How is intensity measured? How to measure relative illumination with a homemade device

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

For the human eye, light is energy waves length from 380 nanometers (nm) (purple) 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 it does not happen uniform distribution waves of different lengths, as in sunlight. Moreover, due to the design of the lamp, some parts of the spectrum may be more intense than others. In addition, the human eye is better at perceiving waves of wavelengths that are not very suitable for plants. As a result, it may turn out that some lighting seems pleasant and bright to us, but for plants it will be inappropriate 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 sunny noon, the light intensity in our latitudes reaches 100,000 lux. In the afternoon, the light brightness decreases 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 lighting intensity 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, directly behind the glass (that is, on the windowsill), the light intensity reaches best case scenario 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 each plant requires to survive is approximately 500 lux. With more low light it will inevitably perish. For normal life and growth, even unpretentious plants with little 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, since it is endowed with the ability to adapt to lighting. In addition, the human eye better perceives waves of such lengths that are not very suitable for plants.

What to do? A special device - a lux meter - can help. When purchasing it, it is very important to pay attention to what range light spectrum(wavelength) he is able to measure. Otherwise, it may happen that when measuring you end up at a wavelength unsuitable for plants. Remember - a lux meter, although more accurate than the human eye, also perceives a limited range of light waves.

A camera or photo exposure meter is suitable for assessing light intensity. But since when photographing, illumination is not measured in “lux,” you will have to carry out an appropriate recalculation.

The measurement is carried out as follows:

1.Set ISO to 100 and Aperture to 4.

2. Place a white piece of paper in the place where you want to measure the light intensity and point the camera at it.

3. Determine shutter speed.

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

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

Based on 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”, M.: ZAO “Fiton+”, 2004;

Bespalchenko E. A. “Tropical ornamental plants for home, apartment and office”, LLC PKF “BAO”, Donetsk, 2005;

D. Gosse, “Even the sun needs help”, magazine “Vestnik Florist”, No. 3, 2005.

Let us establish the relationship between the displacement x of 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, let us consider transverse wave, although all subsequent arguments

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

where A is the amplitude, circular frequency of oscillations. Then all particles of the medium will also come into harmonic vibration 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 similar in appearance to the graph harmonic vibration(Fig. 46), but essentially they are different. The oscillation 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 oscillations in at the moment time. It is like a snapshot of a wave.

Let us consider a certain particle C located at a distance y from the source of oscillations (particle O). It is obvious that if particle O is already oscillating, then particle C is still oscillating only where is the time of propagation of oscillations from to C, i.e., the time during which the wave travels the path y. Then the equation of vibration of particle C should be written as follows:

But where is the speed of wave propagation? Then

Relationship (23), which allows us to determine the displacement of any point on the wave at any time, is called the wave equation. By introducing the wavelength X into consideration as the distance between the two closest points of the wave that are in the same phase, for example, between two adjacent wave crests, we can give the wave equation a different form. Obviously, the wavelength is equal to the distance over which the oscillation propagates during 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 vibrations of particles of the medium, the energy of vibrations moves in space along with the wave. The energy transferred by a wave per unit time through a unit area perpendicular to the beam is called wave intensity (or energy flux density). We obtain an expression for the wave intensity

where is the Poynting vector $\backslash vec\; S(t)=\backslash frac(c)(4\backslash pi)\backslash left[\backslash vec\; E(t)\backslash times\backslash vec\; B(t)\backslash right],$(in the GHS system), $E$- tension electric field, A $B$- magnetic induction.

For a monochromatic linearly polarized wave with an electric field strength amplitude $E\_0$ intensity is equal to:

$I\; =\; \backslash frac(\backslash epsilon\_0cE\_0^2)(8\backslash pi).$

For a monochromatic circularly polarized wave this value is twice as large:

$I\; =\; \backslash frac(\backslash epsilon\_0cE\_0^2)(4\backslash pi).$

Sound intensity

Sound is a wave mechanical vibrations environment. Sound intensity can be expressed in terms of amplitude sound pressure values p and vibrational speed of the medium v:

$I\; =\; \backslash frac(pv)(2).$

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Notes

Excerpt characterizing Intensity (physics)

The wave process is associated with the propagation of energy (E) in space. The quantitative energy characteristic of this process is flow of energy(F) -the ratio of the energy transferred by a wave through some surface to time (t),for which this transfer is made. If energy transfer occurs uniformly, then: Ф = E/t, and for the general case the flow represents the derivative of energy with respect to time - Ф = 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) -flow ratioenergy to the area (S) of the surface located perpendicular to the direction of wave propagation. To distribute energy evenly over the surface through which the wave passes: I =F/S, and in general case -I = dФ / dS. Intensity is measured in W/m2.

Note that intensity is the physical parameter, which at the primary level determines the degree of physiological sensation arising under the influence wave process(such as sound or light).

Let's imagine it as a parallelepiped with length l area of ​​the medium in which the wave propagates. The area of ​​the parallelepiped face that is perpendicular to the direction of the wave speed v , denote by S(see Fig.9) . Let's introduce volumetric energy density of vibrational motion w , representing the amount of energy per unit volume: w = E /V . During the time t through the platform S energy will pass equal to the product of the volume V = l S =v t S on volumetric energy density:

E =w v t S .(25)

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

. (26)

Formula (26) can be presented in a slightly different form. Considering that the energy of harmonic vibrations (see formula (7))
and expressing the mass m through the density of matter  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 the speed of its propagation, the square of the amplitude of particle oscillations and the square of the oscillation frequency.

8. Doppler effect

The Doppler effect consists of changing the frequency of waves perceived by some receiver (observer) depending on relative speed movements of the wave source and observer.

When the source and receiver are stationary (Fig. 10.a), then, naturally, the frequency of the waves recorded by some receiver coincides with the frequency of the waves emitted by the source:  ist =  pr . If a source approaches a stationary receiver at a certain speed v ist, then its movement causes “compression” of the wave - the distance between the wave crests decreases - the period and wavelength decrease  pr, registered by the receiver. There is an increase in the frequency of the perceived wave process:  pr > ist(see Fig. 10.b).

For this case, a quantitative relationship between the frequency of the emitted waves, the speed of the source and the frequency recorded by a stationary receiving device can be established from the following considerations.

Wavelength perceived by the receiver:

 pr = (v V - v ist ) T ist , (28)

Where v V - the speed of wave propagation relative to a stationary source, T ist- the period of these waves. Thus, for a source approaching the receiver, the wavelength shortens. Perceived frequency increases:

 pr =
or  pr =
 ist . (29)

When moving the source away from the receiver (Fig. 10.c):

 pr =
 ist . (30)

For the general case when the source and receiver are moving:

 pr =
 ist (31)

The plus sign in the numerator of formula (30) and the minus sign in its denominator correspond to the convergence of the source and receiver, and the opposite signs correspond to their mutual distance.

A.4. Radiation transfer in the atmosphere

Main physical characteristics radiation fields are – intensity, density, flow.

Intensity (brightness) radiation is the amount of light energy that falls perpendicularly onto a unit area area (emitted from a unit area of ​​the visible surface of the source) from a unit solid angle per unit time:

In this expression dE– amount of light energy, dS – energy receiving area dΩ- solid angle from which radiation energy comes, dt– time interval during which the radiation operates. It is assumed that the solid angle is sufficiently small and the area is perpendicular to the direction of radiation propagation.

In general, we should consider the so-called spectral intensity - intensity per unit interval of radiation wavelengths I λ or frequencies I ν(here the subscripts indicate wavelength or frequency). According to the definition, intensity is a function of the coordinates of a point in the medium r, direction of propagation and time (here the angles are defined in spherical system coordinates k – unit vector, which determines the direction of propagation of radiation). For a solid angle element in a spherical coordinate system we have

.

The above definition of brightness makes sense when we're talking about about a surface source, for which the concept of a radiation source surface unit is quite obvious. In the case when we are talking about the brightness of a volumetric radiation source (the brightness of the sky), such a definition is, at least, unclear. Let us show that the brightness of the source is numerically equal to the intensity of radiation recorded at a certain distance, when the angle dΩ less angular dimensions source. Let us assume that the named angle covers the area of ​​the radiation source located at a distance r from the observation point, and the angle between the direction of radiation propagation and the normal to the site is equal to α. Then . Substituting this expression into the definition of intensity, we get

where indicated, is the solid angle through which the emitted radiation propagates. Thus , the brightness of an extended source is numerically equal to the radiation intensity of this source at some distance from it. In this formulation there is no mention of the surface of the source, therefore it is also applicable to sources that do not have a clearly defined emitting surface, for example, to such a volumetric source of scattered solar radiation like the atmosphere. It is assumed, of course, that on the way from the source to the observation point the medium does not introduce additional attenuation of the radiation.

Volume radiation densityρ is the amount of light energy per unit volume of the medium. Spreading at the speed of light c, radiation I in the direction k in time dt occupies volume dV= cdtdS, and the energy entering the volume is dE=IdSdΩdt. Here ds- elementary site, perpendicular to direction propagation of radiation. Consequently, the contribution to the value ρ from radiation coming from dΩ in the direction k, equals

.

The total radiation density is obtained by summing the individual contributions from different directions:

.

If I does not depend on the direction, the radiation is said to be isotropic. Then

For example, bulk density black body radiation

,

and intensity.

Radiation flux is the amount of light energy incident on a selected area per unit of time from all directions. The flow through a unit area is called flux density. By direction k, in particular, energy falls on a unit area in an elementary solid angle

Therefore, the flux density will be equal to

.

To get the flow value through the site arbitrary area, the given expression should be integrated over this area. Here it is assumed that the axis z coordinate system coincides with the direction of the normal to the site n. Then the dependence on the orientation of the radiation k in relation to the site is “hidden” in the angles and φ spherical coordinate system defining the direction k.

The expression for the flux density can also be rewritten as follows: N =H + -H - Where,

.

Here, a division is made into flows falling onto the platform from the upper and lower hemispheres (if the platform is oriented horizontally). If I does not depend on the direction, then such fluxes are equal, and the total flux density is zero. Flux density from the upper hemisphere H+ also called illumination(the amount of radiation energy falling from the upper hemisphere onto a horizontal area of ​​a unit area per unit time).