Today we will talk about the unit of measurement of luminous intensity. This article will reveal to readers the properties of photons that will help determine why light comes in different brightnesses.
Particle or wave?
At the beginning of the twentieth century, scientists were puzzled by the behavior of light quanta - photons. On the one hand, interference and diffraction spoke of their wave essence. Therefore, light was characterized by such properties as frequency, wavelength, and amplitude. On the other hand, Lebedev’s experiments convinced the scientific community that photons transmit momentum to surfaces. It would be impossible if the particle had no mass. Thus, physicists had to admit: electromagnetic radiation is both a wave and a material object.
Photon energy
As Einstein proved, mass is energy. This fact proves our central luminary, the Sun. The thermonuclear reaction turns a mass of highly compressed gas into clean energy. But how to determine the power of the emitted radiation? Why in the morning, for example, the light intensity of the sun is lower than at noon? The characteristics described in the previous paragraph are interconnected by specific relationships. And all of them indicate the energy that electromagnetic radiation carries. This value changes upward when:
- wavelength reduction;
- increasing frequency.
What is the energy of electromagnetic radiation?
Photon is different from other particles. Its mass and, therefore, energy exist only as long as it moves through space. When colliding with an obstacle, a quantum of light increases its internal energy or gives it a kinetic moment. But the photon itself ceases to exist. Depending on what exactly is an obstacle, various changes occur.
- If the obstacle is a solid, then most often the light heats it. The following scenarios are also possible: a photon changes the direction of motion, stimulates a chemical reaction, or forces one of the electrons to leave its orbit and switch to another state (photoelectric effect).
- If the obstacle is the only molecule, for example, from a rarefied gas cloud in outer space, then the photon makes all its bonds oscillate more strongly.
- If the obstacle is a massive body (for example, a star or even a galaxy), then the light is distorted and changes direction. The ability to “peek” into the distant past of space is based on this effect.
Science and humanity
Scientific evidence often seems to be something abstract, not applicable to life. This also happens with the characteristics of light. If we are talking about an experiment or measuring the radiation of stars, scientists need to know the absolute values (they are called photometric). These concepts are usually expressed in terms of energy and power. Recall that power means the rate of change of energy per unit time, and in general it shows the amount of work that the system can produce. But man is limited in his ability to sense reality. For example, the skin feels warm, but the eye does not see a photon of infrared radiation. The same problem is with light intensity units: the power that the radiation actually shows is different from the power that the human eye can perceive.
Spectral sensitivity of the human eye
We remind you that the discussion below will focus on averaged indicators. All people are different. Some do not perceive individual colors (color blind) at all. For others, the color culture does not coincide with the generally accepted scientific point of view. For example, the Japanese do not distinguish between green and blue, and the British - blue and blue. In these languages, different colors are denoted by one word.
The unit of light intensity depends on the spectral sensitivity of the average human eye. The maximum of daylight falls on a photon with a wavelength of 555 nanometers. This means that in the light of the sun a person sees green best of all. The maximum of night vision is a photon with a wavelength of 507 nanometers. Therefore, with the moon, people see blue objects better. At dusk, everything depends on the lighting: the better it is, the more “green” the maximum color that a person perceives becomes.
The structure of the human eye
Almost always, when it comes to vision, we say that we see the eye. This is a false statement, because it perceives the brain in the first place. The eye is only a tool that transfers information about the light flux to the host computer. And, like any tool, the entire color perception system has its limitations.
There are two different types of cells in the human retina - cones and rods. The former are responsible for daytime vision and perceive colors better. The latter provide night vision, thanks to sticks a person distinguishes between light and shadow. But they do not perceive colors well. The sticks are also more sensitive to movements. That is why, if a person walks through a moonlit park or forest, he notices every swing of the branches, every breath of wind.
The evolutionary reason for this separation is simple: we have one sun. The moon shines with reflected light, which means that its spectrum is not much different from the spectrum of the central luminary. Therefore, the day is divided into two parts - illuminated and dark. If people lived in a system of two or three stars, then our vision would probably have more components, each of which was adapted to the spectrum of one star.
I must say, on our planet there are beings whose vision is different from human. Desert residents, for example, catch infrared light through their eyes. Some fish see near-ultraviolet, as this radiation penetrates deep into the water column. Our pets, cats and dogs, perceive colors differently, and their spectrum is trimmed: they are better suited to chiaroscuro.
But people are all different, as we mentioned above. Some representatives of humanity see the near infrared light. This is not to say that they would not need thermal imagers, but they are able to perceive slightly more red shades than most. Others have developed the ultraviolet part of the spectrum. Such a case is described, for example, in the movie Planet Ka-Pax. The main character claims that he came from another star system. The examination revealed his ability to see ultraviolet radiation.
Does this prove that Prot is an alien? No. Some people can do it. In addition, the near ultraviolet is closely adjacent to the visible spectrum. It is not surprising that someone perceives a little more. But Superman is definitely not from Earth: the X-ray spectrum is too far from the visible to such a vision could be explained from a human point of view.
Absolute and relative units for determining luminous flux
A value independent of spectral sensitivity, which shows the light flux in a known direction, is called “candela”. A unit of power with a more “human” attitude is pronounced in the same way. The only difference is the mathematical designation of these concepts: the subscript “e” has absolute value, and “υ” relative to the human eye. But do not forget that the values of these categories will vary greatly. This must be taken into account when solving real problems.
Enumeration and comparison of absolute and relative values
To understand what the power of light is measured in, it is necessary to compare the "absolute" and "human" values. On the right are purely physical concepts. On the left are the quantities into which they turn when passing through the system of the human eye.
- The power of radiation becomes the power of light. Concepts are measured in candelas.
- Energy brightness turns into brightness. Values are expressed in candelas per square meter.
Surely the reader saw familiar words here. Many times in their life people say: “A very bright sun, let’s go into the shade” or “Make the monitor brighter, the film is too dark and dark.” We hope the article will clarify slightly where this concept came from, as well as the name of the unit of light intensity.
Features of the concept of "candela"
We already mentioned this term a little higher. We also explained why the same word refers to completely different concepts of physics related to the power of electromagnetic radiation. So, the unit of measurement of light intensity is called "candela." But what is it equal to? One candela is the intensity of light in a known direction from a source that emits strictly monochromatic radiation with a frequency of 5.4 * 10 14 , and the energy force of the source in this direction is 1/683 watts per unit solid angle. The reader may well translate the frequency into the wavelength himself, the formula is very light. Let me tell you: the result lies in the visible region.
The unit of measurement of luminous intensity is called "candela" for a reason. Those who know English remember that candle is a candle. Previously, many areas of human activity were measured in natural parameters, for example, horsepower, millimeters of mercury. So it is not surprising that the unit of measurement of luminous intensity is Candela, one candle. Only a candle is very peculiar: with a strictly specified wavelength, and producing a specific number of photons per second.