JPH0455314Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Technical Field) The present invention relates to an overhead projector using a thermochromic film capable of writing and erasing images on the film.

(Prior Art) The present inventors have proposed an overhead projector device that first writes and forms an image read from a document onto a thermochromic film, and projects and displays the image by transmitting light onto the film (patent application). (See No. 1983-309334).

The thermochromic film becomes optically transparent when heated to a first temperature, for example, about 60°C, and remains transparent even after returning to room temperature, and when heated to a second temperature, for example, 72°C. It has the property of becoming optically opaque and remaining opaque even after returning to room temperature. Therefore, if the first temperature (transparenting temperature) is applied to the entire surface of the film on which an image has already been formed, the image will be erased. Further, if a second temperature is selectively applied to the erased and transparent film in response to a signal from which the original image is read, a new image is formed. In this way, by erasing unnecessary images and forming desired images, it is possible to create a large number of projection films using a limited length of film.

By the way, in a thermochromic film, a thermochromic layer is formed on a base film, and when the second temperature is applied, that portion of the thermochromic layer becomes cloudy. However, if the thermochromic layer is thin, that is, if the cloudy layer is thin, some light will pass through it and will not completely block it. Therefore, in order to obtain sufficient contrast, it is necessary to increase the thickness of the thermochromic layer and the thickness of the cloudy layer.

However, by thickening the thermochromic layer,
And when heated by the printing element (heating element) of the thermal head so that the deep layer becomes cloudy,
The surface portion of the thermochromic layer is exposed to even higher temperatures, causing the problem that the surface portion softens and changes in quality. This state will be explained using FIGS. 3 and 4. FIG. 3 shows a thermochromic layer 2 formed on a base film 1.
The figure shows a state in which the thermal head 3 is in contact with the heat generating element of the thermal head 3, and is heated by the heat generating element of the thermal head 3, and ~ is the heat generating element of the thermal head 3,
Each of the layers divided into four layers or the upper layer of the base film 1 is shown. Figure 4a
4 shows the temperature changes followed by each of the above layers when the printing pulse shown in FIG. 4b is applied to the heating element of the thermal head 3. When a printing pulse is applied to the heating element, the temperature of the heating element portion rises with the rise of the pulse, and decreases with the fall of the pulse. On the other hand, the thermochromic layer ~ takes time to conduct heat, so the temperature rises with a sequential delay. Therefore, if an attempt is made to heat the deep layers of the thermochromic layer to reach the clouding temperature, the temperature of the surface layer will rise too much and exceed the alteration temperature (for example, 90°C). The parts that exceed the alteration temperature soften and become transparent and
There is also a possibility that the reversibility of cloudiness may be lost.

(Purpose of the invention) The present invention is an overhead projector that solves the above-mentioned problems and is capable of clouding the deep layer without causing the surface layer of the thermochromic layer to exceed the alteration temperature. It provides:

(Structure of the invention) In order to achieve the above object, a circuit means is provided which intermittently applies a printing pulse to the printing element of the thermal head once or a plurality of times during printing of one pixel.

According to the above configuration, by applying a printing pulse as shown in FIG. 5b to the printing element of the thermal head, each layer from the surface layer to the deep layer of the thermochromic layer is heated to the desired temperature for clouding. The temperature of the heating element can be maintained within the required temperature range for the time it takes to reach the temperature, thereby suppressing the temperature of the surface layer of the thermochromic layer below the deterioration temperature, preventing deterioration, and preventing deterioration of the surface layer of the thermochromic layer. Since the area can be made cloudy, this area completely blocks light and provides sufficient contrast.

Furthermore, by selectively changing the number of printing pulses supplied for each pixel, as shown in Figure 6, the clouding of the thermochromic layer can be suppressed from only the surface layer (section A) to the middle layer (section B). part), to the deep part (part C)
In this way, the thickness can be controlled arbitrarily, and it is also possible to express intermediate tones by changing the light transmittance.

(Example) Hereinafter, an example will be described in detail based on the drawings. FIG. 1 shows an embodiment of the present invention, in which 11 to 15 are monostable multivibrators;
6 is an OR circuit, and 17 is a thermistor for detecting the temperature of the thermal head.

Next, the operation of this embodiment will be explained with reference to the timing chart of FIG. First, the multivibrator 11 operates at the falling edge of the print timing pulse a and outputs the pulse b. This pulse b passes through the OR circuit 16 and becomes the first printing pulse of the printing pulses g. Next, the multivibrator 12 operates at the falling edge of pulse b and outputs pulse c which determines the pause time. Next, the multivibrator 13 operates at the falling edge of pulse c and outputs pulse d. This pulse d passes through the OR circuit 16 and becomes the second printing pulse of the printing pulse g. Furthermore, the multivibrator 14 operates at the falling edge of the pulse d, and outputs a pulse e that determines the pause time. Next, the multivibrator 15 operates at the falling edge of the pulse e, and the pulse f
Output. This pulse f passes through an OR circuit 16 and becomes the third printing pulse of the printing pulse g. In this way, a plurality of printing pulses as shown in FIG. 5b are generated and printed on the heating element, thereby realizing clouding that reaches the deep part of the thermochromic layer.

Note that the width of the pulse b output from the multivibrator 11 is controlled by the temperature of the thermal head detected by the thermistor 17. That is, when the temperature of the head is high, the pulse width becomes short, and the operation is performed so that the temperature of the heating element reaches a predetermined temperature as shown in the peak value of in Fig. 5 at the end of heating by the first printing pulse. .

Next, by selectively changing the number of times printing pulses are supplied for each printing dot, it becomes possible to change the thickness of the cloudy part of the thermochromic layer, and it becomes possible to change not only the binary value of light transmission and non-transmission. , an intermediate state (halftone) can also be realized. FIG. 7 shows the circuit configuration for this purpose, in which 21 is a photoelectric conversion unit that reads the original image;
2 is the CCD section of the image sensor, 23 is the shift register of the thermal head, 24 is a driver that drives the heating element 25, 26 is a comparator, and 27 is an analog switch that selects the reference voltages E ₁ , E ₂ , and E ₃ . Switch.

The operation will be explained below with reference to the timing chart of FIG. First, the image sensor
The voltage of each dot, which is proportional to the brightness, taken into the CCD section 22 is compared with the first reference voltage _E1 and converted into a binary value.
3, and the dot portion corresponding to black is printed by the first printing pulse. This first printing pulse has a pulse width sufficient to make the surface layer of the thermochromic layer cloudy as shown in section A of FIG. Next, the voltage of each dot of the CCD section 22 is
It is binarized using the second reference voltage _E2 , transferred to the shift register 23 of the thermal head, and the dot portion corresponding to black is printed using the second printing pulse.
That is, a dot corresponding to a part blacker (or darker) than part A in FIG. 6 is selected, and the second printing pulse is applied to make the middle part of the thermochromic layer cloudy as shown in B in FIG. This operation is set to be performed within a period of time after the heating by the first printing pulse ends, so that the temperature of the heating element section does not drop too much. Furthermore, as before, the third reference voltage
By performing binarization with _E3 and printing with the third printing pulse, the blackest dots are clouded down to the deep part of the thermochromic layer, as shown in C in FIG.

In this way, if the thickness of the cloudy thermochromic layer is changed depending on the brightness of the original to be read, the light transmittance will differ when projection light is applied to it.
This means that intermediate gradations can be expressed.

(Effects of the invention) As explained above, according to the invention, it is possible to make the deep layers cloudy without causing any alteration of the thermochromic layer, and therefore the cloudy parts are exposed to light. Since transmission can be completely blocked, sufficient contrast can be obtained. Further, by controlling the thickness of the clouding of the thermochromic layer for dots, it is possible to express up to halftones.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the circuit configuration of an embodiment of the present invention, FIG. 2 is a timing chart for explaining the operation, and FIG. 3 is a diagram showing a thermochromic film and a thermal head in contact with it. , FIG. 4 is a diagram showing the temperature change of the heating element part of the thermal head and each layer of the thermochromic layer in the conventional printing pulse supply method, and FIG. Diagram showing temperature changes in each layer of thermochromic layer, No. 6
The figure shows thickness control for clouding in a thermochromic layer, FIG. 7 is a block diagram of a thickness control circuit for clouding, and FIG. 8 is a timing chart for explaining the operation of thickness control. 11-15... Monostable multivibrator, 1
6... OR circuit, 17... Temperature detection thermistor, 21... Photoelectric conversion section, 22... CCD section of image sensor, 23... Shift register of thermal head, 24... Driver, 25... Heat generating element, 26... Comparator, 27... Analog switch.

Claims (1)

[Claims for Utility Model Registration] (1) A thermochromic film that becomes optically transparent when heated at a first predetermined temperature and becomes optically opaque when heated at a second predetermined temperature. In an overhead projector in which a projection film is formed by applying the second temperature using a thermal head in response to a reading signal of an original image, the thermal head prints while printing one pixel. 1. An overhead projector comprising circuit means for intermittently applying a printing pulse to an element once or multiple times. (2) The scope of the utility model registration claim, which is characterized in that halftones are expressed by changing the thickness of the opaque portion of the thermochromic layer by selectively changing the number of times printing pulses are supplied for each pixel. The overhead projector described in paragraph (1).