Modern plate making equipment (cameras, duplicators, plate making machines, etc.), where exposure to photosensitive materials (film or PS plates) is required, almost all use an integrating light meter to control exposure. Why do you have to use a cumulative light meter? What are the disadvantages of using the delayed exposure method in the past? Discuss this issue below.
The degree of sensitivity of a photosensitive material is measured by the amount of total energy received by a photon per unit area. This quantity is known as exposure. In addition to the exposure time, it is also related to the illuminance projected on the surface of the photosensitive material. This can be expressed in a mathematical formula:
H=L·t
In the formula
H—The energy of photons received on the surface of the photosensitive material, commonly referred to as exposure;
L—the intensity of light projected onto the surface of the sensing material, commonly referred to as illumination;
T—exposure time.
According to the formula frog above, the amount of light exposure varies with the light intensity and the exposure time on the surface of the photosensitive material. The stronger the light, the shorter the exposure time. Conversely, if the light is weak, the exposure time is long.
The so-called illuminance refers to the photon energy received by an object per unit area in unit time, that is:
L = △F/ΔS=h·U2/R·cosq/4pr2 -
Where △ F - the energy of the light passing through ΔS;
H—luminous luminous efficiency;
U—power supply voltage of the lamp;
R—Equivalent impedance of the lamp;
R—the distance of the point source to the surface of the object being photographed;
q - the angle of incidence of the light.
By-type can be out of the year, the energy of the photosensitive material surface and the grid voltage, the old and new degree of the tube and the light source to the photosensitive surface distance and included angle (see Figure 1).
1. It is proportional to the square of the voltage of the grid. Assuming other conditions remain unchanged, considering only the influence of the voltage fluctuation of the grid on the exposure, the formula can be obtained:
(â–³L/L)u= 2 â–³U/U ?
It can be seen from the formula that the grid voltage fluctuates by 10% and the illuminance must fluctuate by 20%. In some areas of our country, voltage fluctuations of more than 20% during the day and at night occur. If exposure is performed by a time-delay method, the same exposure time will cause a difference of 40% between daytime and nighttime exposures. Also, the version ideas made during the day are well restored, and the evening version is lost. This is one of the major drawbacks of using delayed exposure - poor resistance to grid voltage fluctuations.
2. It is inversely proportional to the resistance of the lamp (the oldness of the lamp). Assuming that other conditions remain the same, considering only the effect of the lamp's age on the exposure, the formula can be obtained:
(△L/L)R = -ΔR/R ˉ
From the formula, if the lamp resistance increases by 10%, the illumination decreases by 10%. During the use of the lamp, the electrode must be continuously evaporated to reduce the degree of vacuum in the lumen, resulting in an increase in the equivalent impedance of the tube. For experimental frogs, their light intensity should be reduced by about 30% after the lamp has exceeded its allowable lifetime. Therefore, if the old and new tubes are exposed with the same exposure time, the results obtained will be very different. This is the second major drawback of using a time-delay exposure - the poor resistance to lamp aging.
3. It is inversely proportional to the square of the distance from the light source to the photoreceptor. Assuming other conditions remain the same, considering only the effect of the lamp distance on the exposure to the exposure, the formula can be obtained:
(△L/L)r = -2Δr/r °
By the formula, the distance between the lamp and the photosensitive spot increases by 10%, and the illumination decreases by 20%. The effect of changes in lamp distance on exposure is no less than the effect of grid voltage fluctuations on exposure.
4. It is proportional to the cosine of the angle of incidence of the light source. Assuming other conditions remain the same, only consider the impact of the position of the manuscript on the exposure. The formula can be obtained by:
(△L/L)q = -tgq·Δq ±
From the ± formula, we can see that when q <45 °, tgq <1, which means that the size of the photosensitive material should not be greater than 2 times the distance from the lamp to the photosensitive surface, otherwise, the center of the photosensitive material and the edge of the exposure difference is greater.
The above-mentioned four factors affecting the exposure amount cannot be controlled by the plate-making workers in actual production. Therefore, if you can find a controller that can make the exposure constant, it is ideal. That is, regardless of what causes the projection skin projected onto the photosensitive surface to change, the exposure time can be appropriately lengthened or shortened so that the photosensitive surface always maintains the received light at a constant total energy. Accumulated light meter can do this.
The integrating light meter (see Figure 2) consists of the metering head BS, signal amplifier IC3, analog-to-digital converter IC6, and preset counters ICl and IC2. The amount of light makes the counters IC1 and IC2 preset the exposure value by dialing the dial. When the exposure starts, the CR8 relay contact is closed, IC4-13 is high, and the BS is the photocell. The weak signal passes through the amplifier IC3 and then passes through the amplifier. IC6 carries on the analog to digital conversion, the counter ICl, IC2 subtraction, when reduces to zero makes IC4-12 become the high level, causes the transistor TR2 to lead, the Ry relay pulls shuts the shutter, the exposure ends.
If the light is increased, the counting pulse is increased, which corresponds to the shortening of the exposure time; if the light is decreased, the counting pulse is decreased, which corresponds to a longer exposure time, and the total number of pulses is not changed. In this way, it is ensured that the total amount of light irradiated onto the surface of the photosensitive material does not change.
Although the integrating light meter is simple, it does a great job of improving the quality of the plate. No matter how the voltage of the grid fluctuates, or how old or new the lamp is, even if there is a lot of dust on the insulating glass in the light box, the impact on the quality of the exposure is not significant. Therefore, it is advisable that users with broken light meters repair quickly and do not make use of time delay relays; printing equipment that is not equipped with a light meter is quickly matched, which is of great benefit to improving the quality of plate making.
In the selection of the cumulative light timing, please pay attention to the light source used. There are two types of plate-making light sources, one is a weak light source for darkroom films, and the other is a strong light source for a bright room film or PS plate. Different light sources have different spectral ranges (see Table 1). The first five are strong light sources, and the spectrum is purple at 320-450 nm; incandescent lamps are weak light sources, and the spectrum is reddish at 400-600 nm. Different light sources use different measuring heads. The structure of the photometric head is shown in Fig. 3. A set of color filters is installed above the photosensitive element. For the weak light source, only one C-50S blue V-type filter is installed. The characteristics of this filter are shown in Figure 4. The light is limited to 350~600nm. Red light and ultraviolet light are filtered out, and only green light is allowed to pass. This method is used to increase the sensitivity of photometry. For strong light sources, because the light is strong, in addition to installing a V to the filter, plus an ND gray filter to prevent the photosensitive element from saturating.
The following describes a type of LI-100 type integrating light meter used in the factory (see Figure 5).
(1) "COUNT" count display. The amount of light from the start of exposure is counted down each time, and it flashes once when the set value of light decreases to zero.
(2) "OVER" incident light is too strong. When the light intensity exceeds the saturation value of the photosensitive element, it lights up. If the exposure starts, it lights up to indicate that the measuring head is too close to the light source, and the measuring head is properly away from the light source, and the fault can be eliminated. If it is still bright with the metering head attached, it means that the integrated circuit's internal circuitry has broken components.
(3) "UNDER" light is too weak to show. When it is not exposed, it is always on. When the exposure is on, it should be extinguished immediately. If it is still on, it means that the light is too weak or the lamp is not on. If you move the measuring head closer to the light source, the integral light meter internal circuit There are broken components.
(4) "GAIN" sensitivity adjustment potentiometer URl. Use it to adjust the "COUNT" flash speed, generally adjusted to flash once per second.
(5) Light volume setting dial. Press the + key when adding numbers, and press the - key when decreasing numbers.
There are two sockets and external connections behind the integrated light meter (see Figure 6). The power supply AC l00V is tapped from CN1-(1,2). The start signal is shorted by CN1-(7,8). When the light meter counts back to zero, its internal relay is turned on. If the exposure is controlled by the shutter, the constant open point CN1-(4,5) is used; if it is directly lighted, the normally closed point CNl-(4, 6). CN2 is a metering signal input socket.
The degree of sensitivity of a photosensitive material is measured by the amount of total energy received by a photon per unit area. This quantity is known as exposure. In addition to the exposure time, it is also related to the illuminance projected on the surface of the photosensitive material. This can be expressed in a mathematical formula:
H=L·t
In the formula
H—The energy of photons received on the surface of the photosensitive material, commonly referred to as exposure;
L—the intensity of light projected onto the surface of the sensing material, commonly referred to as illumination;
T—exposure time.
According to the formula frog above, the amount of light exposure varies with the light intensity and the exposure time on the surface of the photosensitive material. The stronger the light, the shorter the exposure time. Conversely, if the light is weak, the exposure time is long.
The so-called illuminance refers to the photon energy received by an object per unit area in unit time, that is:
L = △F/ΔS=h·U2/R·cosq/4pr2 -
Where △ F - the energy of the light passing through ΔS;
H—luminous luminous efficiency;
U—power supply voltage of the lamp;
R—Equivalent impedance of the lamp;
R—the distance of the point source to the surface of the object being photographed;
q - the angle of incidence of the light.
By-type can be out of the year, the energy of the photosensitive material surface and the grid voltage, the old and new degree of the tube and the light source to the photosensitive surface distance and included angle (see Figure 1).
1. It is proportional to the square of the voltage of the grid. Assuming other conditions remain unchanged, considering only the influence of the voltage fluctuation of the grid on the exposure, the formula can be obtained:
(â–³L/L)u= 2 â–³U/U ?
It can be seen from the formula that the grid voltage fluctuates by 10% and the illuminance must fluctuate by 20%. In some areas of our country, voltage fluctuations of more than 20% during the day and at night occur. If exposure is performed by a time-delay method, the same exposure time will cause a difference of 40% between daytime and nighttime exposures. Also, the version ideas made during the day are well restored, and the evening version is lost. This is one of the major drawbacks of using delayed exposure - poor resistance to grid voltage fluctuations.
2. It is inversely proportional to the resistance of the lamp (the oldness of the lamp). Assuming that other conditions remain the same, considering only the effect of the lamp's age on the exposure, the formula can be obtained:
(△L/L)R = -ΔR/R ˉ
From the formula, if the lamp resistance increases by 10%, the illumination decreases by 10%. During the use of the lamp, the electrode must be continuously evaporated to reduce the degree of vacuum in the lumen, resulting in an increase in the equivalent impedance of the tube. For experimental frogs, their light intensity should be reduced by about 30% after the lamp has exceeded its allowable lifetime. Therefore, if the old and new tubes are exposed with the same exposure time, the results obtained will be very different. This is the second major drawback of using a time-delay exposure - the poor resistance to lamp aging.
3. It is inversely proportional to the square of the distance from the light source to the photoreceptor. Assuming other conditions remain the same, considering only the effect of the lamp distance on the exposure to the exposure, the formula can be obtained:
(△L/L)r = -2Δr/r °
By the formula, the distance between the lamp and the photosensitive spot increases by 10%, and the illumination decreases by 20%. The effect of changes in lamp distance on exposure is no less than the effect of grid voltage fluctuations on exposure.
4. It is proportional to the cosine of the angle of incidence of the light source. Assuming other conditions remain the same, only consider the impact of the position of the manuscript on the exposure. The formula can be obtained by:
(△L/L)q = -tgq·Δq ±
From the ± formula, we can see that when q <45 °, tgq <1, which means that the size of the photosensitive material should not be greater than 2 times the distance from the lamp to the photosensitive surface, otherwise, the center of the photosensitive material and the edge of the exposure difference is greater.
The above-mentioned four factors affecting the exposure amount cannot be controlled by the plate-making workers in actual production. Therefore, if you can find a controller that can make the exposure constant, it is ideal. That is, regardless of what causes the projection skin projected onto the photosensitive surface to change, the exposure time can be appropriately lengthened or shortened so that the photosensitive surface always maintains the received light at a constant total energy. Accumulated light meter can do this.
The integrating light meter (see Figure 2) consists of the metering head BS, signal amplifier IC3, analog-to-digital converter IC6, and preset counters ICl and IC2. The amount of light makes the counters IC1 and IC2 preset the exposure value by dialing the dial. When the exposure starts, the CR8 relay contact is closed, IC4-13 is high, and the BS is the photocell. The weak signal passes through the amplifier IC3 and then passes through the amplifier. IC6 carries on the analog to digital conversion, the counter ICl, IC2 subtraction, when reduces to zero makes IC4-12 become the high level, causes the transistor TR2 to lead, the Ry relay pulls shuts the shutter, the exposure ends.
If the light is increased, the counting pulse is increased, which corresponds to the shortening of the exposure time; if the light is decreased, the counting pulse is decreased, which corresponds to a longer exposure time, and the total number of pulses is not changed. In this way, it is ensured that the total amount of light irradiated onto the surface of the photosensitive material does not change.
Although the integrating light meter is simple, it does a great job of improving the quality of the plate. No matter how the voltage of the grid fluctuates, or how old or new the lamp is, even if there is a lot of dust on the insulating glass in the light box, the impact on the quality of the exposure is not significant. Therefore, it is advisable that users with broken light meters repair quickly and do not make use of time delay relays; printing equipment that is not equipped with a light meter is quickly matched, which is of great benefit to improving the quality of plate making.
In the selection of the cumulative light timing, please pay attention to the light source used. There are two types of plate-making light sources, one is a weak light source for darkroom films, and the other is a strong light source for a bright room film or PS plate. Different light sources have different spectral ranges (see Table 1). The first five are strong light sources, and the spectrum is purple at 320-450 nm; incandescent lamps are weak light sources, and the spectrum is reddish at 400-600 nm. Different light sources use different measuring heads. The structure of the photometric head is shown in Fig. 3. A set of color filters is installed above the photosensitive element. For the weak light source, only one C-50S blue V-type filter is installed. The characteristics of this filter are shown in Figure 4. The light is limited to 350~600nm. Red light and ultraviolet light are filtered out, and only green light is allowed to pass. This method is used to increase the sensitivity of photometry. For strong light sources, because the light is strong, in addition to installing a V to the filter, plus an ND gray filter to prevent the photosensitive element from saturating.
The following describes a type of LI-100 type integrating light meter used in the factory (see Figure 5).
(1) "COUNT" count display. The amount of light from the start of exposure is counted down each time, and it flashes once when the set value of light decreases to zero.
(2) "OVER" incident light is too strong. When the light intensity exceeds the saturation value of the photosensitive element, it lights up. If the exposure starts, it lights up to indicate that the measuring head is too close to the light source, and the measuring head is properly away from the light source, and the fault can be eliminated. If it is still bright with the metering head attached, it means that the integrated circuit's internal circuitry has broken components.
(3) "UNDER" light is too weak to show. When it is not exposed, it is always on. When the exposure is on, it should be extinguished immediately. If it is still on, it means that the light is too weak or the lamp is not on. If you move the measuring head closer to the light source, the integral light meter internal circuit There are broken components.
(4) "GAIN" sensitivity adjustment potentiometer URl. Use it to adjust the "COUNT" flash speed, generally adjusted to flash once per second.
(5) Light volume setting dial. Press the + key when adding numbers, and press the - key when decreasing numbers.
There are two sockets and external connections behind the integrated light meter (see Figure 6). The power supply AC l00V is tapped from CN1-(1,2). The start signal is shorted by CN1-(7,8). When the light meter counts back to zero, its internal relay is turned on. If the exposure is controlled by the shutter, the constant open point CN1-(4,5) is used; if it is directly lighted, the normally closed point CNl-(4, 6). CN2 is a metering signal input socket.
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