Light intensity and how to measure it. wave optics

Replacing the plant with the sun is very difficult. Try to turn on a lamp in the room on a sunny day, and you will understand how little light it can give plants.

To the human eye, light is energy waves ranging from 380 nanometers (nm) (violet) to 780 nm (red). The wavelengths important for photosynthesis lie between 700 nm (red) and 450 nm (blue). This is especially important to know when using artificial lighting, because in this case there is no uniform distribution of waves of different lengths, as in sunlight. Moreover, due to the design of the lamp, certain parts of the spectrum may be more intense, others less so. In addition, the human eye perceives better just wavelengths that are not very suitable for plants. As a result, it may turn out that some lighting will seem pleasant and bright to us, but for plants it will be unsuitable and weak.

Light intensity indoors and outdoors

The intensity of light falling on a certain plane is measured in the unit "lux". In summer, at solar noon, the light intensity in our latitudes reaches 100,000 lux. In the afternoon, the brightness of the light is reduced to 25,000 lux. At the same time, in the shade, depending on its density, it will be only a tenth of this value or even less.

In houses, the intensity of illumination is even less, since the light does not fall there directly, but is weakened by other houses or trees. In summer, on the south window, right behind the glass (that is, on the windowsill), the light intensity reaches at best from 3000 to 5000 lux, and quickly decreases towards the middle of the room. At a distance of 2-3 meters from the window, it will be about 500 lux.

The minimum amount of light that each plant needs to survive is approximately 500 lux. With weaker lighting, it will inevitably die. For normal life and growth, even unpretentious plants with a small need for light need at least 800 lux.

How to measure illumination?

The human eye is not able to determine the absolute intensity of light, because it is endowed with the ability to adapt to lighting. In addition, the human eye perceives better just the wavelengths that are not very suitable for plants.

What to do? A special device - a light meter - can help. When buying it, it is very important to pay attention to what range of the light spectrum (wavelength) it is able to measure. Otherwise, it may happen that during the measurement you get to a wavelength unsuitable for plants. Remember - the luxmeter, although more accurate than the human eye, also perceives a limited range of light waves.

To assess the intensity of illumination, a camera or photo exposure meter is suitable. But since when photographing, the illumination is not measured in "lux", you will have to make an appropriate conversion.

The measurement is carried out as follows:

1.Set ISO to 100 and Aperture to 4.

2. Place a white sheet of paper where you want to measure the light intensity and aim your camera at it.

3. Set your shutter speed.

4. The shutter speed denominator multiplied by 10 will give the approximate lux value.

Example: if the exposure time was 1/60 of a second, this corresponds to 600 lux.

According to materials:

Paleeva T. V. “Your flowers. Care and treatment”, M.: Eksmo, 2003;

Anita Paulisen "Flowers in the House", M .: Eksmo, 2004;

Vorontsov V. V. “Care for indoor plants. Practical advice for flower lovers”, Moscow: ZAO Fiton+, 2004;

Bespalchenko E. A. "Tropical ornamental plants for home, apartment and office", OOO PKF "BAO", Donetsk, 2005;

D. Gosse, “Even the sun must be helped”, the magazine “Bulletin of the Florist”, No. 3, 2005

Let us establish the relationship between the displacement x of the particles of the medium participating in the wave process and the distance y of these particles from the source of oscillations O for any moment of time. For greater clarity, we consider a transverse wave, although all subsequent reasoning

will also be true for the longitudinal wave. Let the source oscillations be harmonic (see § 27):

where A is the amplitude, the circular frequency of oscillations. Then all the particles of the medium will also come into harmonic oscillation with the same frequency and amplitude, but with different phases. A sinusoidal wave appears in the medium, shown in Fig. 58.

The wave graph (Fig. 58) is outwardly similar to the harmonic oscillation graph (Fig. 46), but in essence they are different. The wobble graph represents the displacement of a given particle as a function of time. The wave graph represents the dependence of the displacement of all particles of the medium on the distance to the source of vibrations at a given time. It is, as it were, a snapshot of a wave.

Consider some particle C located at a distance y from the source of oscillations (particles O). Obviously, if the particle O already oscillates, then the particle C oscillates only where the propagation time of the oscillations is from to C, i.e., the time during which the wave has traveled the path y. Then the equation for the oscillation of particle C should be written as follows:

But where is the speed of wave propagation. Then

Relation (23), which makes it possible to determine the displacement of any point of the wave at any time, is called the wave equation. By introducing the wavelength X as the distance between two nearest points of the wave that are in the same phase, for example, between two adjacent wave crests, one can give the wave equation a different form. Obviously, the wavelength is equal to the distance over which the oscillation propagates over a period with a speed

where is the frequency of the wave. Then, substituting into the equation and taking into account that we obtain other forms of the wave equation:

Since the passage of waves is accompanied by oscillations of the particles of the medium, the energy of oscillations also moves in space along with the wave. The energy transferred by a wave per unit of time through a unit area perpendicular to the beam is called the intensity of the wave (or energy flux density). We obtain an expression for the wave intensity

The wave process is associated with the propagation of energy (E) in space. The quantitative energy characteristic of this process is energy flow (F) - the ratio of the energy transferred by a wave through a certain surface to the time (t) during which this transfer takes place. If the energy transfer is uniform, then: F \u003d E / t, and for the general case, the flow represents the derivative of energy with respect to time - F \u003d d E / d t. The unit of energy flow is the same as the unit of power J/s = W.

Wave intensity (or energy flux density) (I)-the ratio of the energy flux to the area (S) of the surface located perpendicular to the direction of wave propagation. For uniform distribution of energy over the surface through which the wave passes I = F / S, and in the general case - I = dF / dS. The intensity is measured in W / m 2.

Note that the intensity is the physical parameter that at the primary level determines the degree of physiological sensation that occurs under the action of the wave process (for example, sound or light).

Represent in the form of a parallelepiped with length l part of the medium in which the wave propagates. The area of ​​the face of the parallelepiped, which is perpendicular to the direction of the wave velocity v, will be denoted by S(see fig. 9) . Let us introduce the volume energy density of oscillatory motion w, representing the amount of energy per unit volume:
w = E / V. During t through the site S an energy equal to the product of the volume V = l S = v tS on the volume energy density:

E = w v t S .(25)

Dividing the left and right parts of formula (25) by time and area, we obtain an expression relating the intensity of the wave and the speed of its propagation. The vector , whose modulus is equal to the intensity of the wave, and the direction coincides with the direction of its propagation, is called Umov vector :

Formula (26) can be represented in a slightly different form. Considering that the energy of harmonic oscillations (see formula (7)) and expressing the mass m through the density of matter r and volume V, for the volumetric energy density we obtain: w =. Then formula (26) takes the form:

. (27)

So, the intensity of an elastic wave, determined by the Umov vector, is directly proportional to its propagation velocity, the square of the particle oscillation amplitude, and the square of the oscillation frequency.

Light plays a huge role not only in the interior, but also in our life in general. After all, the efficiency of work, as well as our psychological state, depends on the correct illumination of the room. Light gives a person the opportunity not only to see, but also to evaluate the colors and shapes of surrounding objects.

Of course, natural light is most comfortable for human eyes. In such lighting, everything is seen very well and without distortion of colors. But not always natural lighting is present, in the dark, for example, you have to make do with artificial light sources.

So that the eyes do not strain, and vision does not deteriorate, it is necessary to create optimal conditions for light and shadow, creating the most comfortable lighting.

For the eyes, the most pleasant lighting is natural

Lighting, as well as many other factors, is evaluated in terms of quantitative and qualitative parameters. Quantitative characteristics are determined by the intensity of light, while qualitative characteristics are determined by its spectral composition and distribution in space.

How and in what way is light intensity measured?

Light has many characteristics and each has its own unit of measurement:

• The intensity of light characterizes the amount of light energy that is transferred in a certain time in any direction. It is measured in candelas (cd), 1 cd is approximately equal to the intensity of light emitted by one burning candle;
• Brightness is also measured in candela, in addition, there are such units as stilb, apostilb and lambert;
• Illumination is the ratio of the luminous flux that falls on a certain area to its surface. It is measured in lux.

It is illumination that is an important indicator for the proper functioning of vision. In order to determine this value, a special measuring device is used. It's called a luxometer.

A luxometer is a device for measuring illumination.

This device consists of a light receiver and a measuring part, it can be of the pointer type or electronic. The light receiver is a photocell that converts the light wave into an electrical signal and sends it to the measuring part. This device is a photometer and has a specified spectral sensitivity. With it, you can measure not only visible light, but also infrared radiation, etc.

This device is used both in industrial premises and in educational institutions, as well as at home. For each type of activity and occupation, there are standards for what the intensity of light should be.

Comfortable light intensity

Visual comfort depends on many factors. By far, the most pleasing to the human eye is sunlight. But the modern rhythm of life dictates its own rules, and very often you have to work or just be in artificial lighting.

Manufacturers of lighting fixtures and lamps are trying to create such light sources that would correspond to the peculiarities of the visual perception of people and would create the most comfortable light in terms of intensity.

Light from an incandescent lamp most accurately reproduces natural shades

Conventional incandescent lamps use an incandescent spring as a light source, and therefore, this light is most similar to natural light.

Lamps are divided into the following categories according to the type of light they give:

• warm light with reddish hues, it is well suited for a home environment;
• neutral light, white, used to illuminate workplaces;
• cold light, bluish, intended for places where high precision work is performed or for places with a hot climate.

It is important not only what type of lamps belong to, but also the design of the lamp itself. lamp or chandeliers: how many light bulbs are screwed in where the light is directed, the shades are closed or open - all these features must be taken into account when choosing a lighting device.

Lighting standards are fixed in several documents, the most important are: SNiP (building codes and regulations) and SanPiN (sanitary rules and regulations). There are also MGSN (Moscow city building codes), as well as its own set of rules for each region.

It is on the basis of all these documents that a decision is made about what the intensity of lighting should be.

Of course, when thinking about which chandelier to hang in the living room, bedroom or kitchen, no one measures the intensity of lighting with a luxmeter. However, it is very useful to know in general terms which light will be more comfortable for the eyes.

Table 1 shows the illumination standards for residential premises:

Table 1

Table 2 shows the lighting standards for offices

At home, without special equipment, it is difficult to measure indoor lighting, and therefore, in order to understand which lamp to choose, you should pay attention to the color (cold, neutral or warm) and the number of watts. In recreation rooms, it is better to use not too bright ones, and in workrooms - with more intense light.

Since natural light is most pleasant for the eyes, lamps that give warm light should be preferred in the home environment. When we come home, our eyes definitely need a rest after a busy day at work. Properly sized lamps for chandeliers and lamps will help create the right intensity of lighting.