresponds to wavelengths between 10 nm and m, and includes the regions commonly called the ultraviolet, the visible, and the infrared.
Typical radiometric units include watt radiant flux , watt per steradian radiant intensity , watt per square meter irradiance , and watt per square meter per steradian radiance. Photometry is the measurement of light, which is defined as electromagnetic radiation detectable by the human eye.
It is thus restricted to the visible region of the spectrum wavelength range from nm to nm , and all the quantities are weighted by the spectral response of the eye. Photometry uses either optical radiation detectors constructed to mimic the spectral response of the eye, or spectroradiometry coupled with appropriate calculations for weighting by the spectral response of the eye.
Typical photometric units include lumen luminous flux , candela luminous intensity , lux illuminance , and candela per square meter luminance.
0コメント